how to compare two static functions in javascript equal or not equal?
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why do you want to compare them?Toskan– Toskan2012-11-01 13:30:22 +00:00Commented Nov 1, 2012 at 13:30
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4 Answers
String(f1) === String(f2)
3 Comments
Andy E
I think you mean
String(f1) === String(f2) unless you have a custom string() method that I'm unaware of :)Andy E
Another way of writing it, for the byte-savers out there:
""+f1 === ""+f2
Martin Meeser
Does it work with
(String(f1.bind(this)) === String(f1.bind(this)))? I think not?var f1 = f2 = function( a ){ return a; };
here, you can use f1 === f2 because they're pointing to the same memory and they're the same type
var f1 = function( a ){ return a; },
f2 = function( a ){ return a; };
here you can use that byte-saver Andy E used (which is implicitly converting the function to it's body's text as a String),
''+f1 == ''+f2.
this is the gist of what is happening behind the scences:
f1.toString( ) == f2.toString( )
Edit: Looking back on this post over a year after, I agree with @kangax - you should probably never do this.
12 Comments
Andy E
+1 for a more elaborate answer
kangax
Note that
var f1 = f2 = function(){ ... } results in f2 becoming a global property, due to undeclared assignment of function to f2. Undeclared assignments are generally harmful, so it's better to avoid that pattern. Also, don't rely on Function.prototype.toString — it's not standardized and varies across browsers.
Dan Beam
I just tested the above code in IE 6, 7, 8, FF 2, 3.0, 3.5 (Windows / Linux), Opera 10 (Windows / Linux), Safari 4, and Chrome (Windows / Linux). Which browsers do you develop for exactly? Konqueror, haha? here's the code, someone prove me wrong
javascript:(function(){var f1=function(a){return a;},f2=function(a){return a;};alert(f1.toString());alert(f2.toString());alert(""+f1==""+f2);})();kangax
Here are few I stumbled upon — perfectionkills.com/those-tricky-functions , so not something I would rely on. YMMV, of course ;)
kangax
I simply try to avoid design which would lead to using function decompilation. You can certainly decompile functions when working in closed environments (e.g. in intranet apps) or for debugging/profiling purposes (during development). Anything that's meant for public web is better left without it, and, as a result, have less chances of blowing up in some less-common environments.
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Whenever I need to compare functions I make sure there is no scope ambiguity and use the same function object.
Say I have some a pair of library functions that take a callback as one of the parameters. For the sake of this example create1minuteCallback function will set a 1 minute timer and call the callback on each tick. kill1minuteCallback will turn off the callback and you must pass the same callback function as you did for create1minuteCallback.
function create1minuteCallback(callback){
//implementation
}
function kill1minuteCallback(callback){
//implementation
}
Quite clearly this will not work as the function we are passing is different on the second line:
create1minuteCallback(function(){alert("1 minute callback is called");});
kill1minuteCallback(function(){alert("1 minute callback is called");});
I normally do this:
function callbackFunc(){alert("1 minute callback is called");}
create1minuteCallback(callbackFunc);
kill1minuteCallback(callbackFunc);
Comments
Well, as simply as that - if you are going to compare functions, you do it for a reason I assume. What is your reason? My reason was to not run a certain function twice. I did it this way (just snippet code to get the idea)
var x = function(){
console.error("i am a functionX");
}
var y = function(){
console.error("i am a functionX");
}
var z = function(){
console.error("i am a functionZ");
}
var x2= x;
var obj = new Object();
obj[x] = "";
obj[x2] = "";
obj[y] = "";
obj[z] = "";
obj.abc = "xaxa";
for (prop in obj) {
if (obj.hasOwnProperty(prop)) {
console.error(obj[prop] + " hello " + prop);
}
}
Function x and y are the same, even though they have different whitespaces. x and y are not the same as z, since z has a different console.error. Btw open your firebug console to see it in the jsbin example
