appear

Track the visibility of dom elements and fire user defined callbacks as they appear and disappear.

Demos

• appear.js simple test page
• appearlazy - a full fledged lazy image loader that uses appear.js

Be sure to view the documentation on the project page

Usage

Include appear.js in your page, it has no dependencies.

Call appear() passing in an object with the following:

• init function to run when dom is interactive, but before appear.js has started tracking.
• elements required function that returns an htmlcollection of elements to track. The dom will be interactive at this point. Can alternatively be an existing htmlcollection instead of a function.
• appear function to run when an element is in view, passed the element that has come into view. If defined then appear.js will track elements until they come into view.
• disappear function to run when an element goes out of view, passed the element that has gone out of view. If defined then appear.js will track elements until they go out of view.
• reappear if true appear.js will keep tracking elements for successfuive appears and dissappears. Default is false.
• bounds increase, in pixels, to the threshold size of the element so it can be "viewable" before it is actually in the viewport.
• debounce appear.js tracks elements on browser scroll and resize, for performance reasons this check is "debounced" to only happen once for multiple events, 50ms after the last event ends. You can override this value here.
• deltaSpeed in addition to debouncing, appear.js will also check for items during continuous slow scrolling, you can controll how slow the scrolling should be via this value. Default is 50 (pixels).
• deltaTimeout after a succesful delta speed check, appear.js will not check for viewable items again until this timeout passes. Default is 500 (ms).
• done function called when appear.js is no longer tracking any items and event listeners have been removed.

Example

appear({
  init: function init(){
    console.log('dom is ready');
  },
  elements: function elements(){
    // work with all elements with the class "track"
    return document.getElementsByClassName('track');
  },
  appear: function appear(el){
    console.log('visible', el);
  },
  disappear: function disappear(el){
    console.log('no longer visible', el);
  },
  bounds: 200,
  reappear: true
});

api

appear() will return an object with the following:

• trigger() force a check for viewable elements.
• pause() temporarily stop tracking elements.
• resume() resume tracking elements.
• destroy() permanently stop tracking elements.

Download

source or minified

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