Display a loading bar before the entire page is loaded

I would like to display a loading bar before the entire page is loaded. For now, I'm just using a small delay:

$(document).ready(function(){
    $('#page').fadeIn(2000);
});

The page already uses jQuery.

Note: I have tried this, but it didn't work for me: loading bar while the script runs

I also tried other solutions. In most cases, the page loads as usual, or the page won't load/display at all.

Use a div #overlay with your loading info / .gif that will cover all your page:

<div id="overlay">
     <img src="loading.gif" alt="Loading" />
     Loading...
</div>

jQuery:

$(window).load(function(){
   // PAGE IS FULLY LOADED  
   // FADE OUT YOUR OVERLAYING DIV
   $('#overlay').fadeOut();
});

Here's an example with a Loading bar:

jsBin demo

;(function(){
  function id(v){ return document.getElementById(v); }
  function loadbar() {
    var ovrl = id("overlay"),
        prog = id("progress"),
        stat = id("progstat"),
        img = document.images,
        c = 0,
        tot = img.length;
    if(tot == 0) return doneLoading();

    function imgLoaded(){
      c += 1;
      var perc = ((100/tot*c) << 0) +"%";
      prog.style.width = perc;
      stat.innerHTML = "Loading "+ perc;
      if(c===tot) return doneLoading();
    }
    function doneLoading(){
      ovrl.style.opacity = 0;
      setTimeout(function(){ 
        ovrl.style.display = "none";
      }, 1200);
    }
    for(var i=0; i<tot; i++) {
      var tImg     = new Image();
      tImg.onload  = imgLoaded;
      tImg.onerror = imgLoaded;
      tImg.src     = img[i].src;
    }    
  }
  document.addEventListener('DOMContentLoaded', loadbar, false);
}());

*{margin:0;}
body{ font: 200 16px/1 sans-serif; }
img{ width:32.2%; }

#overlay{
  position:fixed;
  z-index:99999;
  top:0;
  left:0;
  bottom:0;
  right:0;
  background:rgba(0,0,0,0.9);
  transition: 1s 0.4s;
}
#progress{
  height:1px;
  background:#fff;
  position:absolute;
  width:0;                /* will be increased by JS */
  top:50%;
}
#progstat{
  font-size:0.7em;
  letter-spacing: 3px;
  position:absolute;
  top:50%;
  margin-top:-40px;
  width:100%;
  text-align:center;
  color:#fff;
}

<div id="overlay">
  <div id="progstat"></div>
  <div id="progress"></div>
</div>

<div id="container">
  <img src="http://placehold.it/3000x3000/cf5">
</div>

HTML

<div class="preload">
<img src="http://i.imgur.com/KUJoe.gif">
</div>

<div class="content">
I would like to display a loading bar before the entire page is loaded. 
</div>

JAVASCRIPT

$(function() {
    $(".preload").fadeOut(2000, function() {
        $(".content").fadeIn(1000);        
    });
});

CSS

.content {display:none;}
.preload { 
    width:100px;
    height: 100px;
    position: fixed;
    top: 50%;
    left: 50%;
}

DEMO

Whenever you try to load any data in this window this gif will load.

HTML

Make a Div

<div class="loader"></div>

CSS .

.loader {
    position: fixed;
    left: 0px;
    top: 0px;
    width: 100%;
    height: 100%;
    z-index: 9999;
    background: url('https://lkp.dispendik.surabaya.go.id/assets/loading.gif') 50% 50% no-repeat rgb(249,249,249);

jQuery

$(window).load(function() {
        $(".loader").fadeOut("slow");
});
<script src="https://code.jquery.com/jquery-1.9.1.min.js"></script>

enter image description here

I've recently made a page loader in VanillaJS for a project, just wanted to share it as, all the other answers, are jQuery based. It's a plug and play, one-liner.

let settings={backgroundColor:"#2774ab",filterBrightness:2,timeOnScreen:100},u=document.querySelector("*"),s=document.createElement("style"),a=document.createElement("div"),m="http://www.w3.org/2000/svg",g=document.createElementNS(m,"svg"),c=document.createElementNS(m,"circle");document.head.appendChild(s),s.innerHTML="@keyframes swell{to{transform:rotate(360deg)}}",a.setAttribute("style","background-color:"+settings.backgroundColor+";color:"+settings.backgroundColor+";display:flex;align-items:center;justify-content:center;position:fixed;top:0;height:100vh;width:100vw;z-index:2147483647"),document.body.prepend(a),g.setAttribute("style","height:50px;filter:brightness("+settings.filterBrightness+");animation:.3s swell infinite linear"),g.setAttribute("viewBox","0 0 100 100"),a.prepend(g),c.setAttribute("cx","50"),c.setAttribute("cy","50"),c.setAttribute("r","35"),c.setAttribute("fill","none"),c.setAttribute("stroke","currentColor"),c.setAttribute("stroke-dasharray","165 57"),c.setAttribute("stroke-width","10"),g.prepend(c),u.style.pointerEvents="none",u.style.userSelect="none",u.style.cursor="wait",window.onload=(()=>{setTimeout(()=>{u.style.pointerEvents="",u.style.userSelect="",u.style.cursor="",a.remove()},settings.timeOnScreen)});

Fundamentals

Generate a <script> element appended to the <head> element, containing any required styling.
Generate a <div> element, acting as overlay, prepended to the <body> element.
Generate a <svg> element, acting as loader, prepended to the previously generated <div> element.
On window.onload self generated elements are automatically removed.

The latest version is available on my GitHub.

Demo

Settings

let settings = {
    backgroundColor: "#2774ab",
    filterBrightness: 2,
    timeOnScreen: 100
}, //...

Option	Description
`backgroundColor`	Refer to MDN Web Docs color for acceptable values. The background-color CSS property sets the background color of an element. Default to Wordpress deep blue accent `#2774ab`
`filterBrightness`	number or percentage. The brightness of the `svg` loader element (brightness() CSS function). A value under `100%` darkens the loader, while a value over `100%` brightens it. The lacuna value for interpolation is `1`. Default to `2`.
`timeOnScreen`	Positive `integer`. The time on screen is appended to the page loading time. Default to `100` milliseconds.