How to determine whether a function exists in javascript

In JavaScript, functions can be passed and manipulated as variables. In actual development, we often need to determine whether a function has been defined or exists to avoid unnecessary exceptions or errors.

The following are several common ways to determine whether a JavaScript function exists:

typeof determination

The typeof operator in JavaScript is used to determine the type of an object. If If the object is of function type, the string "function" is returned, otherwise the corresponding type string is returned. Therefore, you can use the typeof operator to determine whether a function already exists. For example:

if(typeof myFunction === "function") { // myFunction 已经定义 } else { // myFunction 未定义 }

in operator judgment

Use the in operator to determine whether an object contains a certain attribute (including a function). Therefore, you can determine whether the function has been defined by determining whether the function name exists in the object. For example:

if("myFunction" in window) { // myFunction 已经定义 } else { // myFunction 未定义 }

Among them, the window object is the global object of the browser, including all global variables and functions.

Function body toString judgment

The function object in JavaScript has a built-in toString method, which is used to convert the function into a string. Therefore, you can determine whether the function has been defined by determining whether the function body string contains a certain keyword. For example:

if(myFunction.toString().indexOf("function myFunction(") != -1) { // myFunction 已经定义 } else { // myFunction 未定义 }

try-catch judgment

Use the try-catch statement to catch exceptions that may be thrown when JavaScript is running. Therefore, you can place the function call in a try block. If the function exists, it will be executed normally; otherwise, an exception will be thrown and caught by the catch block. For example:

try { myFunction(); // myFunction 已经定义 } catch(e) { // myFunction 未定义 }

It should be noted that if the function exists but other exceptions are thrown during execution, it will also be caught by the catch block, so you need to use this method with caution.

To sum up, the above are several common ways to determine whether a JavaScript function exists. In actual development, you can choose a suitable method according to the specific situation to determine whether the function has been defined, so as to avoid unnecessary errors and exceptions.

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