How do I check a variable if it\'s null
or undefined
and what is the difference between the null
and undefined
?<
How do I check a variable if it's null or undefined
just check if a variable has a valid value like this :
if(variable)
it will return true if variable does't contain :
How do I check a variable if it's
null
orundefined
...
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
andundefined
?
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:
==
, also called "loose" equality)===
)