images

A React Image component let you handle image UX and performance

May 19, 2020
A React Image component let you handle image UX and performance

React Cool Img

This is a lightweight React component, which helps you handle image UX (user experience) and performance optimization as a professional guy nerd_face.

It empowers the standard img tag by many cool features without breaking your original development experience. Ideally, it can be an img tag replacement for React.js.

View Demo View Github

Features

Requirement

react-cool-img is based on React Hooks. It requires react v16.8+.

Installation

This package is distributed via npm.

$ yarn add react-cool-img # or $ npm install --save react-cool-img

Quick Start

The default props of the component has been fine-tuned for the purpose of loading optimization. Let's start it as the following example.

import Img from "react-cool-img"; // Suggest to use low quality or vector images import loadingImage from "./images/loading.gif"; import errorImage from "./images/error.svg"; const App = () => ( <Img placeholder={loadingImage} src="https://the-image-url" error={errorImage} alt="React Cool Img" /> );

Don't want an image placeholder? No worries, you can use inline styles or CSS for it. The component is fully compatible with the development experience of normal img tag.

import Img from "react-cool-img"; const App = () => ( <Img style={{ backgroundColor: "grey", width: "480", height: "320" }} src="https://the-image-url" alt="React Cool Img" /> );

API

The image component working similar with standard img tag and with the following props.

PropTypeDefaultDescription
srcstringImage source. It's required.
Support formats
srcSetstringImage sources for responsive images. For src prop only.
Reference article
sizesstringImage sizes for responsive images. For src prop only.
Reference article
widthstringWidth of the image in px.
heightstringHeight of the image in px.
placeholderstringPlaceholder image source.
Support formats
errorstringError image source. It'll replace Placeholder image.
Support formats
altstringAn alternate text for an image section.
decodebooleantrueUse img.decode() to pre-decode the image before render it. Useful to prevent main thread from blocking by decoding of large image.
lazybooleantrueTurn on/off lazy loading.
Using Intersection Observer
cachebooleantrueInstantly load images which have been cached when possible to abort the lazy loading behavior.
Reference article
debouncenumber300How much to wait in milliseconds that the image has to be in viewport before starting to load. This can prevent images from being downloaded while the user scrolls quickly past them.
observerOptionsobject{ root: window, rootMargin: '50px', threshold: 0.01 }See the observerOptions section.
retryobject{ count: 3, delay: 2, acc: '*' }See the retry section.
...Find more props and events.

observerOptions

All the properties are optional.

  • root: Element | null - the element that is used as the viewport for checking visibility of the target. Must be the ancestor of the target. Defaults to the browser viewport if not specified or if null.
  • rootMargin: string - margin around the root. Can have values similar to the CSS margin property, e.g. "10px 20px 30px 40px" (top, right, bottom, left). The values can be percentages. This set of values serves to grow or shrink each side of the root element's bounding box before computing intersections.
  • threshold: number - a single number between 0 and 1, which indicate at what percentage of the target's visibility the observer's callback should be executed. A value of 0 means as soon as even one pixel is visible, the callback will be run. 1 means that the threshold isn't considered passed until every pixel is visible.

retry

All the properties are optional.

  • count: number - specifies the number of times you want to retry. Set it to 0 will disable auto-retry.
  • delay: number - specifies the delay between retries in seconds.
  • acc: string | false - specifies how the delay should be accumulated with each retry. It accepts the following values:
    • '*' (default) - multiply delay after each subsequent retry by the given delay value, e.g. delay: 2 means retry will run after 2 seconds, 4 seconds, 8 seconds, and so on.
    • '+' - increment delay after each retry by the given delay value, e.g. delay: 2 means retry will run after 2 seconds, 4 seconds, 6 seconds, and so on.
    • false - keep the delay constant between retries, e.g. delay: 2 means retry will run every 2 seconds.

The Smart Way to Load Images

Lazy image loading via the Intersection Observer API is good. But could it be greater to download an image only when user want to see it? Or bypass lazy loading for cached images? The answer is yes and these features already be built into react-cool-img by the debounce and cache props.

By the debounce prop, an image can wait to be downloaded while it's in the viewport for a set time. In cases where you have a long list of images that the user might scroll through inadvertently. At this time loading images can cause unnecessary waste of bandwidth and processing time.

import Img from "react-cool-img"; import defaultImg from "./images/default.svg"; const App = () => ( <Img placeholder={defaultImg} src="https://the-image-url" debounce={1000} // Default is 300 (ms) alt="React Cool Img" /> );

By the cache prop, images you already have cached will abort lazy loading until user visit your app next time. Lazy loading is set up for any remaining images which were not cached. This is helpful for UX, because there's not much extra work to load cached images immediately and is an easy win for making the UI looks more intuitive.

import Img from "react-cool-img"; import defaultImg from "./images/default.svg"; const App = () => ( <Img placeholder={defaultImg} src="https://the-image-url" cache // Default is true, just for demo alt="React Cool Img" /> );

JavaScript Availability and SEO

There're two challenges when doing lazy image loading with server-side rendering. One is Javascript availability the other is SEO. Fortunately, we can use <noscript> tag to solve these problems. It will render the actual image as fallback if Javascript is disabled thus user won't see the image which be stuck with the placeholder. Moreover, the <noscript> tag ensure the image is indexed by search engine bots even if they cannot fully understand our JavaScript code. Take a look at how magic happens.

// src/Img.tsx const Img = () => { // ... return ( <> <img class="image" src="https://the-placeholder-image" alt="There's no magic" /> <noscript> <img class="image" src="https://the-actual-image" alt="The magic begins in here..." /> </noscript> </> ); };

Intersection Observer Polyfill

Intersection Observer has good support amongst browsers, but it's not universal. You'll need to polyfill browsers that don't support it. Polyfills is something you should do consciously at the application level. Therefore react-cool-img doesn't include it.

You can use W3C's polyfill:

$ yarn add intersection-observer # or $ npm install --save intersection-observer

Then import it at your app's entry point:

import "intersection-observer";

Or load the polyfill only if needed:

if (!window.IntersectionObserver) require("intersection-observer");

Polyfill.io is an alternative way to add the polyfill when needed.

GitHub

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