How can I check for “undefined” in JavaScript? [duplicate]

Answer 1

If it is undefined, it will not be equal to a string that contains the characters "undefined", as the string is not undefined.

You can check the type of the variable:

if (typeof(something) != "undefined") ...

Sometimes you don't even have to check the type. If the value of the variable can't evaluate to false when it's set (for example if it's a function), then you can just evalue the variable. Example:

if (something) {
  something(param);
}

Answer 2

Since none of the other answers helped me, I suggest doing this. It worked for me in Internet Explorer 8:

if (typeof variable_name.value === 'undefined') {
    // variable_name is undefined
}

Answer 3

    var x;
    if (x === undefined) {
        alert ("I am declared, but not defined.")
    };
    if (typeof y === "undefined") {
        alert ("I am not even declared.")
    };

    /* One more thing to understand: typeof ==='undefined' also checks 
       for if a variable is declared, but no value is assigned. In other 
       words, the variable is declared, but not defined. */

    // Will repeat above logic of x for typeof === 'undefined'
    if (x === undefined) {
        alert ("I am declared, but not defined.")
    };
    /* So typeof === 'undefined' works for both, but x === undefined 
       only works for a variable which is at least declared. */

    /* Say if I try using typeof === undefined (not in quotes) for 
       a variable which is not even declared, we will get run a 
       time error. */

    if (z === undefined) {
        alert ("I am neither declared nor defined.")
    };
    // I got this error for z ReferenceError: z is not defined 

Answer 4

On the contrary of @Thomas Eding answer:

If I forget to declare myVar in my code, then I'll get myVar is not defined.

Let's take a real example:

I've a variable name, but I am not sure if it is declared somewhere or not.

Then @Anurag's answer will help:

var myVariableToCheck = 'myVar';
if (window[myVariableToCheck] === undefined)
    console.log("Not declared or declared, but undefined.");

// Or you can check it directly 
if (window['myVar'] === undefined) 
    console.log("Not declared or declared, but undefined.");