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HTML

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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

About the <head>

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. If you need things in the <head> of the document, put that code here.

!
Insecure Resource

The resource you are linking to is using the 'http' protocol, which may not work when the browser is using https.

CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

We offer two popular choices: Autoprefixer (which processes your CSS server-side) and -prefix-free (which applies prefixes via a script, client-side).

Add External Stylesheets/Pens

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You can apply CSS to your Pen from any stylesheet on the web. Just put a URL to it here and we'll apply it, in the order you have them, before the CSS in the Pen itself.

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient. For instance, CoffeeScript can help prevent easy-to-make mistakes and offer a cleaner syntax and Babel can bring ECMAScript 6 features to browsers that only support ECMAScript 5.

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Babel is required to process package imports. If you need a different preprocessor remove all packages first.

Add External Scripts/Pens

Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

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About External Resources

You can apply a script from anywhere on the web to your Pen. Just put a URL to it here and we'll add it, in the order you have them, before the JavaScript in the Pen itself.

If the script you link to has the file extension of a preprocessor, we'll attempt to process it before applying.

You can also link to another Pen here, and we'll pull the JavaScript from that Pen and include it. If it's using a matching preprocessor, we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

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HTML 

              
                <div id="app">
  <div class="item" v-for="(item, index) in items" :key="item">
    <h2>Image {{ index + 1 }}</h2>
    
    <img :data-src="item" />
  </div>
</div>
!

CSS 

              
                html, body {
  margin: 0;
  padding: 0;
  background: #232323;
  color: #ececec;
  font-family: sans-serif;
}

#app {
  width: 100%;
}

.item {
  width: 90%;
  max-width: 650px;
  min-height: 500px;
  margin: 75px auto;
}

.item h2 {
  text-transform: uppercase;
}

.item img {
  position: relative;
  width: 100%;
  
  background: #ececec;
  
  transition: opacity 1s ease;
}

.item img[data-src] {
  height: 400px;
  opacity: 0.0;
}
!

JS 

              
                /// ScrollTrigger v1.0.2 since it's not hosted on cdnjs yet
/// Scroll down for the Vue App


!function(t,e){"object"==typeof exports&&"object"==typeof module?module.exports=e():"function"==typeof define&&define.amd?define("ScrollTrigger",[],e):"object"==typeof exports?exports.ScrollTrigger=e():t.ScrollTrigger=e()}(window,(function(){return function(t){var e={};function i(n){if(e[n])return e[n].exports;var o=e[n]={i:n,l:!1,exports:{}};return t[n].call(o.exports,o,o.exports,i),o.l=!0,o.exports}return i.m=t,i.c=e,i.d=function(t,e,n){i.o(t,e)||Object.defineProperty(t,e,{enumerable:!0,get:n})},i.r=function(t){"undefined"!=typeof Symbol&&Symbol.toStringTag&&Object.defineProperty(t,Symbol.toStringTag,{value:"Module"}),Object.defineProperty(t,"__esModule",{value:!0})},i.t=function(t,e){if(1&e&&(t=i(t)),8&e)return t;if(4&e&&"object"==typeof t&&t&&t.__esModule)return t;var n=Object.create(null);if(i.r(n),Object.defineProperty(n,"default",{enumerable:!0,value:t}),2&e&&"string"!=typeof t)for(var o in t)i.d(n,o,function(e){return t[e]}.bind(null,o));return n},i.n=function(t){var e=t&&t.__esModule?function(){return t.default}:function(){return t};return i.d(e,"a",e),e},i.o=function(t,e){return Object.prototype.hasOwnProperty.call(t,e)},i.p="",i(i.s=2)}([function(t,e){
/*!
 * object-extend
 * A well-tested function to deep extend (or merge) JavaScript objects without further dependencies.
 *
 * http://github.com/bernhardw
 *
 * Copyright 2013, Bernhard Wanger <mail@bernhardwanger.com>
 * Released under the MIT license.
 *
 * Date: 2013-04-10
 */
t.exports=function t(e,i){return null==e||null==i?e:(Object.keys(i).forEach((function(n){"[object Object]"==Object.prototype.toString.call(i[n])?"[object Object]"!=Object.prototype.toString.call(e[n])?e[n]=i[n]:e[n]=t(e[n],i[n]):e[n]=i[n]})),e)}},function(t,e){Array.prototype.each=function(t){for(var e=this.length,i=0;i<e;i++){var n=this[i];n&&t(n,i)}},NodeList.prototype.each=Array.prototype.each,NodeList.prototype.filter=Array.prototype.filter},function(t,e,i){"use strict";i.r(e);var n=function(){this.trigger={once:!1,offset:{viewport:{x:0,y:0},element:{x:0,y:0}},toggle:{class:{in:"visible",out:"invisible"},callback:{in:null,visible:null,out:null}}},this.scroll={sustain:300,element:window,callback:function(){},start:function(){},stop:function(){},directionChange:function(){}}},o=i(0),s=i.n(o);i(1);function r(t,e){for(var i=0;i<e.length;i++){var n=e[i];n.enumerable=n.enumerable||!1,n.configurable=!0,"value"in n&&(n.writable=!0),Object.defineProperty(t,n.key,n)}}function l(t){return Number(t)===t&&t%1==0}function c(t){return Number(t)===t&&t%1!=0}var a=function(){function t(e,i){!function(t,e){if(!(t instanceof e))throw new TypeError("Cannot call a class as a function")}(this,t),this.element=e,i=s()((new n).trigger,i),this.offset=i.offset,this.toggle=i.toggle,this.once=i.once,this.visible=null,this.active=!0}var e,i,o;return e=t,(i=[{key:"checkVisibility",value:function(t,e){if(!this.active)return this.visible;var i={w:t.offsetWidth||t.innerWidth||0,h:t.offsetHeight||t.innerHeight||0},n=this.getBounds(),o=this._checkVisibility(n,i,e);if(o!==this.visible){this.visible=o;var s=this._toggleCallback();s instanceof Promise?s.then(this._toggleClass.bind(this)).catch((function(t){console.error("Trigger promise failed"),console.error(t)})):this._toggleClass(),this.visible&&this.once&&(this.active=!1)}else if(o&&"function"==typeof this.toggle.callback.visible)return this.toggle.callback.visible.call(this.element,this);return o}},{key:"getBounds",value:function(){return this.element.getBoundingClientRect()}},{key:"_getElementOffset",value:function(t,e){var i={x:0,y:0};return"function"==typeof this.offset.element.x?i.x=t.width*this.offset.element.x(this,t,e):c(this.offset.element.x)?i.x=t.width*this.offset.element.x:l(this.offset.element.x)&&(i.x=this.offset.element.x),"function"==typeof this.offset.element.y?i.y=t.height*this.offset.element.y(this,t,e):c(this.offset.element.y)?i.y=t.height*this.offset.element.y:l(this.offset.element.y)&&(i.y=this.offset.element.y),i}},{key:"_getViewportOffset",value:function(t,e){var i={x:0,y:0};return"function"==typeof this.offset.viewport.x?i.x=t.w*this.offset.viewport.x(this,t,e):c(this.offset.viewport.x)?i.x=t.w*this.offset.viewport.x:l(this.offset.viewport.x)&&(i.x=this.offset.viewport.x),"function"==typeof this.offset.viewport.y?i.y=t.h*this.offset.viewport.y(this,t,e):c(this.offset.viewport.y)?i.y=t.h*this.offset.viewport.y:l(this.offset.viewport.y)&&(i.y=this.offset.viewport.y),i}},{key:"_checkVisibility",value:function(t,e,i){var n=this._getElementOffset(t,i),o=this._getViewportOffset(e,i),s=!0;return t.x-o.x<-(t.width-n.x)&&(s=!1),t.x+o.x>e.w-n.x&&(s=!1),t.y-o.y<-(t.height-n.y)&&(s=!1),t.y+o.y>e.h-n.y&&(s=!1),s}},{key:"_toggleClass",value:function(){var t=this;if(this.visible)return Array.isArray(this.toggle.class.in)?this.toggle.class.in.each((function(e){t.element.classList.add(e)})):this.element.classList.add(this.toggle.class.in),void(Array.isArray(this.toggle.class.out)?this.toggle.class.out.each((function(e){t.element.classList.remove(e)})):this.element.classList.remove(this.toggle.class.out));Array.isArray(this.toggle.class.in)?this.toggle.class.in.each((function(e){t.element.classList.remove(e)})):this.element.classList.remove(this.toggle.class.in),Array.isArray(this.toggle.class.out)?this.toggle.class.out.each((function(e){t.element.classList.add(e)})):this.element.classList.add(this.toggle.class.out)}},{key:"_toggleCallback",value:function(){if(this.visible){if("function"==typeof this.toggle.callback.in)return this.toggle.callback.in.call(this.element,this)}else if("function"==typeof this.toggle.callback.out)return this.toggle.callback.out.call(this.element,this)}}])&&r(e.prototype,i),o&&r(e,o),t}();function u(t,e){for(var i=0;i<e.length;i++){var n=e[i];n.enumerable=n.enumerable||!1,n.configurable=!0,"value"in n&&(n.writable=!0),Object.defineProperty(t,n.key,n)}}var h=function(){function t(e){!function(t,e){if(!(t instanceof e))throw new TypeError("Cannot call a class as a function")}(this,t),this.triggers=e instanceof Array?e:[]}var e,i,n;return e=t,(i=[{key:"add",value:function(t){var e=this;if(t instanceof a)return this.triggers.push(t);t.each((function(t){t instanceof a?e.triggers.push(t):console.error("Object added to TriggerCollection is not a Trigger. Object: ",t)}))}},{key:"remove",value:function(t){t instanceof a&&(t=[t]),this.triggers=this.triggers.filter((function(e){var i=!1;return t.each((function(t){t==e&&(i=!0)})),!i}))}},{key:"query",value:function(t){return this.triggers.filter((function(e){var i=e.element,n=i.parentNode;return[].slice.call(n.querySelectorAll(t)).indexOf(i)>-1}))}},{key:"search",value:function(t){var e=this.triggers.filter((function(e){if(t instanceof NodeList||Array.isArray(t)){var i=!1;return t.each((function(t){e.element==t&&(i=!0)})),i}return e.element==t}));return 0==e.length?null:e.length>1?e:e[0]}},{key:"call",value:function(t){this.triggers.each(t)}}])&&u(e.prototype,i),n&&u(e,n),t}();function f(t,e){for(var i=0;i<e.length;i++){var n=e[i];n.enumerable=n.enumerable||!1,n.configurable=!0,"value"in n&&(n.writable=!0),Object.defineProperty(t,n.key,n)}}var g=function(){function t(e,i){!function(t,e){if(!(t instanceof e))throw new TypeError("Cannot call a class as a function")}(this,t),this._parseOptions(e),"function"==typeof i&&(this.callback=i),this.direction="none",this.position=this.getPosition(),this.lastAction=this._getTimestamp(),this._startRun(),this._boundListener=this._didScroll.bind(this),this.element.addEventListener("scroll",this._boundListener)}var e,i,o;return e=t,(i=[{key:"_parseOptions",value:function(t){var e=(new n).scroll;"function"!=typeof t?(e.callback=function(){},e=s()(e,t)):e.callback=t,this.element=e.element,this.sustain=e.sustain,this.callback=e.callback,this.startCallback=e.start,this.stopCallback=e.stop,this.directionChange=e.directionChange}},{key:"_didScroll",value:function(){var t=this.getPosition();if(this.position!==t){var e=this.direction;(e=t.x!==this.position.x?t.x>this.position.x?"right":"left":t.y!==this.position.y?t.y>this.position.y?"bottom":"top":"none")!==this.direction&&(this.direction=e,"function"==typeof this.directionChange&&this.directionChange(this.direction)),this.position=t,this.lastAction=this._getTimestamp()}else this.direction="none";this.running||this._startRun()}},{key:"_startRun",value:function(){this.running=!0,"function"==typeof this.startCallback&&this.startCallback(),this._loop()}},{key:"_stopRun",value:function(){this.running=!1,"function"==typeof this.stopCallback&&this.stopCallback()}},{key:"getPosition",value:function(){return{x:this.element.pageXOffset||this.element.scrollLeft||document.documentElement.scrollLeft||0,y:this.element.pageYOffset||this.element.scrollTop||document.documentElement.scrollTop||0}}},{key:"_getTimestamp",value:function(){return Number(Date.now())}},{key:"_tick",value:function(){this.callback(this.position,this.direction),this._getTimestamp()-this.lastAction>this.sustain&&this._stopRun(),this.running&&this._loop()}},{key:"_loop",value:function(){(window.requestAnimationFrame||window.webkitRequestAnimationFrame||window.mozRequestAnimationFrame||window.msRequestAnimationFrame||window.oRequestAnimationFrame||function(t){setTimeout(t,1e3/60)})(this._tick.bind(this))}},{key:"kill",value:function(){this.running=!1,this.element.removeEventListener("scroll",this._boundListener)}}])&&f(e.prototype,i),o&&f(e,o),t}();function y(t,e){for(var i=0;i<e.length;i++){var n=e[i];n.enumerable=n.enumerable||!1,n.configurable=!0,"value"in n&&(n.writable=!0),Object.defineProperty(t,n.key,n)}}i.d(e,"Trigger",(function(){return p})),i.d(e,"TriggerCollection",(function(){return v})),i.d(e,"ScrollAnimationLoop",(function(){return d})),i.d(e,"default",(function(){return m}));
/*!
 * ScrollTrigger
 *
 *
 * http://github.com/terwanerik
 *
 * Copyright 2017, Erik Terwan <erik@erikterwan.com>
 * Released under the MIT license.
 *
 * Date: 2017-07-09
 */
var p=a,v=h,d=g,m=function(){function t(e){!function(t,e){if(!(t instanceof e))throw new TypeError("Cannot call a class as a function")}(this,t),this._parseOptions(e),this._initCollection(),this._initLoop()}var e,i,o;return e=t,(i=[{key:"_parseOptions",value:function(t){t=s()(new n,t),this.defaultTrigger=t.trigger,this.scrollOptions=t.scroll}},{key:"_initCollection",value:function(){var t=document.querySelectorAll("[data-scroll]"),e=[];t.length>0&&(e=this.createTriggers(t)),this.collection=new v(e)}},{key:"_initLoop",value:function(){var t=this;this.loop=new d({sustain:this.scrollOptions.sustain,element:this.scrollOptions.element,callback:function(e,i){t._scrollCallback(e,i)},start:function(){t._scrollStart()},stop:function(){t._scrollStop()},directionChange:function(e){t._scrollDirectionChange(e)}})}},{key:"_scrollCallback",value:function(t,e){var i=this;this.collection.call((function(t){t.checkVisibility(i.scrollOptions.element,e)})),this.scrollOptions.callback(t,e)}},{key:"_scrollStart",value:function(){this.scrollOptions.start()}},{key:"_scrollStop",value:function(){this.scrollOptions.stop()}},{key:"_scrollDirectionChange",value:function(t){this.scrollOptions.directionChange(t)}},{key:"createTrigger",value:function(t,e){return new p(t,s()(this.defaultTrigger,e))}},{key:"createTriggers",value:function(t,e){var i=this,n=[];return t.each((function(t){n.push(i.createTrigger(t,e))})),n}},{key:"add",value:function(t,e){return t instanceof HTMLElement?(this.collection.add(this.createTrigger(t,e)),this):t instanceof p?(this.collection.add(t),this):t instanceof NodeList?(this.collection.add(this.createTriggers(t,e)),this):Array.isArray(t)&&t.length&&t[0]instanceof p?(this.collection.add(t),this):Array.isArray(t)&&t.length&&t[0]instanceof HTMLElement?(this.collection.add(this.createTriggers(t,e)),this):(this.collection.add(this.createTriggers(document.querySelectorAll(t),e)),this)}},{key:"remove",value:function(t){return t instanceof p?(this.collection.remove(t),this):Array.isArray(t)&&t.length&&t[0]instanceof p?(this.collection.remove(t),this):t instanceof HTMLElement?(this.collection.remove(this.search(t)),this):Array.isArray(t)&&t.length&&t[0]instanceof HTMLElement?(this.collection.remove(this.search(t)),this):t instanceof NodeList?(this.collection.remove(this.search(t)),this):Array.isArray(t)&&t.length&&t[0]instanceof p?(this.collection.remove(t),this):(this.collection.remove(this.query(t.toString())),this)}},{key:"query",value:function(t){return this.collection.query(t)}},{key:"search",value:function(t){return this.collection.search(t)}},{key:"listen",value:function(){this.loop||this._initLoop()}},{key:"kill",value:function(){this.loop.kill(),this.loop=null}}])&&y(e.prototype,i),o&&y(e,o),t}()}])}));
//# sourceMappingURL=ScrollTrigger.min.js.map






// Super Simple image loading example.

var app = new Vue({
  el: '#app',
  data() {
    return {
      items: [
        'https://source.unsplash.com/random?1',
        'https://source.unsplash.com/random?2',
        'https://source.unsplash.com/random?3',
        'https://source.unsplash.com/random?4',
        'https://source.unsplash.com/random?5',
        'https://source.unsplash.com/random?6',
        'https://source.unsplash.com/random?7',
        'https://source.unsplash.com/random?8',
        'https://source.unsplash.com/random?9',
        'https://source.unsplash.com/random?10'
      ],
      trigger: null
    }
  },
  mounted() {
    this.trigger = new ScrollTrigger.default()
    this.trigger.add('[data-src]', {
      once: true,
      toggle: {
        callback: {
          in: this.loadImage.bind(this)
        }
      }
    })
  },
  beforeDestroy() {
    this.trigger.kill()
    this.trigger = null
  },
  methods: {
    loadImage(trigger) {
      const el = trigger.element
      
      if (!el.hasAttribute('data-src')) { return }
      
      const source = el.getAttribute('data-src')
      
      el.setAttribute('src', source)
      
      el.addEventListener('load', () => {
        el.removeAttribute('data-src')
      })
    }
  }
})
!
999px

Console