What is the difference between null and undefined in JavaScript?

Questions : What is the difference between null and undefined in JavaScript?

I want to know what the difference is between null and undefined in JavaScript.

Total Answers: 38 Answers 38

Popular Answers:

  1. undefined means a variable has been declared but has not yet been assigned a value :

    var testVar; alert(testVar); //shows undefined alert(typeof testVar); //shows undefined

    null is an assignment value. It can be assigned to a variable as a representation of no value :

    var testVar = null; alert(testVar); //shows null alert(typeof testVar); //shows object

    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.

    Proof :

    console.log(null === undefined) // false (not the same type) console.log(null == undefined) // true (but the "same value") console.log(null === null) // true (both type and value are the same)


    null = 'value' // Uncaught SyntaxError: invalid assignment left-hand side undefined = 'value' // 'value' 
  2. 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.

    • A zero value is like a holder with an empty toilet tissue tube.

    • A null value is like a holder that doesn’t even have a tissue tube.

    • An undefined value is similar to the holder itself being missing.

  3. I picked this from here

    The undefined value is a primitive value used when a variable has not been assigned a value.

    The null value is a primitive value that represents the null, empty, or non-existent reference.

    When you declare a variable through var and do not give it a value, it will have the value undefined. By itself, if you try to WScript.Echo() or alert() this value, you won’t see anything. However, if you append a blank string to it then suddenly it’ll appear:

    var s; WScript.Echo(s); WScript.Echo("" + s); 

    You can declare a variable, set it to null, and the behavior is identical except that you’ll see “null” printed out versus “undefined”. This is a small difference indeed.

    You can even compare a variable that is undefined to null or vice versa, and the condition will be true:

    undefined == null null == undefined 

    They are, however, considered to be two different types. While undefined is a type all to itself, null is considered to be a special object value. You can see this by using typeof() which returns a string representing the general type of a variable:

    var a; WScript.Echo(typeof(a)); var b = null; WScript.Echo(typeof(b)); 

    Running the above script will result in the following output:

    undefined object 

    Regardless of their being different types, they will still act the same if you try to access a member of either one, e.g. that is to say they will throw an exception. 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).

    You can explicitely set a variable to be undefined, but I highly advise against it. I recommend only setting variables to null and leave undefined the value for things you forgot to set. At the same time, I really encourage you to always set every variable. 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.

    Another instance where you will see undefined pop up is when using the delete operator. Those of us from a C-world might incorrectly interpret this as destroying an object, but it is not so. What this operation does is remove a subscript from an Array or a member from an Object. For Arrays it does not effect the length, but rather that subscript is now considered undefined.

    var a = [ 'a', 'b', 'c' ]; delete a[1]; for (var i = 0; i < a.length; i++) WScript.Echo((i+".) "+a[i]); 

    The result of the above script is:

    0.) a 1.) undefined 2.) c 

    You will also get undefined returned when reading a subscript or member that never existed.

    The difference between null and undefined is: JavaScript will never set anything to null, that’s usually what we do. While we can set variables to undefined, we prefer null because it’s not something that is ever done for us. When you’re debugging this means that anything set to null is of your own doing and not JavaScript. Beyond that, these two special values are nearly equivalent.

  4. Please read the following carefully. It should remove all your doubts regarding the difference between null and undefined in JavaScript. Also, you can use the utility function at the end of this answer to get more specific types of variables.

    In JavaScript we can have the following types of variables:

    1. Undeclared Variables
    2. Declared but Unassigned Variables
    3. Variables assigned with literal undefined
    4. Variables assigned with literal null
    5. Variables assigned with anything other than undefined or null

    The following explains each of these cases one by one:

    1. Undeclared Variables

      • Can only be checked with the typeof operator which returns string ‘undefined’
      • Cannot be checked with the loose equality operator ( == undefined ), let alone the strict equality operator ( === undefined ),
        as well as if-statements and ternary operators ( ? : ) — these throw Reference Errors
    2. Declared but Unassigned Variables

      • typeof returns string ‘undefined’
      • == check with null returns true
      • == check with undefined returns true
      • === check with null returns false
      • === check with undefined returns true
      • Is falsy to if-statements and ternary operators ( ? : )
    3. Variables assigned with literal undefined
      These variables are treated exactly the same as Declared But Unassigned Variables.

    4. Variables assigned with literal null

      • typeof returns string ‘object’
      • == check with null returns true
      • == check with undefined returns true
      • === check with null returns true
      • === check with undefined returns false
      • Is falsy to if-statements and ternary operators ( ? : )
    5. Variables assigned with anything other than undefined or null

      • typeof returns one of the following strings: ‘bigint’, ‘boolean’, ‘function’, ‘number’, ‘object’, ‘string’, ‘symbol’

    Following provides the algorithm for correct type checking of a variable:

    1. Get the typeof our variable and return it if it isn’t ‘object’
    2. Check for null, as typeof null returns ‘object’ as well
    3. Evaluate Object.prototype.toString.call(o) with a switch statement to return a more precise value. Object‘s toString method returns strings that look like ‘[object ConstructorName]’ for native/host objects. For all other objects (user-defined objects), it always returns ‘[object Object]’
    4. If that last part is the case (the stringified version of the variable being ‘[object Object]’) and the parameter returnConstructorBoolean is true, it will try to get the name of the constructor by toString-ing it and extracting the name from there. If the constructor can’t be reached, ‘object’ is returned as usual. If the string doesn’t contain its name, ‘anonymous’ is returned

    (supports all types up to ECMAScript 2020)

    function TypeOf(o, returnConstructorBoolean) { const type = typeof o if (type !== 'object') return type if (o === null) return 'null' const toString = Object.prototype.toString.call(o) switch (toString) { // Value types: 6 case '[object BigInt]': return 'bigint' case '[object Boolean]': return 'boolean' case '[object Date]': return 'date' case '[object Number]': return 'number' case '[object String]': return 'string' case '[object Symbol]': return 'symbol' // Error types: 7 case '[object Error]': return 'error' case '[object EvalError]': return 'evalerror' case '[object RangeError]': return 'rangeerror' case '[object ReferenceError]': return 'referenceerror' case '[object SyntaxError]': return 'syntaxerror' case '[object TypeError]': return 'typeerror' case '[object URIError]': return 'urierror' // Indexed Collection and Helper types: 13 case '[object Array]': return 'array' case '[object Int8Array]': return 'int8array' case '[object Uint8Array]': return 'uint8array' case '[object Uint8ClampedArray]': return 'uint8clampedarray' case '[object Int16Array]': return 'int16array' case '[object Uint16Array]': return 'uint16array' case '[object Int32Array]': return 'int32array' case '[object Uint32Array]': return 'uint32array' case '[object Float32Array]': return 'float32array' case '[object Float64Array]': return 'float64array' case '[object ArrayBuffer]': return 'arraybuffer' case '[object SharedArrayBuffer]': return 'sharedarraybuffer' case '[object DataView]': return 'dataview' // Keyed Collection types: 2 case '[object Map]': return 'map' case '[object WeakMap]': return 'weakmap' // Set types: 2 case '[object Set]': return 'set' case '[object WeakSet]': return 'weakset' // Operation types: 3 case '[object RegExp]': return 'regexp' case '[object Proxy]': return 'proxy' case '[object Promise]': return 'promise' // Plain objects case '[object Object]': if (!returnConstructorBoolean) return type const _prototype = Object.getPrototypeOf(o) if (!_prototype) return type const _constructor = _prototype.constructor if (!_constructor) return type const matches = Function.prototype.toString.call(_constructor).match(/^functions*([^s(]+)/) return matches ? matches[1] : 'anonymous' default: return toString.split(' ')[1].slice(0, -1) } } 
  5. null is a special keyword that indicates an absence of value.

    think about it as a value, like:

    • “foo” is string,
    • true is boolean ,
    • 1234 is number,
    • null is undefined.

    undefined property indicates that a variable has not been assigned a value including null too . Like

    var foo; 

    defined empty variable is null of datatype undefined

    Both of them are representing a value of a variable with no value

    AND null doesn’t represent a string that has no value – empty string-


    var a = ''; console.log(typeof a); // string  console.log(a == null); //false  console.log(a == undefined); // false  

    Now if

    var a; console.log(a == null); //true console.log(a == undefined); //true  


    var a; console.log(a === null); //false  console.log(a === undefined); // true 

    SO each one has it own way to use

    undefined use it to compare the variable data type

    null use it to empty a value of a variable

    var a = 'javascript'; a = null ; // will change the type of variable "a" from string to object  
  6. null: absence of value for a variable; undefined: absence of variable itself;

    ..where variable is a symbolic name associated with a value.

    JS could be kind enough to implicitly init newly declared variables with null, but it does not.

  7. You might consider undefined to represent a system-level, unexpected, or error-like absence of value and null to represent program-level, normal, or expected absence of value.

    via JavaScript:The Definitive Guide

  8. The best way to understand the difference is to first clear your mind of the inner workings of JavaScript and just understand the differences in meaning between:

    let supervisor = "None" // I have a supervisor named "None" let supervisor = null // I do NOT have a supervisor. It is a FACT that I do not. let supervisor = undefined // I may or may not have a supervisor. I either don't know // if I do or not, or I am choosing not to tell you. It is // irrelevant or none of your business. 

    There is a difference in meaning between these three cases, and JavaScript distinguishes the latter two cases with two different values, null and undefined. You are free to use those values explicitly to convey those meanings.

    So what are some of the JavaScript-specific issues that arise due to this philosophical basis?

    1. A declared variable without an initializer gets the value undefined because you never said anything about the what the intended value was.

      let supervisor; assert(supervisor === undefined); 
    2. A property of an object that has never been set evaluates to undefined because no one ever said anything about that property.

      const dog = { name: 'Sparky', age: 2 }; assert(dog.breed === undefined); 
    3. null and undefined are “similar” to each other because Brendan Eich said so. But they are emphatically not equal to each other.

      assert(null == undefined); assert(null !== undefined); 
    4. null and undefined thankfully have different types. null belongs to the type Null and undefined to the type Undefined. This is in the spec, but you would never know this because of the typeof weirdness which I will not repeat here.

    5. 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.

    By the way, there are other forms of “nothingness” in JavaScript (it’s good to have studied Philosophy….)

    • NaN
    • Using a variable that has never been declared and receiving a ReferenceError
    • Using a let or const defined local variable in its temporal dead zone and receiving a ReferenceError
    • Empty cells in sparse arrays. Yes these are not even undefined although they compare === to undefined.

      $ node > const a = [1, undefined, 2] > const b = [1, , 2] > a [ 1, undefined, 2 ] > b [ 1, <1 empty item>, 2 ] 
  9. I’ll explain undefined, null and Uncaught ReferenceError:

    1 – Uncaught ReferenceError : variable has not been declared in your script, there is no reference to this varaible
    2 – undefined: Variable declared but does not initialised
    3 – null : Variable declared and is an empty value

  10. null and undefined are two distinct object types which have the following in common:

    • both can only hold a single value, null and undefined respectively;
    • 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 undefined if you try to read a non-existent property);
    • values null and undefined are considered equal to each other and to nothing else by == and != operators.

    The similarities however end here. For once, there is a fundamental difference in the way how keywords null and undefined are implemented. This is not obvious, but consider the following example:

    var undefined = "foo"; WScript.Echo(undefined); // This will print: foo 

    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.

    Now, let’s try the same thing with null:

    var null = "foo"; // This will cause a compile-time error WScript.Echo(null); 

    Oops! null, true and false are reserved keywords – compiler won’t let you use them as variable or property names

    Another difference is that undefined is a primitive type, while null is an object type (indicating the absense of an object reference). Consider the following:

    WScript.Echo(typeof false); // Will print: boolean WScript.Echo(typeof 0); // Will print: number WScript.Echo(typeof ""); // Will print: string WScript.Echo(typeof {}); // Will print: object WScript.Echo(typeof undefined); // Will print: undefined WScript.Echo(typeof null); // (!!!) Will print: object 

    Also, there is an important difference in the way null and undefined are treated in numeric context:

    var a; // declared but uninitialized variables hold the value undefined WScript.Echo(a === undefined); // Prints: -1 var b = null; // the value null must be explicitly assigned  WScript.Echo(b === null); // Prints: -1 WScript.Echo(a == b); // Prints: -1 (as expected) WScript.Echo(a >= b); // Prints: 0 (WTF!?) WScript.Echo(a >= a); // Prints: 0 (!!!???) WScript.Echo(isNaN(a)); // Prints: -1 (a evaluates to NaN!) WScript.Echo(1*a); // Prints: -1.#IND (in Echo output this means NaN) WScript.Echo(b >= b); // Prints: -1 (as expected) WScript.Echo(isNaN(b)); // Prints: 0 (b evaluates to a valid number) WScript.Echo(1*b); // Prints: 0 (b evaluates to 0) WScript.Echo(a >= 0 && a <= 0); // Prints: 0 (as expected) WScript.Echo(a == 0); // Prints: 0 (as expected) WScript.Echo(b >= 0 && b <= 0); // Prints: -1 (as expected) WScript.Echo(b == 0); // Prints: 0 (!!!) 

    null becomes 0 when used in arithmetic expressions or numeric comparisons – similarly to false, it is basically just a special kind of “zero”. undefined, on the other hand, is a true “nothing” and becomes NaN (“not a number”) when you try to use it in numeric context.

    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).

  11. null is a special value meaning “no value”. null is a special object because typeof null returns ‘object’.

    On the other hand, undefined means that the variable has not been declared, or has not been given a value.

  12. 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.

    However, I would like to add the following thoughts, especially when you’re writing TypeScript code as part of a bigger project / (enterprise) application:

    • 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
    • Now, while “null” may mean that the value is missing deliberately, more often it does not convey this meaning. Most databases use “null” just because they don’t have an “undefined” type. But the meaning really just is “undefined”.
    • Because of that, you can never know if a “null” value really means deliberate absence. Therefore “null” cannot really mean the deliberate choice of “missing value”. It is undecidable in general.
    • As a consequence, semantically, “null” and “undefined” are exactly the same thing in practice.

    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. It’s far easier than you might think. Don’t get me wrong. I’m not talking about not handling “null” values, only to avoid explicitly using them in your code. Put differently: your code should still be able to work with accidentally passed “null” values coming from outside your application, e.g. via a 3rd party lib like Angular, or a 3rd party backend.

    Here are the guidelines that make it possible:

    • avoid direct undefined type guards (e.g. if (value === undefined) { ... }.
    • Instead, use indirect type guards (aka truthiness checks) e.g. if (value) { ... }
      • Whenever 0 or empty strings are meaningful, use either
        • an explicit helper method like Lodash’s isNil
        • or include the meaningful value in the comparison (e.g. if (!value && value !== 0) { ... })
    • Consider using a lint rule that disallows the usage of null
  13. Undefined means a variable has been declared but has no value:

    var var1; alert(var1); //undefined alert(typeof var1); //undefined 

    Null is an assignment:

    var var2= null; alert(var2); //null alert(typeof var2); //object 
  14. tl;dr

    Use null for set a variable you know it is an Object.

    Use undefined for set a variable whose type is mixed.

    This is my usage of both 5 primitives and Object type, and that explain the difference between « use case » of undefined or null.


    If you know a variable is only a string while all lifecycle, by convention, you could initialize it, to "":

    ("") ? true : false; // false typeof ""; // "string"; ("Hello World") ? true : false; // true typeof "Hello World"; // "string" 


    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):

    (0) ? true : false; // false typeof 0; // "number"; (16) ? true : false; // true typeof 16; // "number" 


    (NaN) ? true : false; // false typeof NaN; // "number"; (16) ? true : false; // true typeof 16; // "number" 


    If you know a variable is only a boolean while all lifecycle, by convention, you could initialize it, to false:

    (false) ? true : false; // false typeof false; // "boolean"; (true) ? true : false; // true typeof true; // "boolean" 


    If you know a variable is only an Object while all lifecycle, by convention, you could initialize it, to null:

    (null) ? true : false; // false typeof null; // "object"; ({}) ? true : false; // true typeof {}; // "object" 

    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. That means typeof myVarObject return consistent value for both Object and null type.


    If you know a variable has a mixed type (any type while all lifecycle), by convention, you could initialize it, to undefined.

  15. In JavasScript there are 5 primitive data types: String, Number, Boolean, null and undefined. I will try to explain with some simple examples.

    Let’s say we have a simple function

     function test(a) { if(a == null) { alert("a is null"); } else { alert("The value of a is " + a); } } 

    Also, in above function if(a == null) is the same as if(!a).

    Now when we call this function without passing the parameter a

    test(); // will alert "a is null"; test(4); // will alert "The value of a is " + 4; 


    var a; alert(typeof a); 

    This will give undefined; we have declared a variable but we have not asigned any value to this variable;

    but if we write

    var a = null; alert(typeof a); // will give alert as object 

    so null is an object. In a way we have assigned a value null to ‘a’

  16. In addition to a different meaning there are other differences:

    1. Object destructuring works differently for these two values:
      const { a = "default" } = { a: undefined }; // a is "default" const { b = "default" } = { b: null }; // b is null 
    2. JSON.stringify() keeps null but omits undefined
      const json = JSON.stringify({ undefinedValue: undefined, nullValue: null }); console.log(json); // prints {"nullValue":null} 
    3. typeof operator
      console.log(typeof undefined); // "undefined" console.log(typeof null); // "object" instead of "null" 
  17. 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'.

    typeof null; //"object" typeof undefined; //"undefined"; 

    But if you check them with == as below, you see they are both falsy:

    null==undefined; //true 

    Also you can assign null to an object property or to a primitive, while undefined can simply be achieved by not assigning to anything.

    I create a quick image to show the differences for you at a glance.

    Null and Undefined

  18. For the undefined type, there is one and only one value: undefined.
  19. var a = null;
  20. Per Ryan Morr’s thorough article on this subject…

    “Generally, if you need to assign a non-value to a variable or property, pass it to a function, or return it from a function, null is almost always the best option. To put it simply, JavaScript uses undefined and programmers should use null.”

    See Exploring the Eternal Abyss of Null and Undefined

  21. In javascript all variables are stored as key value pairs. Each variable is stored as variable_name : variable_value/reference.

    undefined means a variable has been given a space in memory, but no value is assigned to it. As a best practice, you should not use this type as an assignment.

    In that case how to denote when you want a variable to be without value at a later point in the code? You can use the type 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. That value is null

    Both are similar but usage and meaning are different.

  22. I want to add a very subtle difference between null and undefined which is good to know when you are trying to learn Vanilla JavaScript(JS) from ground up:

    • null is a reserved keyword in JS while undefined is a variable on the global object of the run-time environment you’re in.

    While writing code, this difference is not identifiable as both null and undefined are always used in RHS of a JavaScript statement. But when you use them in LHS of an expression then you can observe this difference easily. So JS interpreter interprets the below code as error:

    var null = 'foo' 

    It gives below error:

    Uncaught SyntaxError: Unexpected token null

    While below code runs successfully although I won’t recommend doing so in real life:

    var undefined = 'bar' 

    This works because undefined is a variable on the global object (browser window object in case of client-side JS)

  23. 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).

  24. null – It is an assignment value, which is used with variable to represent no value (it’s an object).

    undefined – It is a variable which does not have any value assigned to it, so JavaScript will assign an undefined to it (it’s a data type).

    undeclared – If a variable is not created at all, it is known as undeclared.

  25. Check this out. The output is worth thousand words.

    var b1 = document.getElementById("b1"); checkif("1, no argument" ); checkif("2, undefined explicitly", undefined); checkif("3, null explicitly", null); checkif("4, the 0", 0); checkif("5, empty string", ''); checkif("6, string", "string"); checkif("7, number", 123456); function checkif (a1, a2) { print("ncheckif(), " + a1 + ":"); if (a2 == undefined) { print("==undefined: YES"); } else { print("==undefined: NO"); } if (a2 === undefined) { print("===undefined: YES"); } else { print("===undefined: NO"); } if (a2 == null) { print("==null: YES"); } else { print("==null: NO"); } if (a2 === null) { print("===null: YES"); } else { print("===null: NO"); } if (a2 == '') { print("=='': YES"); } else { print("=='': NO"); } if (a2 === '') { print("==='': YES"); } else { print("==='': NO"); } if (isNaN(a2)) { print("isNaN(): YES"); } else { print("isNaN(): NO"); } if (a2) { print("if-?: YES"); } else { print("if-?: NO"); } print("typeof(): " + typeof(a2)); } function print(v) { b1.innerHTML += v + "n"; }
    <!DOCTYPE html> <html> <body> <pre id="b1"></pre> </body> </html>

    See also:


  26. The difference in meaning between undefined and null is an accident of JavaScript’s design, and it doesn’t matter most of the time. In cases where you actually have to concern yourself with these values, I recommend treating them as mostly interchangeable.

    From the Eloquent Javascript book

  27. The difference between undefined and null is minimal, but there is a difference. A variable whose value is undefined has never been initialized. 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. If you compare undefined and null by using the null==undefined expression, they will be equal.

  28. 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).

    Let’s take an example:

     var x; //we declared a variable x, but no value has been assigned to it. document.write(x) //let's print the variable x 

    Undefined that’s what you will get as output.


     x=5; y=null; z=x+y; 

    and you will get 5 as output. That’s the main difference between the Undefined and null

  29. Both special values imply an empty state.

    The main difference is that undefined represents the value of a variable that wasn’t yet initialized, while null represents an intentional absence of an object.

    The variable number is defined, however, is not assigned with an initial value:

    let number; number; // => undefined 

    number variable is undefined, which clearly indicates an uninitialized variable

    The same uninitialized concept happens when a non-existing object property is accessed:

    const obj = { firstName: 'Dmitri' }; obj.lastName; // => undefined 

    Because lastName property does not exist in obj, JavaScript correctly evaluates obj.lastName to undefined.

    In other cases, you know that a variable expects to hold an object or a function to return an object. But for some reason, you can’t instantiate the object. In such a case null is a meaningful indicator of a missing object.

    For example, clone() is a function that clones a plain JavaScript object. The function is expected to return an object:

    function clone(obj) { if (typeof obj === 'object' && obj !== null) { return Object.assign({}, obj); } return null; } clone({name: 'John'}); // => {name: 'John'} clone(15); // => null clone(null); // => null 

    However, clone() might be invoked with a non-object argument: 15 or null (or generally a primitive value, null or undefined). In such case, the function cannot create a clone, so it returns null – the indicator of a missing object.

    typeof operator makes the distinction between the two values:

    typeof undefined; // => 'undefined' typeof null; // => 'object' 

    The strict quality operator === correctly differentiates undefined from null:

    let nothing = undefined; let missingObject = null; nothing === missingObject; // => false 
  30. Quote from “Eloquent Javascript” 3rd edition by Marijn Haverbeke:

    The difference in meaning between undefined and null is an accident of Javascript’s design, and it doesn’t matter most of the time. In cases where you actually have to concern yourself with these values, I recommend treating them as mostly interchangeable

    Honestly, at first, I am a bit skeptical about this advice. However, in my own interpretation, it is a lazy (vs eager) way to deal with their differences. Maybe, we don’t have to deal with the differences at all. If we have to, we can delay our concern (util we have to) and not hyperactively/defensively worry about it every step of the way as those values (null and undefined) flow through our code.

    PS: This is not a direct answer to your question. This is just a related opinion.

  31. The type of null is Object, while the type of undefined is undefined. Null means ‘no value’, while undefined means ‘not existing’.

    typeof undefined; //undefined typeof null; // Object undefined !== null; //true undefined == null; //true undefined === null; //false var var1; var1; //undefined  var var2 = null; var2; //null 
  32. Generally – don’t use null to avoid confusion.

    1. Standard library methods return undefined, not null
    let a = [10]; console.log(a[1]) //=> undefined console.log(a.find(value => value === 5)) //=> undefined 
    1. I see often in people’s code that some variable was undefined at first, then assigned to some value, then cleared by setting to null. That’s not consistent, better to set back to undefined.

    Still, null makes sense if framework uses it, or for json serialization.

  33. const data = { banners: null } const { banners = [] } = data; console.log(data) // null const data = { banners: undefined } const { banners = [] } = data; console.log(data) // [] 
  34. let a = null; console.log(a); // null let b; console.log(b); // undefined
  35. If a variable is not initialized then it is undefined. undefined is not a object. Example: var MyName; console.log(typeof MyName);

    Check the console log in development tool, it will be printed as undefined.

    null is a a object.If you want some variable to be null then null is used.null variable exists but value is not known.It should be assigned to a variable pro grammatically. null is not automatically initialized.

    Example : var MyName = null; console.log(typeof MyName); Check the csole log in development tool, it will be an object.

  36. Just to add my views –

    A variable that is declared only, and never used anywhere, is removed off by an optimizing compiler in case of compiled languages like C++[or a warning is flagged in the IDE]. It ultimately means that the variable is non-existent because it’s memory is never allocated.

    In case of javascript interpreter, [I guess] a variable is treated as existing only from the point onwards where it is given a value. Before that point, it’s type is “undefined”, and no memory is allocated for it. And, so its type is undefined.

    A null in javascript is a value that represents an address, but that address points to nothing yet[non-existent reference]. Nevertheless, its a value.

  37. Null is ALWAYS an unknown object that exists in memory, whereas undefined is not.

Tasg: javascript, null