7 Essential JavaScript Functions

essential-javascript-functions.js

// http://davidwalsh.name/essential-javascript-functions

/*
  debounce

  The debounce function can be a game-changer when it comes to event-fueled performance.  
  If you aren't using a debouncing function with a scroll, resize, key* event, you're 
  probably doing it wrong.  Here's a debounce function to keep your code efficient:
*/

// Returns a function, that, as long as it continues to be invoked, will not
// be triggered. The function will be called after it stops being called for
// N milliseconds. If `immediate` is passed, trigger the function on the
// leading edge, instead of the trailing.
function debounce(func, wait, immediate) {
  var timeout;
  return function() {
    var context = this, args = arguments;
    var later = function() {
      timeout = null;
      if (!immediate) func.apply(context, args);
    };
    var callNow = immediate && !timeout;
    clearTimeout(timeout);
    timeout = setTimeout(later, wait);
    if (callNow) func.apply(context, args);
  };
};

// Usage
var myEfficientFn = debounce(function() {
  // All the taxing stuff you do
}, 250);
window.addEventListener('resize', myEfficientFn);

//The debounce function will not allow a callback to be used more than once per given time frame.  
//This is especially important when assigning a callback function to frequently-firing events.



/*
  poll

  As I mentioned with the debounce function, sometimes you don't get to plug into an event 
  to signify a desired state -- if the event doesn't exist, you need to check for your 
  desired state at intervals:
*/

function poll(fn, callback, errback, timeout, interval) {
    var endTime = Number(new Date()) + (timeout || 2000);
    interval = interval || 100;

    (function p() {
            // If the condition is met, we're done! 
            if(fn()) {
                callback();
            }
            // If the condition isn't met but the timeout hasn't elapsed, go again
            else if (Number(new Date()) < endTime) {
                setTimeout(p, interval);
            }
            // Didn't match and too much time, reject!
            else {
                errback(new Error('timed out for ' + fn + ': ' + arguments));
            }
    })();
}

// Usage:  ensure element is visible
poll(
    function() {
        return document.getElementById('lightbox').offsetWidth > 0;
    },
    function() {
        // Done, success callback
    },
    function() {
        // Error, failure callback
    }
);

// Polling has long been useful on the web and will continue to be in the future!



/*
  once

  There are times when you prefer a given functionality only happen once, similar to 
  the way you'd use an onload event.  This code provides you said functionality:
*/

function once(fn, context) { 
  var result;

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

    return result;
  };
}

// Usage
var canOnlyFireOnce = once(function() {
  console.log('Fired!');
});

canOnlyFireOnce(); // "Fired!"
canOnlyFireOnce(); // nada
// The once function ensures a given function can only be called once, thus prevent duplicate initialization!



/*
  getAbsoluteUrl

  Getting an absolute URL from a variable string isn't as easy as you think.  There's the URL constructor 
  but it can act up if you don't provide the required arguments (which sometimes you can't).  Here's a 
  suave trick for getting an absolute URL from and string input:
*/

var getAbsoluteUrl = (function() {
  var a;

  return function(url) {
    if(!a) a = document.createElement('a');
    a.href = url;

    return a.href;
  };
})();

// Usage
getAbsoluteUrl('/something'); // http://davidwalsh.name/something
//The "burn" element href handles and URL nonsense for you, providing a reliable absolute URL in return.



/*
  isNative

  Knowing if a given function is native or not can signal if you're willing to override it.  
  This handy code can give you the answer:
*/

;(function() {

  // Used to resolve the internal `[[Class]]` of values
  var toString = Object.prototype.toString;
  
  // Used to resolve the decompiled source of functions
  var fnToString = Function.prototype.toString;
  
  // Used to detect host constructors (Safari > 4; really typed array specific)
  var reHostCtor = /^\[object .+?Constructor\]$/;

  // Compile a regexp using a common native method as a template.
  // We chose `Object#toString` because there's a good chance it is not being mucked with.
  var reNative = RegExp('^' +
    // Coerce `Object#toString` to a string
    String(toString)
    // Escape any special regexp characters
    .replace(/[.*+?^${}()|[\]\/\\]/g, '\\$&')
    // Replace mentions of `toString` with `.*?` to keep the template generic.
    // Replace thing like `for ...` to support environments like Rhino which add extra info
    // such as method arity.
    .replace(/toString|(function).*?(?=\\\()| for .+?(?=\\\])/g, '$1.*?') + '$'
  );
  
  function isNative(value) {
    var type = typeof value;
    return type == 'function'
      // Use `Function#toString` to bypass the value's own `toString` method
      // and avoid being faked out.
      ? reNative.test(fnToString.call(value))
      // Fallback to a host object check because some environments will represent
      // things like typed arrays as DOM methods which may not conform to the
      // normal native pattern.
      : (value && type == 'object' && reHostCtor.test(toString.call(value))) || false;
  }
  
  // export however you want
  module.exports = isNative;
}());

// Usage
isNative(alert); // true
isNative(myCustomFunction); // false
//The function isn't pretty but it gets the job done!



/*
  insertRule

  We all know that we can grab a NodeList from a selector (via document.querySelectorAll) and 
  give each of them a style, but what's more efficient is setting that style to a selector 
  (like you do in a stylesheet):
*/

var sheet = (function() {
  // Create the <style> tag
  var style = document.createElement('style');

  // Add a media (and/or media query) here if you'd like!
  // style.setAttribute('media', 'screen')
  // style.setAttribute('media', 'only screen and (max-width : 1024px)')

  // WebKit hack :(
  style.appendChild(document.createTextNode(''));

  // Add the <style> element to the page
  document.head.appendChild(style);

  return style.sheet;
})();

// Usage
sheet.insertRule("header { float: left; opacity: 0.8; }", 1);
//This is especially useful when working on a dynamic, AJAX-heavy site.  If you set the style 
//to a selector, you don't need to account for styling each element that may match that selector (now or in the future).



/*
  matchesSelector

  Oftentimes we validate input before moving forward; ensuring a truthy value, ensuring forms 
  data is valid, etc.  But how often do we ensure an element qualifies for moving forward?  
  You can use a matchesSelector function to validate if an element is of a given selector match:
*/

function matchesSelector(el, selector) {
  var p = Element.prototype;
  var f = p.matches || p.webkitMatchesSelector || p.mozMatchesSelector || p.msMatchesSelector || function(s) {
    return [].indexOf.call(document.querySelectorAll(s), this) !== -1;
  };
  return f.call(el, selector);
}

// Usage
matchesSelector(document.getElementById('myDiv'), 'div.someSelector[some-attribute=true]')