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 Operator
The 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 var
Parameters:
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 JavaScript
Example 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.
let defined = 'Not defined';
if (typeof fun === 'function') {
defined = "Defined";
}
console.log("Function " + defined);
Output
Function 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.
function isFunction(possibleFunction) {
return typeof possibleFunction === 'function';
}
function fun() { }
let defined = 'Not defined';
if (isFunction(fun)) {
defined = "Defined";
}
console.log("Function " + defined);
Output
Function Defined