Function in JavaScript that can be called only once

I need to create a function which can be executed only once, in each time after the first it won't be executed. I know from C++ and Java about static variables that can do the work but I would like to know if there is a more elegant way to do this?

Answers:

Answer

If by "won't be executed" you mean "will do nothing when called more than once", you can create a closure:

var something = (function() {
    var executed = false;
    return function() {
        if (!executed) {
            executed = true;
            // do something
        }
    };
})();

something(); // "do something" happens
something(); // nothing happens

In answer to a comment by @Vladloffe (now deleted): With a global variable, other code could reset the value of the "executed" flag (whatever name you pick for it). With a closure, other code has no way to do that, either accidentally or deliberately.

As other answers here point out, several libraries (such as Underscore and Ramda) have a little utility function (typically named once()[*]) that accepts a function as an argument and returns another function that calls the supplied function exactly once, regardless of how many times the returned function is called. The returned function also caches the value first returned by the supplied function and returns that on subsequent calls.

However, if you aren't using such a third-party library, but still want such a utility function (rather than the nonce solution I offered above), it's easy enough to implement. The nicest version I've seen is this one posted by David Walsh:

function once(fn, context) { 
    var result;
    return function() { 
        if (fn) {
            result = fn.apply(context || this, arguments);
            fn = null;
        }
        return result;
    };
}

I would be inclined to change fn = null; to fn = context = null;. There's no reason for the closure to maintain a reference to context once fn has been called.

[*] Be aware, though, that other libraries, such as this Drupal extension to jQuery, may have a function named once() that does something quite different.

Answer

Replace it with a reusable NOOP (no operation) function.

// this function does nothing
function noop() {};

function foo() {
    foo = noop; // swap the functions

    // do your thing
}

function bar() {
    bar = noop; // swap the functions

    // do your thing
}
Answer

Point to an empty function once it has been called:

function myFunc(){
     myFunc = function(){}; // kill it as soon as it was called
     console.log('call once and never again!'); // your stuff here
};
<button onClick=myFunc()>Call myFunc()</button>


Or, like so:

var myFunc = function func(){
     if( myFunc.fired ) return;
     myFunc.fired = true;
     console.log('called once and never again!'); // your stuff here
};

// even if referenced & "renamed"
((refToMyfunc)=>{
  setInterval(refToMyfunc, 1000);
})(myFunc)

Answer

UnderscoreJs has a function that does that, underscorejs.org/#once

  // Returns a function that will be executed at most one time, no matter how
  // often you call it. Useful for lazy initialization.
  _.once = function(func) {
    var ran = false, memo;
    return function() {
      if (ran) return memo;
      ran = true;
      memo = func.apply(this, arguments);
      func = null;
      return memo;
    };
  };
Answer

Talking about static variables, this is a little bit like closure variant:

var once = function() {
    if(once.done) return;
    console.log('Doing this once!');
    once.done = true;
};

once(); once(); 

You could then reset a function if you wish:

once.done = false;
Answer
var quit = false;

function something() {
    if(quit) {
       return;
    } 
    quit = true;
    ... other code....
}
Answer

try this

var fun = (function() {
  var called = false;
  return function() {
    if (!called) {
      console.log("I  called");
      called = true;
    }
  }
})()
Answer

From some dude named Crockford... :)

function once(func) {
    return function () {
        var f = func;
        func = null;
        return f.apply(
            this,
            arguments
        );
    };
}
Answer

Here is an example JSFiddle - http://jsfiddle.net/6yL6t/

And the code:

function hashCode(str) {
    var hash = 0, i, chr, len;
    if (str.length == 0) return hash;
    for (i = 0, len = str.length; i < len; i++) {
        chr   = str.charCodeAt(i);
        hash  = ((hash << 5) - hash) + chr;
        hash |= 0; // Convert to 32bit integer
    }
    return hash;
}

var onceHashes = {};

function once(func) {
    var unique = hashCode(func.toString().match(/function[^{]+\{([\s\S]*)\}$/)[1]);

    if (!onceHashes[unique]) {
        onceHashes[unique] = true;
        func();
    }
}

You could do:

for (var i=0; i<10; i++) {
    once(function() {
        alert(i);
    });
}

And it will run only once :)

Answer

Initial setup:

var once = function( once_fn ) {
    var ret, is_called;
    // return new function which is our control function 
    // to make sure once_fn is only called once:
    return function(arg1, arg2, arg3) {
        if ( is_called ) return ret;
        is_called = true;
        // return the result from once_fn and store to so we can return it multiply times:
        // you might wanna look at Function.prototype.apply:
        ret = once_fn(arg1, arg2, arg3);
        return ret;
    };
}
Answer

If your using Node.js or writing JavaScript with browserify, consider the "once" npm module:

var once = require('once')

function load (file, cb) {
  cb = once(cb)
  loader.load('file')
  loader.once('load', cb)
  loader.once('error', cb)
}
Answer

Reusable invalidate function which works with setInterval:

var myFunc = function (){
  if (invalidate(arguments)) return;
  console.log('called once and never again!'); // your stuff here
};

const invalidate = function(a) {
  var fired = a.callee.fired;
  a.callee.fired = true;
  return fired;
}

setInterval(myFunc, 1000);

Try it on JSBin: https://jsbin.com/vicipar/edit?js,console

Variation of answer from Bunyk

Answer

If you want to be able to reuse the function in the future then this works well based on ed Hopp's code above (I realize that the original question didn't call for this extra feature!):

   var something = (function() {
   var executed = false;              
    return function(value) {
        // if an argument is not present then
        if(arguments.length == 0) {               
            if (!executed) {
            executed = true;
            //Do stuff here only once unless reset
            console.log("Hello World!");
            }
            else return;

        } else {
            // otherwise allow the function to fire again
            executed = value;
            return;
        }       
    }
})();

something();//Hello World!
something();
something();
console.log("Reset"); //Reset
something(false);
something();//Hello World!
something();
something();

The output look like:

Hello World!
Reset
Hello World!
Answer

Trying to use underscore "once" function:

var initialize = _.once(createApplication);
initialize();
initialize();
// Application is only created once.

http://underscorejs.org/#once

Answer
var init = function() {
    console.log("logges only once");
    init = false;
}; 

if(init) { init(); }

/* next time executing init() will cause error because now init is 
   -equal to false, thus typing init will return false; */
Answer

If you're using Ramda, you can use the function "once".

A quote from the documentation:

once Function (a… ? b) ? (a… ? b) PARAMETERS Added in v0.1.0

Accepts a function fn and returns a function that guards invocation of fn such that fn can only ever be called once, no matter how many times the returned function is invoked. The first value calculated is returned in subsequent invocations.

var addOneOnce = R.once(x => x + 1);
addOneOnce(10); //=> 11
addOneOnce(addOneOnce(50)); //=> 11
Answer

simple decorator that easy to write when you need

function one(func) {
  return function () {
     func && func.apply(this, arguments);
     func = null;
  }
}

using:

var initializer= one( _ =>{
      console.log('initializing')
  })

initializer() // 'initializing'
initializer() // nop
initializer() // nop
Answer

keep it as simple as possible

function sree(){
  console.log('hey');
  window.sree = _=>{};
}

You can see the result

script result

Answer
if (!window.doesThisOnce){
  function myFunction() {
    // do something
    window.doesThisOnce = true;
  };
};
Answer

This one is useful for preventing infinite loops (using jQuery):

<script>
var doIt = true;
if(doIt){
  // do stuff
  $('body').html(String($('body').html()).replace("var doIt = true;", 
                                                  "var doIt = false;"));
} 
</script>

If you're worried about namespace pollution, subsitute a long, random string for "doIt".

Answer

It helps to prevent sticky execution

var done = false;

function doItOnce(func){
  if(!done){
    done = true;
    func()
  }
  setTimeout(function(){
    done = false;
  },1000)
}
Answer

You could simply have the function "remove itself"

?function Once(){
    console.log("run");

    Once = undefined;
}

Once();  // run
Once();  // Uncaught TypeError: undefined is not a function 

But this may not be the best answer if you don't want to be swallowing errors.

You could also do this:

function Once(){
    console.log("run");

    Once = function(){};
}

Once(); // run
Once(); // nothing happens

I need it to work like smart pointer, if there no elements from type A it can be executed, if there is one or more A elements the function can't be executed.

function Conditional(){
    if (!<no elements from type A>) return;

    // do stuff
}
Answer

JQuery allows to call the function only once using the method one():

let func = function() {
  console.log('Calling just once!');
}
  
let elem = $('#example');
  
elem.one('click', func);
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div>
  <p>Function that can be called only once</p>
  <button id="example" >JQuery one()</button>
</div>

Implementation using JQuery method on():

let func = function(e) {
  console.log('Calling just once!');
  $(e.target).off(e.type, func)
}
  
let elem = $('#example');
  
elem.on('click', func);
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div>
  <p>Function that can be called only once</p>
  <button id="example" >JQuery on()</button>
</div>

Implementation using native JS:

let func = function(e) {
  console.log('Calling just once!');
  e.target.removeEventListener(e.type, func);
}
  
let elem = document.getElementById('example');
  
elem.addEventListener('click', func);
<div>
  <p>Functions that can be called only once</p>
  <button id="example" >ECMAScript addEventListener</button>
</div>

Answer

Tossing my hat in the ring for fun, added advantage of memoizing

const callOnce = (fn, i=0, memo) => () => i++ ? memo : (memo = fn());
// usage
const myExpensiveFunction = () => { return console.log('joe'),5; }
const memoed = callOnce(myExpensiveFunction);
memoed(); //logs "joe", returns 5
memoed(); // returns 5
memoed(); // returns 5
...

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