How to execute a Javascript function only after multiple other functions have completed?

Question

My specific problem is that I need to execute a (potentially) large number of Javascript functions to prepare something like a batch file (each function call adds some infor

Answer 1

If you don't use any asynchronous functions and your script doesn't break the order of execution, then the most simple solution is, as stated by Pointy and others:

funcA(); 
funcB();
funcC();

However, since you're using node.js, I believe you're going to use asynchronous functions and want to execute funcC after a async IO request has finished, so you have to use some kind of counting mechanisms, for example:

var call_after_completion = function(callback){
    this._callback = callback;
    this._args = [].slice.call(arguments,1);
    this._queue = {};
    this._count = 0;
    this._run = false;
}

call_after_completion.prototype.add_condition = function(str){
    if(this._queue[str] !== undefined)
        throw new TypeError("Identifier '"+str+"' used twice");
    else if(typeof str !== "String" && str.toString === undefined)
        throw new TypeError("Identifier has to be a string or needs a toString method");

    this._queue[str] = 1;
    this._count++;
    return str;
}

call_after_completion.prototype.remove_condition = function(str){
    if(this._queue[str] === undefined){
        console.log("Removal of condition '"+str+"' has no effect");
        return;
    }
    else if(typeof str !== "String" && str.toString === undefined)
        throw new TypeError("Identifier has to be a string or needs a toString method");

    delete this._queue[str];

    if(--this._count === 0 && this._run === false){
        this._run = true;
        this._callback.apply(null,this._args);
    }
}

You can simplify this object by ignoring the identifier str and just increasing/decreasing this._count, however this system could be useful for debugging.

In order to use call_after_completion you simply create a new call_after_completion with your desired function func as argument and add_conditions. func will only be called if all conditions have been removed.

Example:

var foo = function(){console.log("foo");}
var bar = new call_after_completion(foo);
var i;

bar.add_condition("foo:3-Second-Timer");
bar.add_condition("foo:additional function");
bar.add_condition("foo:for-loop-finished");

function additional_stuff(cond){
    console.log("additional things");
    cond.remove_condition("foo:additional function");
}

for(i = 0; i < 1000; ++i){

}
console.log("for loop finished");
bar.remove_condition("foo:for-loop-finished");
additional_stuff(bar);

setTimeout(function(){
    console.log("3 second timeout");
    bar.remove_condition("foo:3-Second-Timer");
},3000);

JSFiddle Demo