undefined and null variables often go hand-in-hand, and some use the terms interchangeably. Though, there is a difference between them: undefined is a variable that refers to something that doesn't exist, and the variable isn't defined to be anything. So you can assign the value null to any variable which basically means it's blank. let x; console.log(x); // prints undefined. Today, it is rationalized that null is considered a placeholder for an object, even though, technically, it is a primitive value. You can see this by using typeof() which returns a string representing the general type of a variable: Running the above script will result in the following output: Regardless of their being different types, they will still act the same if you try to access a member of either one, e.g. Here's a link Watch all the videos and enjoy :) - CoR Jun 30, 2015 at 14:42 2 I like to use C/C++ as the axiomatic truth for what an undefined and NULL valued variable is, since it's very straightforward. typeof undefined ; //"undefined" typeof null ; //"object" 2) In arithmetic operations But if we compare them with abstract equality ==, we get true. Code snippets Resources and more. 3. I guess it's a matter of preference (unless of course dictated by the code base or team conventions). This is an interesting perspective, and I think it could be expanded; and that's why I'm just posting this comment. Difference between undefined and null is undefined means a variable has been declared but has not yet been assigned a value, where null is an assignment value. Undefined 6. So when favourite object is logged to console it will throw ReferenceError and typeof for it will give undefined. For instance: In the above example, the variable is declared, but it has not been assigned a value. They may look very similar at first, but they are all actually different creatures. An object 2. typeof returns one of the following strings: If that last part is the case (the stringified version of the variable being, Using a variable that has never been declared and receiving a, When talking with a Backend of some kind you'll most probably receive JSON, While some backends correctly avoid the use of "null" in their JSON (removing those properties), others do not. Efficiently match all values of a vector in another vector, How to add a local CA authority on an air-gapped host of Debian. But they are emphatically not equal to each other. null ,which is also a type that is used to define the same thing, absence of a value, but it is not the same as undefined, as in this case you actually have the value in memory. With WSH you will see the dreaded "'varname' is null or not an object" and that's if you're lucky (but that's a topic for another article). both have no properties or methods and an attempt to read any properties of either will result in a run-time error (for all other objects, you get value. Can someone help me by explaining the difference. That must be done by you in your code. Another key difference between undefined and null is that undefined is a primitive value in JavaScript, while null is an object. In such a case null is a meaningful indicator of a missing object. Although they seem similar, they have different implications and should be used accordingly. This is my usage of both 5 primitives and Object type, and that explain the difference between use case of undefined or null. Building a safer community: Announcing our new Code of Conduct, Balancing a PhD program with a startup career (Ep. Helpful links for JavaScript "null" and "undefined" Null 7. null is of type null and undefined is of type undefined. It is an object. When is null or undefined used in JavaScript? Voice of a Developer: JavaScript From Scratch. undefined is a value automatically assigned to variables that have not been initialized or to object properties that do not exist. null is of type object, which is a historical quirk in JavaScript (typeof null === object). Undefined: In JavaScript, a variable that has been declared but has not been assigned a value is considered undefined. These values may seem similar, but understanding the difference between the two is essential for writing clean and reliable code. What does this mean when comparing with null? @Nagev: you need to decide whether you want to handle the error directly in the function or if you would like to have the caller handle it. So when we log value of person.firstName it will log null as it does not have any value with it. So JS interpreter interprets the below code as error: Uncaught SyntaxError: Unexpected token null. console.log(null-undefined). Listing contents of paragraphs using Google Apps Script, https://www.youtube.com/c/LaurenceSvekisCourses. Undefined means variable or object property is declared but hasnt been initialized or assigned any value. Why wouldn't a plane start its take-off run from the very beginning of the runway to keep the option to utilize the full runway if necessary? It also implies that you don't know the value that this variable is going to hold at the time of declaration. However, when you assign null to a variable, you are declaring that this value is explicitly empty. In summary, undefined is automatically assigned to a variable that has not been initialized or to a function parameter that has not been passed a value, while null is a value that represents the intentional absence of any object value. The function is expected to return an object: However, clone() might be invoked with a non-object argument: 15 or null (or generally a primitive value, null or undefined). Variables assigned with anything other than, Cannot be checked with the loose equality operator (. Using the triple equals, the values must be equal in type as well.But not in ==. When a variable is declared but not assigned a value, it is undefined. See The Abstract Equality Comparison Algorithm in the specification: 2. The latter can be done easily by making your function actually throw an Error. 3. Null and undefined values are equal when compared using the JavaScript equality operator. At the same time, below code runs successfully although I won't recommend doing so in real life: This works because undefined is a property on the global object (window object in case of JavaScript running in a browser). In addition to a different meaning there are other differences: In JavasScript there are 5 primitive data types: String, Number, Boolean, null and undefined. I will try to explain with some simple examples. null is a variable that is defined but is missing a value. 2023 - Programming Cube. Note that null and undefined receive a special treatment from == and != operators, but you can test true numeric equality of a and b with the expression (a >= b && a <= b). While undefined is a type all to itself, null is considered to be a special object value. Consider the following: Also, there is an important difference in the way null and undefined are treated in numeric context: null becomes 0 when used in arithmetic expressions or numeric comparisons - similarly to false, it is basically just a special kind of "zero". It represents the absence of a value. For the undefined type, there is one and only one value: undefined. Can I trust my bikes frame after I was hit by a car if there's no visible cracking? Only when using a truthy operator (==) may we see that javascript says its true but a strict comparison (===) is produces a false. 2. The variable number is defined, however, is not assigned with an initial value: number variable is undefined, which clearly indicates an uninitialized variable. That's the main difference between the Undefined and null. In JavaScript, a variable is said to be " undefined " if it has been declared but not initialized. If you compare undefined and null by using the null==undefined expression, they will be equal. The undefined value is a primitive value, which is used when a variable has not been assigned a value. For example, var demo; alert (demo); //shows undefined alert (typeof demo); //shows undefined A null value is like a holder that doesn't even have a tissue tube. For example pulled from the w3c page, given x = 5, x is an int, so x==="5" is false because it is comparing and int to string and x ===5 is true because it is both an int and the right value. What reason is there to use null instead of undefined in JavaScript? The former can be done by either still returning an object of some kind (with maybe only defaults or empty or undefined properties). Is there a grammatical term to describe this usage of "may be"? rev2023.6.2.43474. Unlike null, undefined is of the type undefined: console.log(typeof test2); // undefined. See The Strict Equality Comparison Algorithm in the specification: 1. Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, NaN. While they may seem similar at first glance, there are important differences between them in terms of their meaning and behavior within the language. Another instance where you will see undefined pop up is when using the delete operator. JavaScript. The difference in meaning between undefined and null is an accident of JavaScripts design, and it doesnt matter most of the time. Overview and Key Difference 2. I want to know what the difference is between null and undefined in JavaScript. If you know a variable has a mixed type (any type while all lifecycle), by convention, you could initialize it, to undefined. Also, in above function if(a == null) is the same as if(!a). The difference between null and undefined is NaN. Undefined is a value automatically assigned to a variable that has been declared but has not been initialized with a value, or to a function parameter that has not been passed a value. Always explicitly initialize them with a null value even if youre are not planning to use them too early. Learn about the difference between 'null' and 'undefined' in JavaScript and when to use them. In this example, we declare a variable "x" without initializing it and then log its value to the console, which is "undefined". This website is using a security service to protect itself from online attacks. They have different meanings and use cases, and understanding their differences is important to avoid unexpected behavior in your code. If Type(x) is Null, return true. What is the difference between void, never, null and undefined in TypeScript? Hope you learn something new. and you will get 5 as output. Check this out. This is especially true when we are discussing a program that gets data from multiple sources or you don't have control of the structure of the datasource. What is null? It is typically used when you want to explicitly state that there is no object or value assigned, as opposed to undefined which represents an uninitialized variable. In JavaScript, undefined is also returned when you try to access a property or a method of an object that does not exist. Cloudflare Ray ID: 7d1387800ca5caa9 A lot of "technical" answers have been given, all of them mostly correct from the limited point of view of JS as a mere programming language. Most of the answers ignore the fact that you can, Thank you, and yes, that misconception that something is. String: represents a sequence of characters enclosed in quotes. What is Null? typeof null === 'object' but that's a bug! The object data type can contain: 1. 1. This is in the spec, but you would never know this because of the typeof weirdness which I will not repeat here. In other words, it represents the intentional absence of any object value. Later on, when we assign a value, it will no longer be of undefined type. First story of aliens pretending to be humans especially a "human" family (like Coneheads) that is trying to fit in, maybe for a long time? difference between null and undefined in JavaScript? null is of type object, which is a quirk of JavaScript (typeof null === object). However, I would like to add the following thoughts, especially when you're writing TypeScript code as part of a bigger project / (enterprise) application: Therefore, in an effort to harmonize things I'm strictly against using "null" and want to encourage you to stop using "null" in your code. null is a special object because typeof null returns 'object'. In Portrait of the Artist as a Young Man, how can the reader intuit the meaning of "champagne" in the first chapter? undefined means a variable has been declared but has not yet been assigned a value : null is an assignment value. for example: console.log(typeof(abc)); undefined, The comment from Nir O. is very important. Undefined: Undefined is a built-in value in JavaScript, which is assigned to a variable that is declared but has not been assigned a value. Example of variable the global object of the run-time environment you're in. I hope you found this article useful. rev2023.6.2.43474. null vs. undefined and their behaviour in JavaScript. Each variable is stored as variable_name : variable_value/reference. The difference can be explained with toilet tissue holder: A non-zero value is like a holder with roll of toilet tissue and there's tissue still on the tube. Like, defined empty variable is null of datatype undefined, Both of them are representing a value of a variable with no value, AND It is a value that is automatically assigned to a variable when no other value is assigned. The null value is a primitive value that represents the null, empty, In the above example, the planet variable is directly used without its declaration. Please read the following carefully. What is the difference between Null, NaN and undefined in JavaScript? A function reaching the end of its body without an explicit return statement returns undefined since you don't know anything about what it returned. Oops! Free Videos on YouTube https://www.youtube.com/c/LaurenceSvekisCourses, GitHub Source Code https://github.com/lsvekis, Connect with Laurence Svekis on Linkedin Share your course certificates https://www.linkedin.com/in/svekis/, Udemy Courses https://www.udemy.com/user/lars51/, 2023 Learn to Code Tutorials Resources Tips Google Apps Script JavaScript Web Development. So when we declare as object and initialize, it will work as regular object and will be no more of undefined type. How JS engine distinguish null from undefined, Exists any functional reason to use undefined in TypeScript? 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows. Null is basically an assignment value given to a variable. So JavaScript does consider these to be relatively equal since they both represent an empty value. It's worth noting that while this comment was true in '11, with the advent of optional function params, emergence of type-checking systems like Flow, and pervasiveness of React (all of which treat undefined and null very differently), the old wisdom of generally using null rather than undefined no longer holds so strictly. But as we only have one property firstName whose value is null. Basically, Undefined is a global variable that javascript create at the run time whether null means that no value has assigned to the variable (actually null is itself an object). If this article was helpful, share it. For example, clone() is a function that clones a plain JavaScript object. Heres a detailed explanation of the difference between null and undefined: undefined is a primitive value in JavaScript. As typeof returns undefined, undefined is a type where as null is an initializer indicates the variable points to no object(virtually everything in Javascript is an object). JavaScript has a scope chain different than that of C-style languages, easily confusing even veteran programmers, and setting variables to null is the best way to prevent bugs based on it. In JavaScript, null and undefined are two special values that indicate the absence of a value. Lets understand difference between undefined, null and undeclared with examples. THE BASICS Let us start with some of the raw basics - Just what are NULL, UNDEFINED, and EMPTY in Javascript? A variable whose value is null was explicitly given a value of null, which means that the variable was explicitly set to have no value. You can email the site owner to let them know you were blocked. ), Know about undefined and it's relationship with scope, Quote from the book Professional JS For Web Developers (Wrox): "You may wonder why the typeof operator returns 'object' for a value that is null. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. It is often used to indicate that a variable or object property should have no value or an empty value. It's far easier than you might think. I'm not talking about not handling "null" values, only to avoid explicitly using them in your code. In that case how to denote when you want a variable to be without value at a later point in the code? Various examples of null and undefined in JavaScript will be covered. Connect and share knowledge within a single location that is structured and easy to search. QGIS - how to copy only some columns from attribute table. Connect and share knowledge within a single location that is structured and easy to search. What is null in JavaScript? undefined is actually preferable to null in many cases where you want to explicitly use the default value (e.g. Although they are often used interchangeably, there is a subtle difference between them. null and undefined are two distinct object types which have the following in common: The similarities however end here. The difference between undefined and null is minimal, but there is a difference. Undefined that's what you will get as output. Don't miss out on the latest news. Undefined is a primitive value in JavaScript that represents the absence of a value. But when you use them in left hand side (LHS) of an expression then you can observe this difference easily. In the first example, accessing a property of undefined throws a TypeError, which may cause unexpected behavior in your code. When you declare a variable through var and do not give it a value, it will have the value undefined. for an optional param or optional React prop). Why does this trig equation have only 2 solutions and not 4? Null as an assignment value. Can you identify this fighter from the silhouette? What is the difference between "let" and "var"? In JavaScript, null and undefined are two special values that are used to represent the absence of a value. On the other hand, undefined represents the absence of any value. Lets understand difference between undefined, null and undeclared with examples. Undeclared means variable or object property is used but it hasnt been declared yet. so basically null value means a variable has been explicitly set as (no value = null) or has been initialized and defined to be nothing. He wouldn't know if the developer forgot to initialize this variable or if it didn't have any value. In this guide, we have compared the null and undefined values in JavaScript and we have found out that: both are primitives; the two values are equal but not identical; null is a number whereas undefined is not; null is of the type object whereas undefined is of the type undefined; null plays well with numerical operations whereas undefined . Never leave variables undefined or undeclared. Do "Eating and drinking" and "Marrying and given in marriage" in Matthew 24:36-39 refer to the end times or to normal times before the Second Coming? It's more important to look at this in the context of the ecosystem, you generally cannot garuntee your datasource is differentiating between Unknown and Null in the same way that Javascript the programming language does. Your IP: But just to point out, when u checked "undefined == null" the type checking was not strict. This way - when I mistype "if (myxar == null) {}" - the if block is not executed. When an event is triggered on an element, it first triggers on that element and then "bubbles up" through all its ancestor elements. It can be assigned to a variable as a representation of no value : From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object. Yes these are not even undefined although they compare === to undefined. a === null is true if the value of a is null. Here are a two other examples of how you would get an undefined variable in JavaScript: The same meaning is analogous to the meaning of null in programming. null and undefined are both are used to represent the absence of some value. Undefined is a variable that has been declared but not assigned a value. Later on, when we assign a value to firstName, it will no longer be null or object type. How to store objects in HTML5 localStorage/sessionStorage. To put it simply, JavaScript uses undefined and programmers should use null. If x is null and y is undefined, return true. Variables assigned with anything other than undefined or null. why does javascript have both null and undefined? console.log(typeof obj); // Output: object. It is also returned by functions that do not explicitly return a value. or non-existent reference. 5 Best Free Online Word Processors for You, How to Learn to Code from Scratch for Beginners, How Do I Make the First Letter of a String Uppercase in JavaScript, How Do I Generate Random Integers Within a Specific Range in Java. It is also the default value for function . a == null is true if the value of a is null or undefined. It is also the default return value of a function that does not explicitly return anything. But here, as we only have single property firstName whose value isnt assigned yet. When to use each? If you check "undefined === null", it would return false. The intent of, This is what I think I'm going to use as well. Undefined means a variable has been declared, but the value of that variable has not yet been defined. This is most common interview question in javascript interview questions. The difference between null and undefined is: JavaScript will never set anything to null, that's usually what we do. Unlike undefined, null is a value that represents the absence of a value. been assigned a value. console.log(typeof undefined); // Output: undefined. CONTENTS 1. Objects are the foundation of JavaScript and almost everything in JavaScript is considered as object. null belongs to the type Null and undefined to the type Undefined. Your feedback helps me to become better. On the other hand, null indicates a variable or object property has no value. Example Why is null an object and what's the difference between null and undefined? 12 Null is not an object in JavaScript! [duplicate]. null - It is an assignment value, which is used with variable to represent no value (it's an object). Insufficient travel insurance to cover the massive medical expenses for a visitor to US? typeof null would actually return the string "object", which is more confusing than helping. null, true and false are reserved keywords - compiler won't let you use them as variable or property names. What is the difference between call and apply? For example - Some people don't have a middle name. Don't get me wrong. 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows. You are free to use those values explicitly to convey those meanings. It is often used to assign an empty or non-existent value to an object or a variable. Null and Undefined are both data types in JavaScript. The undefined value is a primitive value used when a variable has not Performance & security by Cloudflare. On the other hand, null is a value explicitly assigned to a variable or object property to indicate that it has no value. Differences: Undefined is automatically assigned to a variable that is declared but has not been assigned a value, while null is assigned manually to indicate the intentional absence of a. In the above example, typeof returns undefined for the value undefined, and object for the value null. It is the global object. Definition: Null: It is the intentional absence of the value. In javascript all variables are stored as key value pairs. null must be assigned. 24 What does this mean when comparing with null? What is the difference between "undefined" and undefined? Same here in the above example, person object is declared with one property firstName, but no value is assigned to it. Now you can explicitly assign a variable null. Both null and undefined have their own significance and should be used appropriately based on the context. Speed. For example: let firstName; console.log(firstName); // Output: undefined I recommend only setting variables to null and leave undefined the value for things you forgot to set. There are several actions that could trigger this block including submitting a certain word or phrase, a SQL command or malformed data. Both special values imply an empty state. So by not declaring a value to a variable, JavaScript automatically assigns the value to undefined. Both will return the same result. It is undecidable in general. For example: let y = null;console.log(y); // Output: null, let person = {name: "John", age: 30};person.age = null; // intentionally setting the age property to nullconsole.log(person.age); // Output: null. In summary, undefined is the default value of a declared but unassigned variable, while null is a value that needs to be explicitly assigned. In other cases, you know that a variable expects to hold an object or a function to return an object. Undefined means variable or object property is declared but hasn't been initialized or assigned any value. In conclusion, understanding the difference between null and undefined in JavaScript is important for writing efficient and effective code. null is also a data type. When a variable is declared but not assigned a value, it is automatically assigned the value undefined. Why does bunched up aluminum foil become so extremely hard to compress? While "null" is a special keyword that indicates an absence of value, "undefined" means "it does not exist". We also declare an object "person" with two properties, "name" and "age", and then intentionally set the "age" property to null to indicate that it has no value. The key difference between null and undefined in JavaScript is that null is used to assign a non-value to a variable while undefined is used when a variable is declared but not assigned with a value. Does the policy change for AI-generated content affect users who (want to) difference between is_null "== NULL" and "=== NULL" in PHP. Learn to Code Tutorials Resources Tips Google Apps Script JavaScript Web Development. In summary, undefined is the default value for uninitialized variables, and it is automatically assigned, whereas null needs to be explicitly assigned to indicate intentional absence or emptiness. Is there any function difference between using != null and != undefined in javascript? a === null is true if the value of a is null. In such case, the function cannot create a clone, so it returns null - the indicator of a missing object. Because of that, you can never know if a "null" value really means deliberate absence. Undefined: It means the value does not exist in the compiler. var myVar = null; console.log (myVar); // null console.log (typeof myVar); // object #foryou #javascript #code #programming #hack #tips #codingtips #techtok. So when we log value of person.firstName it will log null as a value. In the second example, since the first operand is a string, all operands are treated as . In this example, x is declared but not initialized, so its value is automatically set to undefined. Two attempts of an if with an "and" are failing: if [ ] -a [ ] , if [[ && ]] Why? Instead, null expresses a lack of identification, indicating that a variable points to no object. As a best practice, you should not use this type as an assignment. undefined property indicates that a variable has not been assigned a value including null too . 3 - null : Variable declared and is an empty value. The undefined is a global object. How can an accidental cat scratch break skin but not damage clothes? So if you need to check if a value is either null or undefined, you can check for abstract equality and compare it to either null or undefined. From what I understand the === does an exact match but what does this mean when comparing with null ? How to say They came, they saw, they conquered in Latin? Declared but not defined - JavaScript assigns 'undefined' to any object that has been declared but not initialized or defined. === means it checks both the value and the type of the variables. null: absence of value for a variable; undefined: absence of variable itself; ..where variable is a symbolic name associated with a value. In the above example, the variable name is declared but not assigned a value, so it is undefined. While undefined means. Beyond that, these two special values are nearly equivalent. For example: Number: represents both integer and floating-point numbers. undefined It means a variable declared, but no value has been assigned a value. Therefore you cannot assume null is intentional for this reason you must treat them same. 2. For example: Actually, null is a special keyword, not an identifier, and thus you cannot treat it as a variable to assign to. Undefined: represents, In JavaScript, events are actions or occurrences that happen in the browser or web page, such as a user clicking a button or a page finishing loading. Also, you can use the utility function at the end of this answer to get more specific types of variables. The main difference between JavaScript null and undefined is that null is an intentional absence of value, whereas undefined is the value that does not exist in the compiler. When you try to access its value, it returns undefined. The variable y is explicitly set to null. A defined variable starts with the value, Even after these many years of this answer being posted, I think I agree with this answer. We see here that the type of null is an object but the type of undefined is undefined. But if you check them with == as below, you see they are both falsy: Also you can assign null to an object property or to a primitive, while undefined can simply be achieved by not assigning to anything. Null, on the other hand, is a deliberate non-value. It means that the variable does not have any value. We can find the datatypes of both undefined and null using the typeof operator. So in such a case its better to assign the value null to the middlename variable of a person object. This will give undefined; we have declared a variable but we have not asigned any value to this variable; so null is an object. However, they have different meanings and use cases. Boolean: represents a logical entity and can have only two values, either true or false. undefined object properties are also undefined. This is not obvious, but consider the following example: undefined, NaN and Infinity are just names of preinitialized "superglobal" variables - they are initialized at run-time and can be overridden by normal global or local variable with the same names. In JavaScript, null and undefined are both used to represent the absence of a value, but they have slightly different meanings. Important: Don't confuse the internal Type function with the typeof operator. Both undefined and null are data types and values. JS | What's the Difference Between null and undefined in JavaScript? Read next The null value is a primitive value which represents the null, empty, or non-existent reference. Alternatively you can just return, They're all good options, thanks for clarifying. null is a primitive value that represents the intentional absence of any object value. Instead, use indirect type guards (aka truthiness checks) e.g. So by not declaring a value to a variable, JavaScript automatically assigns the value to undefined. If you have any questions or suggestions, please leave comments. null is a special keyword that indicates an absence of value. This was actually an error in the original JavaScript implementation that was then copied in ECMAScript. Typescript - What is the difference between null and undefined? Empty cells in sparse arrays. 185.30.35.2 (Note that this is an attempt at humour, before you flame me for misunderstanding the question. However, they have different use cases and behaviors. So you can assign the value null to any variable which basically means its blank. In JavaScript there are 5 different data types that can contain values: string number boolean object function There are 6 types of objects: Object Date Array String Number Boolean And 2 data types that cannot contain values: null undefined The typeof Operator You can use the typeof operator to find the data type of a JavaScript variable. So, undefined is a value that indicates the absence of a value, while null is a value that indicates the intentional absence of any object reference. Hence it returned "true". We have heard the word " null " many times in English. Whereas " null " is assigned to a variable whose value is absent at the time of initialization. However, if you append a blank string to it then suddenly it'll appear: You can declare a variable, set it to null, and the behavior is identical except that you'll see "null" printed out versus "undefined". Undefined most typically means a variable has been declared, but not defined. What is the difference between null and undefined in JavaScript? Personally, I like throwing exceptions sparingly, as I find it annoying having to wrap everything in, This answer is misleading see the discussion in the accepted answer. Facebook Page: https://www.facebook.com/designTechWorld1, Instagram Page: https://www.instagram.com/techd.esign/, Youtube Channel: https://www.youtube.com/@tech..Design/, Twitter: https://twitter.com/sumit_singh2311, You can prefer React Book: https://amzn.to/3Tw29nx. But for some reason, you cant instantiate the object. QGIS - how to copy only some columns from attribute table. console.log (null-undefined). undefined: It means that the object doesn't have any value, therefore undefined.This occurs when you create a variable and don't assign a value to it. Those of us from a C-world might incorrectly interpret this as destroying an object, but it is not so. Null means variable or object property is declared but hasnt been initialized or assigned any value. If you know a variable is only a string while all lifecycle, by convention, you could initialize it, to "": If you know a variable is only a number while all lifecycle, by convention, you could initialize it, to 0 (or NaN if 0 is an important value in your usage): If you know a variable is only a boolean while all lifecycle, by convention, you could initialize it, to false: If you know a variable is only an Object while all lifecycle, by convention, you could initialize it, to null: Note: the smart usage off null is to be the falsy version of an Object because an Object is always true, and because typeof null return object. How much of the power drawn by a chip turns into heat? You can use the type See The Strict Equality Comparison Algorithm in the specification: 1. However, there is a difference between them: undefined is a value that is assigned to a variable that has been declared but has not been assigned a value. For example: undefined: This is another special value that. It is a type itself. It is also considered falsy in JavaScript. See for yourself. 2.On === operator it gives false. null and undefined are "similar" to each other because Brendan Eich said so. Difference Between undefined and null. Difference between undefined and null. 00:00 / 00:00. Find centralized, trusted content and collaborate around the technologies you use most. undefined means a variable has been given a space in memory, but no value is assigned to it. While writing code, this difference is not identifiable as both null and undefined are always used in right hand side (RHS) of a JavaScript statement. Enabling a user to revert a hacked change in their email. it was probably never initialized or if it was it was never defined. (Note that this is an attempt at humour, before you flame me for misunderstanding the question.) 3. So for both of them, the label is both its type and its value. It means a variable has been declared but has not yet been assigned a value. If it has the value null, then the user can easily infer that middlename doesn't have any value and it is not an untouched variable. Angular and Javascript. In JavaScript, "undefined" and "null" are two distinct values that are often used to represent the absence or emptiness of a value. When a variable is declared but not assigned a value, it is automatically initialized with undefined. It needs to be explicitly assigned to a variable. It can be assigned to a variable as a representation of no value. If Type(x) is different from Type(y), return false. I don't have this advantage with undefined: myvar = undefined; myvar = 4; if (typeof myxar == "undefined") { }. A front-end developer. eg. Can I takeoff as VFR from class G with 2sm vis. OK, we may get confused when we hear about null and undefined, but let's start it simple, they both are falsy and similar in many ways, but weird part of JavaScript, make them a couple of significant differences, for example, typeof null is 'object' while typeof undefined is 'undefined'. Both are similar but usage and meaning are different. How to correctly use LazySubsets from Wolfram's Lazy package? User Interface (UI) events: These events, In JavaScript, event bubbling is a mechanism that describes the way events propagate up through the DOM hierarchy. Here's a detailed explanation of the difference between null and undefined: undefined: undefined is a primitive value in JavaScript. Use the === operator to check whether a variable is null or undefined. When is "undefined" equal to "null" in JavaScript? What are the differences between var, let, and const? Null: represents a deliberate non-value or absence of any object value. So, only if Type(a) is Null, the comparison returns true. 2 - undefined: Variable declared but does not initialised ", the variable might as well not be defined at all. The community reviewed whether to reopen this question last year and left it closed: Original close reason(s) were not resolved. Javascripts actual implementation is unimportant in this context. console.log(doSomething()); // Output: undefined. We also define a function "test" with a parameter "y" but do not pass any value to it when calling the function, which also logs "undefined" to the console. `` similar '' to each other because Brendan Eich said so about not handling null... Of us from a C-world might incorrectly interpret this as destroying an object that does not have any value operator. Value ( e.g service to protect itself from online attacks special keyword that indicates an absence of any object.! When favourite object is logged to console it will log null as it does not initialised ``, the can... 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With undefined of Debian Exists any functional reason to use undefined in JavaScript considered... Is logged to console it will give undefined expression, they have different meanings and cases! & technologists worldwide, NaN and undefined are both used to represent the absence a... Several actions that could trigger this block including submitting a certain word or phrase, a variable points to object! Variable as a best practice, you cant instantiate the object the indicator of a is null or.. For writing efficient and effective code to explicitly use the === operator to check whether a variable expects to at... A detailed explanation of the power drawn by a chip turns into heat is an... (! a ) is null takeoff as VFR from class G with 2sm vis will get as Output some. Not have any value '' to each other because Brendan Eich said so let, and that the... As VFR from class G with 2sm vis empty value Equality operator.... Strict Equality Comparison Algorithm in the spec, but there is a value most typically means a variable been. Js engine distinguish null from undefined, and I think it could be expanded ; that! Location that is structured and easy to search ; and that 's the main between., empty, or non-existent reference them, the comment from Nir is... And is an empty value but the type null and! = null and undefined JavaScript. And! = undefined in JavaScript, undefined is of type object, which is used a... Because typeof null === object ) to represent the absence of some value in == are treated as to explicitly! The BASICS let us start with some simple examples as regular object initialize... False are reserved keywords - compiler wo n't let you use them too early to point,. Much of the run-time environment you 're in s blank only 2 solutions not! Of the variables var, let, and it doesnt matter most of the time of initialization code! Foundation of JavaScript and almost everything in JavaScript is important for writing clean and reliable..
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