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Not Defined vs. Undefined in JavaScript

grepliz profile image Liz Lam ・1 min read

There are two conditions that are closely worded but mean very different things in JavaScript. A variable can be undefined vs. not defined.

Let's open up your browser's console and take a closer look.

Suppose you type a + b and hit ENTER. You will see something like the following:

not defined

This ReferenceError is telling us that we have not declared the variable a and therefore it is not defined.

Let's declare a and see what happens.

undefined

The let (as well as the var and const) keyword is used to declare a variable but since we have not assigned it an actual value, a is undefined.

The difference may feel subtle but they really are different things. In one case, a doesn't even exist (i.e. not defined) and in the other it does but doesn't have a value.

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grepliz profile

Liz Lam

@grepliz

I love open source, coffee and tinkering.

Discussion

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Never thought about it, simple but very interesting. Thanks!

 

This is an interesting topic Liz, if you explain the hoisting this can be more didactic for your readers :)

 

Thanks for the suggestion! I will definitely think on that.