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Asynchronous boundary detection - lazy-loading, infinite scroll, and more
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bounds.js on NPM

Asynchronous boundary detection. 1KB, no dependencies.



Whether you're lazy-loading images, implementing infinite-scroll, or avoiding an ex-lover... it's important to set boundaries.

Historically, boundary detection required a mix of event handlers, loops, and calls to getBoundingClientRect. Since these operations run on the main thread, performance would suffer.

Bounds.js defies these expectations, providing a simple and powerful API. It detects intersections between elements asynchronously, keeping complex operations off the main thread and improving performance.


Bounds.js was developed with a modern JavaScript workflow in mind. To use it, it's recommended you have a build system in place that can transpile ES6 and bundle modules.

Follow these steps to get started:

  1. Install
  2. How to Use
  3. Options
  4. API
  5. Browser Support


Using NPM, install bounds.js, and save it to your package.json dependencies.

$ npm install bounds.js --save

Then import, naming it according to your preference.

import Bound from 'bounds.js'

How to Use

The first step is to create a new boundary using bounds.js. To do so, call it and pass in your desired options. Each option and its default is explained in the options section below.

const boundary = Bound()   // initialize with default options

The second step is to have your new boundary watch for certain elements on you webpage. When these elements intersect with the boundary, a callback is executed. See an example below:

const image = document.querySelector('img')
const whenImageEnters = (ratio) => {}
const whenImageLeaves = (ratio) => {}, whenImageEnters, whenImageLeaves)

Now that we've covered the basics, lets delve into the options and API.


You are not required to pass any options during boundary creation. All options come with sensible defaults, shown below:

// default options
  root: window,       // the top-level document's viewport
  margins: {
    top: 0,
    right: 0,
    bottom: 0,
    left: 0
  threshold: 0.0,
  onEmit: () => {}    // no-op

Explanation of each option follows:


Accepts a DOM node.

The root is the element for which we are creating the boundary. Events will be emitted whenever a watched element enters/exits the root element.


Accepts a mapping, where values are stated in pixels.

You can specify a top, right, bottom, or left margin to add to the root's bounding box. This affects detection, NOT style on the root element. For example:

const boundary = Bound({
  margins: {
    bottom: 100,

The above boundary will fire a callback for any watched element that gets within 100px of its bottom border.


Accepts a number between 0.0 and 1.0.

The ratio of intersecting area required before a callback is made. A threshold of 0.0 means that if even a single pixel of a watched element enters the boundary, a callback is made. A threshold of 1.0 means that every pixel of a watched element must be inside the boundary before a callback is made.


Accepts a function or anonymous function.

The provided callback will be executed whenever any watched element enters or exits the boundary, once all individual callbacks have executed. This is a useful option if you'd like some action to take place no matter what element enters/exits your boundary. Here is an example of how it can be used:

const boundary = Bound({
  onEmit: (actions) => {
    if (actions.some(action => action.inside)) {
      console.log('At least one element is inside my boundary')

As seen above, the onEmit callback will be passed an argument actions, which is an array of objects representing the actions taken directly beforehand. Each object in actions has the following detail:

  el,       // DOM node
  inside,   // boolean
  outside,  // boolean
  ratio     // floating number


When you create a new boundary with bounds.js, an object with a set of methods will be returned. Those methods are:

Additionally, the bounds.js import object has a static property:

watch(el [, onEnter, onLeave])

Calling watch will instruct your boundary to watch the desired element. When the specified element enters your boundary, the onEnter callback will be executed. When the specified element leaves your boundary, the onLeave callback will be executed.

Each callback is passed 1 argument, ratio, which represents the ratio of the element's bounding box that is inside the boundary.

const boundary = Bound()

const img = document.querySelector('img')
const onImgEnter = (ratio) => {}
const onImgLeave = (ratio) => {}, onImgEnter, onImgLeave)

The watch method will return a data object with the assigned properties. These properties can be mutated. For example, assuming we had the same methods as above:

// you can choose to not initially provide callbacks
const imgOptions =

// later, you can assign new callbacks
imgOptions.onEnter = onImgEnter
imgOptions.onLeave = onImgLeave


The unWatch method will instruct your boundary to stop watching a certain element. Callbacks for that element will no longer be executed. The boundary instance will be returned.


The check method will return a boolean, indicating if the provided element is currently inside the boundary. The check is based on history, which starts once you watch the element. If the element is not currently being watched, check will return undefined.


The clear method will effectively unWatch all elements for the boundary, destroy all history for the elements the boundary was watching, and ensure that no events are emitted by the boundary going forward.


The static checkCompatibility method will throw an error if Bounds.js is not supported in the user's browser.

Browser Support

Bounds.js depends on the following browser APIs:

Consequently, it supports the following natively:

  • Chrome 51+
  • Firefox 55+
  • Safari 12.1+
  • Edge 15+
  • iOS Safari 12.2+
  • Chrome for Android 51+
  • Opera - Supported
  • IE - No Support

For browsers that do not currently support IntersectionObserver, consider a popular polyfill that has great browser support.


MIT. © 2019 Christopher Cavalea

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