How to check a function is defined in JavaScript ?
The job is to identify whether a function is defined or not. The JavaScript typeof operator is used to solve the problem described below:
Javascript typeof Operator: This operator can be used to find the type of a JavaScript variable. This operator returns the type of a variable or an expression:
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Syntax:
typeof var
Parameter: It contains single value var which is a Javascript variable.
Return value: It returns the type of a variable or an expression:
Example 1: This example checks the type of the function, If it is function then it is defined otherwise not defined by using typeof operator.
<!DOCTYPE HTML> <html> <head> <title> How to check a function is defined </title> </head> <body style = "text-align:center;"> <h1 style = "color:green;" > GeeksForGeeks </h1> <p id = "GFG_UP" style = "font-size: 16px; font-weight: bold;"> </p> <button onclick = "gfg_Run()"> Click here </button> <p id = "GFG_DOWN" style = "color:green; font-size: 20px; font-weight: bold;"> </p> <script> var el_up = document.getElementById("GFG_UP"); var el_down = document.getElementById("GFG_DOWN"); el_up.innerHTML = "Function named 'fun' which is not defined"; function gfg_Run() { var defined = 'Not defined'; if(typeof fun == 'function') { defined = "Defined"; } el_down.innerHTML = defined; } </script> </body> </html> |
Output:
- Before clicking on the button:
- After clicking on the button:
Example 2: This example checks the type of the function, If it is function then it is defined otherwise not defined by using typeof operator by creating a function.
<!DOCTYPE HTML> <html> <head> <title> How to check a function is defined </title> </head> <body style = "text-align:center;"> <h1 style = "color:green;" > GeeksForGeeks </h1> <p id = "GFG_UP" style = "font-size: 16px; font-weight: bold;"> </p> <button onclick = "gfg_Run()"> Click here </button> <p id = "GFG_DOWN" style = "color:green; font-size: 20px; font-weight: bold;"> </p> <script> var el_up = document.getElementById("GFG_UP"); var el_down = document.getElementById("GFG_DOWN"); el_up.innerHTML = "Function named 'fun' which is defined"; function isFunction(possibleFunction) { return typeof(possibleFunction) === typeof(Function); } function fun() { } function gfg_Run() { var defined = 'Not defined'; if(isFunction(fun)) { defined = "Defined"; } el_down.innerHTML = defined; } </script> </body> </html> |
Output:
- Before clicking on the button:
- After clicking on the button:
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