如何检查JavaScript中是否存在函数？

This will verify if the function exists, if so it will be executed

me.onChange && me.onChange(str);

Thus the error TypeError: me.onChange is not a function is prevent.

And then there is this...

( document.exitPointerLock || Function )();

To illustrate the preceding answers, here a quick JSFiddle snippet :

function test () {
console.log()

}

console.log(typeof test) // >> "function"

// implicit test, in javascript if an entity exist it returns implcitly true unless the element value is false as :
// var test = false
if(test){ console.log(true)}
else{console.log(false)}

// test by the typeof method
if( typeof test === "function"){ console.log(true)}
else{console.log(false)}


// confirm that the test is effective : 
// - entity with false value
var test2 = false
if(test2){ console.log(true)}
else{console.log(false)}

// confirm that the test is effective :
// - typeof entity
if( typeof test ==="foo"){ console.log(true)}
else{console.log(false)}

/* Expected :
function
true 
true 
false
false
*/

    function sum(nb1,nb2){

       return nb1+nb2;
    }

    try{

      if(sum() != undefined){/*test if the function is defined before call it*/

        sum(3,5);               /*once the function is exist you can call it */

      }

    }catch(e){

      console.log("function not defined");/*the function is not defined or does not exists*/
    }

I have also been looking for an elegant solution to this problem. After much reflection, I found this approach best.

const func = me.onChange || (str => {}); func(str);

Put double exclamation mark i.e !! before the function name that you want to check. If it exists, it will return true.

function abc(){
}
!!window.abc; // return true
!!window.abcd; // return false

With no conditions

me.onChange=function(){};

function getID( swfID ){
     if(navigator.appName.indexOf("Microsoft") != -1){
          me = window[swfID];
     }else{
          me = document[swfID];
     }
}

function js_to_as( str ){
     me.onChange(str);
}

I always check like this:

if(!myFunction){return false;}

just place it before any code that uses this function

In a few words: catch the exception.

I am really surprised nobody answered or commented about Exception Catch on this post yet.

Detail: Here goes an example where I try to match a function which is prefixed by mask_ and suffixed by the form field "name". When JavaScript does not find the function, it should throw an ReferenceError which you can handle as you wish on the catch section.

function inputMask(input) {
  try {
    let maskedInput = eval("mask_"+input.name);

    if(typeof maskedInput === "undefined")
        return input.value;
    else
        return eval("mask_"+input.name)(input);

  } catch(e) {
    if (e instanceof ReferenceError) {
      return input.value;
    }
  }
}

This simple jQuery code should do the trick:

if (jQuery.isFunction(functionName)) {
    functionName();
}

function js_to_as( str ){
     if (me && me.onChange)
         me.onChange(str);
}

没看到这个建议：me.onChange && me.onChange（str）;

基本上，如果me.onChange是未定义的（如果尚未启动，它将是未定义的），则它将不执行后一部分。如果me.onChange是一个函数，它将执行me.onChange（str）。

您甚至可以走得更远：

me && me.onChange && me.onChange(str);

万一我也是异步的。

尝试这样的事情：

if (typeof me.onChange !== "undefined") { 
    // safe to use the function
}

或更好（根据UpTheCreek推荐的评论）

if (typeof me.onChange === "function") { 
    // safe to use the function
}