检查javascript函数是否存在变量并执行

Question

I have a popup system in my project. With a name, for example 'addnews', it knows which popup content to add and which script is necessary to handle the form. Up until now after completion I always did the same thing. Now, in some cases, I want to do something different.

Now I want to add a JavaScript function with that same name, like previous example 'addnews()', and check if that function exist. If it does run it, else do the standard things. (1)

Also executing a function with that name as variable doesn't seem to work. (2)

var functionName = 'addnews';

if (typeof functionName == 'function') { // (1) typeof functionName = string
    window[functionName](); // (2) this doesn't work for me
} else {
    // Do something standard
}

function addnews() {
    // Do something special
}

Answer 1

If you use this code in a document and it is not loaded complitelly your code will not find the function addnews() because it is not declare before this code.

Like in the comments: you need to use

if (typeof window[functionName] == 'function') 

Answer 2

This will do it:

if (typeof eval(functionName) == 'function') { 
    eval(functionName+'()');
} else {
    // Do something standard
}

Update: Only use this if the answer of Mic doesnt work (whyever). Like he said eval is pretty dangerous.