Ryan Aghdam

Checking for undefined in JavaScript

JavaScript has many quirks, some of which can be found when checking to see if a value is undefined.

In cases where the value is guaranteed to be declared, it is possible to use triple-equals.

> var x;
undefined
> x === undefined;
true

However, if the value is not declared, an error will be thrown.

> y === undefined;
ReferenceError: y is not defined
    at repl:1:1
    at REPLServer.defaultEval (repl.js:132:27)
    at bound (domain.js:254:14)
    at REPLServer.runBound [as eval] (domain.js:267:12)
    at REPLServer.<anonymous> (repl.js:279:12)
    at REPLServer.emit (events.js:107:17)
    at REPLServer.Interface._onLine (readline.js:214:10)
    at REPLServer.Interface._line (readline.js:553:8)
    at REPLServer.Interface._ttyWrite (readline.js:830:14)
    at ReadStream.onkeypress (readline.js:109:10)

In this case, it’s necessary to use typeof to determine if the value is undefined.

> z === undefined;
ReferenceError: z is not defined
    at repl:1:1
    at REPLServer.defaultEval (repl.js:132:27)
    at bound (domain.js:254:14)
    at REPLServer.runBound [as eval] (domain.js:267:12)
    at REPLServer.<anonymous> (repl.js:279:12)
    at REPLServer.emit (events.js:107:17)
    at REPLServer.Interface._onLine (readline.js:214:10)
    at REPLServer.Interface._line (readline.js:553:8)
    at REPLServer.Interface._ttyWrite (readline.js:830:14)
    at ReadStream.onkeypress (readline.js:109:10)
> typeof z === 'undefined';
true