JavaScript - run once without booleans

Question

Is there a way I can run piece of javascript code ONCE without the use of boolean flag variables to know that it already ran?

Specifically not something like:

var alreadyRan = false;
function runOnce() {
  if (alreadyRan) {
    return;
  }
  alreadyRan = true;

  /* do stuff here */

}

I'm going to have a lot of these types of functions and keeping all the booleans around would be messy...

Answer 1

Just an alternative.

function useThisFunctionOnce(){
   // overwrite this function, so it will be executed only once
   useThisFunctionOnce = Function("");
   // real code below
   alert("Hi!");
}

// displays "Hi!"
useThisFunctionOnce();
// does nothing
useThisFunctionOnce();

'Useful' example:

var preferences = {};
function read_preferences(){
   // we do not want to think, just destroy itself
   read_preferences = Function("");
   // do some nasty things with cookies
}
function readPreference(pref_name){
    read_prefences();
    return pref_name in preferences ? preferences[pref_name] : '';
}
if(readPreference('like_javascript') != 'yes'){
   alert("What's wrong wth you?!");
}
alert(readPreference('is_stupid') ? "Stupid!" : ":)");

---

Edit: as CMS pointed out, just overwriting the old function with function(){} will create a closure in which old variables still exist. To work around that problem, I replaced it with Function(""). This will create a empty function in the global scope, leaving the old function scope.

Answer 2

If not bound to an event, code is usually ran once

Answer 3

I like Lekensteyn's implementation, but you could also just have one variable to store what functions have run. The code below should run "runOnce", and "runAgain" both one time. It's still booleans, but it sounds like you just don't want lots of variables.

var runFunctions = {};

function runOnce() {
  if(!hasRun(arguments.callee)) {
   /* do stuff here */
   console.log("once");
  }
}

function runAgain() {
  if(!hasRun(arguments.callee)) {
   /* do stuff here */
   console.log("again");
  }
}


function hasRun(functionName) {
 functionName = functionName.toString();
 functionName = functionName.substr('function '.length);
 functionName = functionName.substr(0, functionName.indexOf('('));

 if(runFunctions[functionName]) {
   return true;
 } else {
   runFunctions[functionName] = true;
   return false;
 }
}

runOnce();
runAgain();
runAgain();

Answer 4

What about a self-invoking anonymous function?

(function () {

    // code in here to run once

})();

the code will execute immediately and leave no trace in the global namespace.

If this code is going to need to be called from elsewhere, then a closure can be used to ensure that the contents of a function are run only once. Personally, I prefer this to a function that rewrites itself as I feel doing so can cause confusion, but to each their own :) This particular implementation takes advantage of the fact that 0 is a falsy value.

var once = (function() {
  var hasRun = 0;  
  return function () {
    if (!hasRun) {
      hasRun++;   

      // body to run only once

      // log to the console for a test       
      console.log("only ran once");
    }              
  }
})();

// test that the body of the function executes only once
for (var i = 0; i < 5; i++) 
  once();

Answer 5

In addition, the nature of what happens in the "/* do stuff here */" may leave something around that, when present, must mean that the function has run e.g.

var counter = null;

function initCounter() {
  if (counter === null) {
    counter = 0;
  }
}