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Undefined versus null in JavaScript

Undefined versus null in JavaScript

David Y.

The ProblemJump To Solution

What is the difference between the values undefined and null in JavaScript?

The Solution

When a variable has been declared but not yet assigned a value, it will have the type and value undefined. undefined is one of JavaScript’s primitive types.

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let myVar; console.log(myVar); // will print "undefined" console.log(typeof myVar); // will also print "undefined"

When a variable has not been declared or assigned, it will also be of type undefined and throw a ReferenceError when accessed. Therefore, typeof will produce the same result for declared but undefined variables and undeclared variables.

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console.log(undeclaredVar); // will throw a ReferenceError console.log(typeof undeclaredVar); // will print "undefined"

In contrast, null is a value that represents nothing. Think of null as an empty container and undefined as the absence of a container. A variable will only have the value null when it is explicitly assigned, and will be of type object.

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const myVar = null; console.log(myVar); // will print "null" console.log(typeof myVar); // will print "object"

The difference between undefined and null is important when using JavaScript’s equality operators. null and undefined are considered the same when using loose equality (==) but not when using strict equality (===).

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console.log(undefined == null); // will print "true" console.log(undefined === null); // will print "false"
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