How to check for an undefined or null variable in JavaScript?
You have to differentiate between cases:
- Variables can be
undefined
or undeclared. You'll get an error if you access an undeclared variable in any context other thantypeof
.
if(typeof someUndeclaredVar == whatever) // works
if(someUndeclaredVar) // throws error
A variable that has been declared but not initialized is undefined
.
let foo;
if (foo) //evaluates to false because foo === undefined
Undefined properties , like
someExistingObj.someUndefProperty
. An undefined property doesn't yield an error and simply returnsundefined
, which, when converted to a boolean, evaluates tofalse
. So, if you don't care about0
andfalse
, usingif(obj.undefProp)
is ok. There's a common idiom based on this fact:value = obj.prop || defaultValue
which means "if
obj
has the propertyprop
, assign it tovalue
, otherwise assign the default valuedefautValue
".Some people consider this behavior confusing, arguing that it leads to hard-to-find errors and recommend using the
in
operator insteadvalue = ('prop' in obj) ? obj.prop : defaultValue
Is there a standard function to check for null, undefined, or blank variables in JavaScript?
You can just check if the variable has a truthy
value or not. That means
if( value ) {
}
will evaluate to true
if value
is not:
- null
- undefined
- NaN
- empty string ("")
- 0
- false
The above list represents all possible falsy
values in ECMA-/Javascript. Find it in the specification at the ToBoolean
section.
Furthermore, if you do not know whether a variable exists (that means, if it was declared) you should check with the typeof
operator. For instance
if( typeof foo !== 'undefined' ) {
// foo could get resolved and it's defined
}
If you can be sure that a variable is declared at least, you should directly check if it has a truthy
value like shown above.
How can I determine if a variable is 'undefined' or 'null'?
You can use the qualities of the abstract equality operator to do this:
if (variable == null){
// your code here.
}
Because null == undefined
is true, the above code will catch both null
and undefined
.
How do I check for null values in JavaScript?
JavaScript is very flexible with regards to checking for "null" values. I'm guessing you're actually looking for empty strings, in which case this simpler code will work:
if(!pass || !cpass || !email || !cemail || !user){
Which will check for empty strings (""
), null
, undefined
, false
and the numbers 0
and NaN
.
Please note that if you are specifically checking for numbers, it is a common mistake to miss 0
with this method, and num !== 0
is preferred (or num !== -1
or ~num
(hacky code that also checks against -1
)) for functions that return -1
, e.g. indexOf
).
Null and Undefined check on a variable at same time
Use if(var != null)
.
The !=
will make both checks: if not undefined
and not null
.
How to check if a variable is both null and /or undefined in JavaScript
A variable cannot be both null
and undefined
at the same time. However, the direct answer to your question is:
if (variable != null)
One =
, not two.
There are two special clauses in the "abstract equality comparison algorithm" in the JavaScript spec devoted to the case of one operand being null
and the other being undefined
, and the result is true
for ==
and false
for !=
. Thus if the value of the variable is undefined
, it's not != null
, and if it's not null, it's obviously not != null
.
Now, the case of an identifier not being defined at all, either as a var
or let
, as a function parameter, or as a property of the global context is different. A reference to such an identifier is treated as an error at runtime. You could attempt a reference and catch the error:
var isDefined = false;
try {
(variable);
isDefined = true;
}
catch (x) {}
I would personally consider that a questionable practice however. For global symbols that may or may be there based on the presence or absence of some other library, or some similar situation, you can test for a window
property (in browser JavaScript):
var isJqueryAvailable = window.jQuery != null;
or
var isJqueryAvailable = "jQuery" in window;
Checking for null or undefined
Also, I know that
if (value == null) {
}
Will get the job done 90% of the time, unless value is zero... or false... or a number of implicit things that can cause obscure bugs.
No, it gets the job done 100% of the time. The only values that are == null
are null
and undefined.
0 == null
is false. "" == undefined
is false. false == null
is false. Etc. You're confusing == null
with falsiness, which is a very different thing.
That's not to say, though, that it's a good idea to write code expecting everyone to know that. You have a perfectly good, clear check in the code you're already using. Whether you choose to write value == null
or the explicit one you're currently using (or if (value === undefined || value === null)
) is a matter of style and in-house convention. But value == null
does do what you've asked: Checks that value
is null
or undefined
.
The details of ==
are here: Abstract Equality Comparison.
How to check if a value is null or undefined in JavaScript (ES2020)
eslint
will not throw an error for value == null
if smart option is enabled
For project that doesn't have smart
eqeqeq
rule in eslint
config,
There's a shorter way of doing it with Nullish Coalescing.
(value ?? null) !== null
let value
// undefined
console.log((value ?? null) !== null)
// null
value = null
console.log((value ?? null) !== null)
// ''
value = ''
console.log((value ?? null) !== null)
// 0
value = 0
console.log((value ?? null) !== null)
// []
value = []
console.log((value ?? null) !== null)
How can I check for undefined in JavaScript?
If you are interested in finding out whether a variable has been declared regardless of its value, then using the in
operator is the safest way to go. Consider this example:
// global scope
var theFu; // theFu has been declared, but its value is undefined
typeof theFu; // "undefined"
But this may not be the intended result for some cases, since the variable or property was declared but just not initialized. Use the in
operator for a more robust check.
"theFu" in window; // true
"theFoo" in window; // false
If you are interested in knowing whether the variable hasn't been declared or has the value undefined
, then use the typeof
operator, which is guaranteed to return a string:
if (typeof myVar !== 'undefined')
Direct comparisons against undefined
are troublesome as undefined
can be overwritten.
window.undefined = "foo";
"foo" == undefined // true
As @CMS pointed out, this has been patched in ECMAScript 5th ed., and undefined
is non-writable.
if (window.myVar)
will also include these falsy values, so it's not very robust:
false
0
""
NaN
null
undefined
Thanks to @CMS for pointing out that your third case - if (myVariable)
can also throw an error in two cases. The first is when the variable hasn't been defined which throws a ReferenceError
.
// abc was never declared.
if (abc) {
// ReferenceError: abc is not defined
}
The other case is when the variable has been defined, but has a getter function which throws an error when invoked. For example,
// or it's a property that can throw an error
Object.defineProperty(window, "myVariable", {
get: function() { throw new Error("W00t?"); },
set: undefined
});
if (myVariable) {
// Error: W00t?
}
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