How to check if a variable exists or defined in JavaScript

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:

<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.");
    }
</script>