How to check if a variable exists or defined in JavaScript
Topic: JavaScript / jQueryPrev|Next
Answer: Use the typeof operator
If you want to check whether a variable has been initialized or defined (i.e. test whether a variable has been declared and assigned a value) you can use the typeof operator.
The most important reason of using the typeof operator is that it does not throw the ReferenceError if the variable has not been declared. Let's take a look at the following example:
Example
Try this code »<script>
var x;
var y = 10;
if(typeof x !== 'undefined'){
// this statement will not execute
alert("Variable x is defined.");
}
if(typeof y !== 'undefined'){
// this statement will execute
alert("Variable y is defined.");
}
// Attempt to access an undeclared z variable
if(typeof z !== 'undefined'){
// this statement will not execute
alert("Variable z is defined.");
}
/* Throws Uncaught ReferenceError: z is not defined,
and halt the execution of the script */
if(z !== 'undefined'){
// this statement will not execute
alert("Variable z is defined.");
}
/* If the following statement runs, it will also
throw the Uncaught ReferenceError: z is not defined */
if(z){
// this statement will not execute
alert("Variable z is defined.");
}
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