Is it possible to check for both null and undefined in javascript?
if(_var == null || _var == undefined) {
}
5 Answers
In JavaScript (pre ECMAScript 5), undefined is not a constant, but a global variable, and therefore it is possible to change its value. Therefore it would be more reliable to use the typeof operator to check for undefined:
if (typeof _var === 'undefined') { }
In addition your expression would return a ReferenceError if the variable _var is not declared. However you would still be able to test it with the typeof operator as shown above.
Therefore, you may prefer to use the following:
if (typeof _var === 'undefined' || _var === null) { }
3 Comments
Peter Jaric
Your first case is a little risky since _var might be set to some other falsy value than null.
Daniel Vassallo
@snowlord: True. I removed that part.
Christian C. Salvadó
fortunately now on ECMAScript 5,
undefined is a property of the global object which is non-writable, non-configurable (non-deletable) and non-enumerable :)yes
However using the == operator it is not necesary. using foo == null will also be true of foo is undefined. Note however that undefined and null or not(!) the same. It is because that == does type coersion that foo == null is also true for foo is undefined.
1 Comment
Christian C. Salvadó
+1, this is the shortest way to achieve it, and it is completely described in the spec: The Abstract Equality Comparison Algorithm (no worries about type coercion)...
if (_var == null) { ... }if (!_var) {
// Code here.
}
This should work since both undefined and null are type coerced to false.
Of course there is the small problem if _var is actually false but it works since in most cases you would want to know if _var is not true and not an object.
1 Comment
Daniel Vassallo
Note that there are three other falsy values in JavaScript apart from
false, null and undefined: An empty string "", 0, and NaN.you can also use $defined function in mootools (and there must be an equivalent in jquery)
Comments
var valuea: number;
var valueb: number = null;
function check(x, name) {
if (x == null) {
console.log(name + ' == null');
}
if (x === null) {
console.log(name + ' === null');
}
if (typeof x === 'undefined') {
console.log(name + ' is undefined');
}
}
check(a, 'a');
check(b, 'b');
answered Jun 24, 2019 at 8:09
Chanaka Weerasinghe
5,8002 gold badges31 silver badges42 bronze badges
_varisundefined, the_var == undefinedexpression will never be reached, because_var == nulldoes type coercion, as Pim noted below andundefined == nullreturnstrue. The_var == nullwill also be testing forundefined, however I believe relying on type coercion is not generally recommended.