What is the purpose of employing this expression in the context of requestAnimationFrame?

Can you explain the purpose of using this specific "if" statement in relation to requestAnimationFrame?

if (!window.requestAnimationFrame)
        window.requestAnimationFrame = function(callback, element) {
            var currTime = new Date().getTime();
            var timeToCall = Math.max(0, 16 - (currTime - lastTime));
            var id = window.setTimeout(function() { callback(currTime + timeToCall); },
              timeToCall);
            lastTime = currTime + timeToCall;
            return id;
        };

javascriptrequestanimationframe

Answer №1

This code snippet introduces requestAnimationFrame implementation. It creates a global function requestAnimationFrame within the window object if it doesn't already exist.

Highlighted below is a brief explanation of its purpose:

if (!window.requestAnimationFrame) // check if requestAnimationFrame is not available
  window.requestAnimationFrame = 
    function(callback, element) { // define the requestAnimationFrame function
      /* actual code goes here */
    };

Such snippets are commonly referred to as polyfills - they bridge the gap between newer browser/javascript engine features and older browsers/javascript engines.

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