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JavaScript JavaScript Foundations Variables Null and Undefined

Paul Trimor
Paul Trimor
5,388 Points

It seems that even if I change the value of "undefined", it still reads true.

This is the Javascript code.

var myVar;

undefined = true;

console.log( typeof myVar === "undefined");

console.log( myVar === undefined);

This is what prints in the console.


However, on the video, it prints true false.

2 Answers

The ECMAScript 5 standard of JavaScript (the currently widely used standard) defines undefined as an immutable property. This basically means that you can't change its value anymore.

When this video was recorded, Chrome was using the ECMAScript 3 standard where undefined was mutable. That's why your browser prints out true true. In your browser, you can't change undefined's value. In the old one, you could do that.

Darren Kynaston
Darren Kynaston
Courses Plus Student 15,610 Points

Hi Paul,

Whats the question or at what point (time) in the video are you looking at?