Question

Is there a way to run a piece of JavaScript code only ONCE, without using boolean flag variables to remember whether it has already been ran or not?

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 booleans would be messy…

Answer 1

An alternative way that overwrites a function when executed so it will be executed only once.

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(){
   // read preferences once
   read_preferences = Function("");
   // load preferences from storage and save it in 'preferences'
}
function readPreference(pref_name){
    read_prefences();
    return preferences.hasOwnProperty(pref_name) ? 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, function(){} is replaced by Function(""). This will create an empty function in the global scope, avoiding a closure.