JavaScript checking for null vs. undefined and difference between == and ===

2018-06-04 00:55:16

I know, I know there must be some threads covering this topic. But I used the search and didn't get the answer which fits my needs. So here we go:

How do I check a variable if it's null or undefined and what is the difference between the null and undefined ?

What is the difference between "==" and "===" (it's hard to search Google for === )?

How do I check a variable if it's null or undefined ...

Is the variable null :

if (a === null)
// or
if (a == null) // but see note below

...but note the latter will also be true if a is undefined .

Is it undefined :

if (typeof a === "undefined")
// or
if (a === undefined)
// or
if (a == undefined) // but see note below

...but again, note that the last one is vague; it will also be true if a is null .

Now, despite the above, the usual way to check for those is to use the fact that they're falsey:

if (!a) {
    // `a` is falsey, which includes `undefined` and `null`
    // (and `""`, and `0`, and `NaN`, and [of course] `false`)
}

This is defined by ToBoolean in the spec.

...and what is the difference between the null and undefined ?

They're both values usually used to indicate the absence of something. undefined is the more generic one, used as the default value of variables until they're assigned some other value, as the value of function arguments that weren't provided when the function was called, and as the value you get when you ask an object for a property it doesn't have. But it can also be explicitly used in all of those situations. (There's a difference between an object not having a property, and having the property with the value undefined ; there's a difference between calling a function with the value undefined for an argument, and leaving that argument off entirely.)

null is slightly more specific than undefined : It's a blank object reference. JavaScript is loosely typed, of course, but not all of the things JavaScript interacts with are loosely typed. If an API like the DOM in browsers needs an object reference that's blank, we use null , not undefined . And similarly, the DOM's getElementById operation returns an object reference — either a valid one (if it found the DOM element), or null (if it didn't).

Interestingly (or not), they're their own types. Which is to say, null is the only value in the Null type, and undefined is the only value in the Undefined type.

What is the difference between "==" and "==="

The only difference between them is that == will do type coercion to try to get the values to match, and === won't. So for instance "1" == 1 is true, because "1" coerces to 1 . But "1" === 1 is false, because the types don't match. ( "1" !== 1 is true.) The first (real) step of === is "Are the types of the operands the same?" and if the answer is "no", the result is false . If the types are the same, it does exactly what == does.

Type coercion uses quite complex rules and can have surprising results (for instance, "" == 0 is true).

More in the spec:

Abstract Equality Comparison ( == , also called "loose" equality)

Strict Equality Comparison ( === )

The difference is subtle.

In JavaScript an undefined variable is a variable that as never been declared, or never assigned a value. Let's say you declare var a; for instance, then a will be undefined , because it was never assigned any value.

But if you then assign a = null; then a will now be null . In JavaScript null is an object (try typeof null in a JavaScript console if you don't believe me), which means that null is a value (in fact even undefined is a value).

Example:

var a;
typeof a;     # => "undefined"

a = null;
typeof null;  # => "object"

This can prove useful in function arguments. You may want to have a default value, but consider null to be acceptable. In which case you may do:

function doSomething(first, second, optional) {
    if (typeof optional === "undefined") {
        optional = "three";
    }
    // do something
}

If you omit the optional parameter doSomething(1, 2) then optional will be the "three" string but if you pass doSomething(1, 2, null) then optional will be null .

As for the equal == and strictly equal === comparators, the first one is weakly type, while strictly equal also checks for the type of values. That means that 0 == "0" will return true; while 0 === "0" will return false, because a number is not a string.

You may use those operators to check between undefined an null . For example:

null === null            # => true
undefined === undefined  # => true
undefined === null       # => false
undefined == null        # => true

The last case is interesting, because it allows you to check if a variable is either undefined or null and nothing else:

function test(val) {
    return val == null;
}
test(null);       # => true
test(undefined);  # => true

The spec is the place to go for full answers to these questions. Here's a summary:

For a variable x , you can:

check whether it's null by direct comparison using === . Example: x === null

check whether it's undefined by either of two basic methods: direct comparison with undefined or typeof . For various reasons, I prefer typeof x === "undefined" .

check whether it's one of null and undefined by using == and relying on the slightly arcane type coercion rules that mean x == null does exactly what you want.

The basic difference between == and === is that if the operands are of different types, === will always return false while == will convert one or both operands into the same type using rules that lead to some slightly unintuitive behaviour. If the operands are of the same type (eg both are strings, such as in the typeof comparison above), == and === will behave exactly the same.