How to Detect Undefined Variables in JavaScript?

Detecting Undefined Variables in JavaScript

Determining whether a variable is defined or undefined is crucial in JavaScript coding. A common error occurs when accessing an undefined variable, resulting in the "not-defined error."

Catching the Error

To avoid this error, JavaScript has two concepts:

• Null: Represents non-existent values. Null is an object.
• Undefined: Represents values that are not defined.

Checking for Null and Undefined

Unlike many languages, JavaScript does not have a direct comparison for null and undefined. To check for null specifically, use:

if (yourvar === null) // Does not execute if yourvar is `undefined`

To ascertain if a variable exists (not undefined), use:

if (yourvar !== undefined) // Any scope

Legacy Syntax

Previously, it was necessary to use the typeof operator to check for undefined safely:

if (typeof yourvar !== 'undefined') // Any scope

However, this is no longer necessary since ECMAScript 5 (2009).

Alternatives

For checking membership without regard to value, use:

if ('membername' in object) // With inheritance
if (object.hasOwnProperty('membername')) // Without inheritance

To evaluate truthiness (non-falsy values), use:

if (yourvar)

The above is the detailed content of How to Detect Undefined Variables in JavaScript?. For more information, please follow other related articles on the PHP Chinese website!