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:
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)
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