What’s the difference between null and undefined data types?

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Last updated: September 18, 2021.

null and undefined are two different data types in Javascript. But both seem to convey a similar meaning. So what’s the difference?

undefined is used to signify that a variable has been initialised but no value has yet been assigned to it. For example:

let emptyVariable;
alert(emptyVariable); // undefined
alert(typeof emptyVariable); // undefined

null on the other hand is an assigned value that indicates no value:

let nullVariable= null;
alert( nullVariable ); // null
alert(typeof  nullVariable ); // object

You may be surprised null is of the type object and not type null given that it is a primitive object.

According to the book Professional Javascript For Web Developers (Wrox):

You may wonder why the typeof operator returns ‘object’ for a value that is null. This was actually an error in the original JavaScript implementation that was then copied in ECMAScript. Today, it is rationalized that null is considered a placeholder for an object, even though, technically, it is a primitive value.