[Solved-5 Solutions] JavaScript check if variable exists (is defined/initialized) - javascript Tutorial
Problem:
How to check if a variable exists or defined in JavaScript ?
Solution 1:
This solution to check the existence of variable
<html>
<body>
<script>
var myVar = 10;
if(myVar !== undefined && myVar !== null) {
document.write("Exists Variable");
}
</script>
</body>
Solution 2:
We can use typeof operator to check the variable is defined or not.
if (typeof variable !== 'undefined') {
// the variable is defined
}Solution 3:
You can use this code:
if (typeof variable === 'undefined') {
// variable is undefined
}
- The typeof operator, unlike the other operators, doesn't throw a ReferenceError exception when used with an undeclared variable.
-
Note that
typeof nullwill be return as"object". - To avoid the mistake of initializing a variable to
null, we can use like this:
if (typeof variable === 'undefined' || variable === null) {
// variable is undefined or nullSolution 4:
In JavaScript, a variable can be defined, but hold the value undefined.
if (typeof v === "undefined") {
// no variable "v" is defined in the current scope
// *or* some variable v exists and has been assigned the value undefined
} else {
// some variable (global or local) "v" is defined in the current scope
// *and* it contains a value other than undefined
}
The following example has simpler semantics, which makes it easier to describe code's behavior.
if ("v" in window) {
// global variable v is defined
} else {
// global variable v is not defined
}Where window is a name for the global object
Solution 5:
This solution gives if a variable exists and has been initialized.
var setOrNot = typeof variable !== typeof undefined;
It is most commonly used in combination with a ternary operator to set a default as certain variable has not been initialized :
var dark = typeof darkColor !== typeof undefined ? darkColor : "black";