How to get 'typeof' of the value of a variable

This question already has an answer here:

How to execute a JavaScript function when I have its name as a string 29 answers

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Supposing that your function is global you can verify if it exists using window object (the object where are stored the global variables).

 if (typeof window[functionName] === 'function') {
     // function exists
 }

JSFIDDLE

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You can use window[functionName] as follows:

function Result(){}
var functionName = 'Result';
if (typeof window[functionName] == 'function'){
    alert("function exists, don't load it");
          }else{
    alert("function does not exist, load it");
}

However you need to make sure you escape the single quote in don't or use double quotes since it terminates your String literal.

Here is a working example: http://jsfiddle.net/WN92K/

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你可以这样做，看看你的功能是全球性的。

if (window[functionName] instanceof Function){

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