Run function only once - functional programming JavaScript

Christian Sakai

I have a code below,

var a = 0;
var addByOne = doOnce(function() { a += 1; });

// I need to define a doOnce function here


// Run addByOne two times
addByOne();
addByOne();

This will result the variable a holds 2 as its value. My question is, how do I make the doOnce function so that it will result in running the function inside doOnce (in the case above, function () { a += 1; } ) just one time. So no matter how many times addByOne is called, variable a will be incremented just once.

Thanks

soktinpk

Try this:

function doOnce(fn) {
  // Keep track of whether the function has already been called
  var hasBeenCalled = false;
  // Returns a new function
  return function() {
    // If it has already been called, no need to call it again
    // Return (undefined)
    if (hasBeenCalled) return;
    // Set hasBeenCalled to true
    hasBeenCalled = true;
    return fn.apply(this, arguments);
  }
}

If you want, you can keep track of the return value and return that instead of undefined.

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