How to check a function is defined in JavaScript ? Last Updated : 30 Apr, 2024 Comments Improve Suggest changes Like Article Like Report In this article, we will check whether a function is defined or not in JavaScript. The JavaScript typeof operator is used to check whether the function is defined or not. JavaScript typeof OperatorThe typeof operator is used to find the type of a JavaScript variable. This operator returns the type of a variable or an expression. Syntax: typeof varParameters: It contains a single parameter var which is a JavaScript variable. Return value: It returns the type of a variable or an expression: Examples to Check a Function is Defined or Not in JavaScriptExample 1: This example checks the type of the function. If it is a function then it is defined otherwise not defined by using typeof operator. JavaScript let defined = 'Not defined'; if (typeof fun === 'function') { defined = "Defined"; } console.log("Function " + defined); OutputFunction Not defined Example 2: This example checks the type of the function, If it is a function then it is defined otherwise not defined by using typeof operator by creating a function. JavaScript function isFunction(possibleFunction) { return typeof possibleFunction === 'function'; } function fun() { } let defined = 'Not defined'; if (isFunction(fun)) { defined = "Defined"; } console.log("Function " + defined); OutputFunction Defined Comment More infoAdvertise with us Next Article How to check a function is defined in JavaScript ? P PranchalKatiyar Follow Improve Article Tags : JavaScript Web Technologies javascript-functions JavaScript-Questions Similar Reads How to check whether a number is NaN or finite in JavaScript ? When working with numbers in JavaScript, it's important to know how to determine if a value is NaN (Not-a-Number) or finite. 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