ansaurus

Question

how to detect a function was called with javascript

Answer 1

+2 A:

var called = false;
function blah() {
   called = true;
}

if ( !called ) {
   blah();
}

meder 2010-07-26 04:50:47

For those global-variable-nazis [since this is in the global scope], you can use blah.called instead of var blah... assuming blah is already defined. This code is pretty enough for me, though.

ItzWarty 2010-07-26 04:52:11

The disadvantage of this pattern is that you have to wrap the function call in a conditional statement everywhere it gets invoked.

Török Gábor 2010-07-26 11:59:12

Answer 2

+3 A:

That's easy to accomplish, as javascript functions are objects that can have members and properties:

var callMeOnlyOnce=function(){

    if(this.alreadyCalled)return;

    alert('calling for the first time');

    this.alreadyCalled=true;
};

// alert box comes
callMeOnlyOnce();


// no alert box
callMeOnlyOnce();

EDIT:

As pointed out correctly by CMS, using this is not that easy. Here's a revised version that uses a custom namespace instead of this.

if(!window.mynamespace){
    window.mynamespace={};
}

mynamespace.callMeOnlyOnce=function(){

    if(mynamespace.alreadyCalled)return;

    alert('calling for the first time');
    mynamespace.alreadyCalled=true;
};

// alert box comes
mynamespace.callMeOnlyOnce();


// no alert box
mynamespace.callMeOnlyOnce();

seanizer 2010-07-26 04:58:03

The `this` value refers to the Global object, not to the function itself, and `allreadyCalled` will end up being a property of the Global object. This is because the function has been invoked from a reference that doesn't have a base object: `callMeOnlyOnce();`. [More info on `this`](http://stackoverflow.com/questions/3320677/this-operator-in-javascript/3320706#3320706)

CMS 2010-07-26 05:34:38

Answer 3

+2 A:

Use decorator pattern.

// your function definition
function yourFunction() {}

// decorator
function callItOnce(fn) {
    var called = false;
    return function() {
        if (!called) {
            called = true;
            return fn();
        }
        return;
    }
}

yourFunction(); // it runs
yourFunction(); // it runs    
yourFunction = callItOnce(yourFunction);
yourFunction(); // it runs
yourFunction(); // null

This solution provides a side-effect free way for achieving your goal. You don't have to modify your original function. It works nice even with library functions. You may assign a new name to the decorated function to preserve the original function.

var myLibraryFunction = callItOnce(libraryFunction);
myLibraryFunction(); // it runs
myLibraryFunction(); // null
libraryFunction(); // it runs

Török Gábor 2010-07-26 07:47:32

Answer 4

A:

If (!your_func.called) {
    your_func.called = true;
    your_func();
}

Thevs 2010-07-26 10:17:28

ansaurus

tags:

views:

answers:

how to detect a function was called with javascript

related questions