Why typeof undefined is return string?

Question

When I run from the browser

typeof undefined

I got "undefined" string. That is why? Most of the time I check my variable something like

if (a == 'undefined') {
  do something
}

is I am checking my variable a is equal to undefined? I don't understand about that. Can someone explain me? Another way to check undefined in javascript?

`typeof` returns the **name** of the datatype of the value (more or less). And the names are represented as strings. — Felix Kling, Jun 17 '14 at 07:39
You can also compare the value against `undefined`. E.g. `v === undefined`, `v === void 0`, `v == null` (Catches undefined and null). — Felix Kling, Jun 17 '14 at 07:43

score 3 · Accepted Answer · edited Jan 25 '19 at 11:42

3

Because that is how typeof works:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof

if (a == undefined) { // instead its a type comparison

edited Jan 25 '19 at 11:42

Luke Garrigan

2,141
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answered Jun 17 '14 at 07:35

alessandro

9,507
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score 1 · Answer 2 · answered Jun 17 '14 at 07:37

1

typeof get's you the type as a string.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof

answered Jun 17 '14 at 07:37

Harazi

122
10

score 0 · Answer 3 · answered Jun 17 '14 at 07:37

undefined just happens to be a global name that holds the undefined value.

Quoting the linked page,

undefined is a property of the global object, i.e. it is a variable in global scope.
The initial value of undefined is the primitive value undefined.

typeof always returns a string describing the value of the name passed to it.

score 0 · Answer 4 · answered Jun 17 '14 at 07:37

The typeof operator returns a string indicating the type of the unevaluated operand.

It changes like this

Undefined   :"undefined"
Null    :"object"
Boolean :"boolean"
Number  "number"
String  :"string"
Host object (provided by the JS environment)    :Implementation-dependent
Function object (implements [[Call]] in ECMA-262 terms):    "function"
Any other object    :"object"

DOC

Why typeof undefined is return string?

4 Answers4