├── .eslintrc.json ├── .github ├── FUNDING.yml └── workflows │ ├── release.yml │ └── test.yml ├── .gitignore ├── CHANGELOG.md ├── LICENSE ├── README.md ├── examples ├── gluon-lazy-image.js ├── hybrids-lazy-image.js ├── lit-lazy-image.js └── polymer-lazy-image.js ├── index.html ├── lazy-image.js ├── lazy-image.test.js ├── logo.png ├── package-lock.json └── package.json /.eslintrc.json: -------------------------------------------------------------------------------- 1 | { 2 | "extends": ["eslint:recommended"], 3 | "env": { 4 | "browser": true, 5 | "es6": true 6 | }, 7 | "parserOptions": { 8 | "sourceType": "module", 9 | "ecmaVersion": "2018" 10 | }, 11 | "rules": { 12 | "max-len": ["error", { "ignoreComments": true, "ignoreTemplateLiterals": true }] 13 | }, 14 | "overrides": [{ 15 | "files": ["*.test.js"], 16 | "env": { 17 | "browser": true, 18 | "mocha": true 19 | } 20 | }] 21 | } 22 | -------------------------------------------------------------------------------- /.github/FUNDING.yml: -------------------------------------------------------------------------------- 1 | # These are supported funding model platforms 2 | 3 | github: [bennypowers] 4 | patreon: # Replace with a single Patreon username 5 | open_collective: # Replace with a single Open Collective username 6 | ko_fi: # Replace with a single Ko-fi username 7 | tidelift: # Replace with a single Tidelift platform-name/package-name e.g., npm/babel 8 | community_bridge: # Replace with a single Community Bridge project-name e.g., cloud-foundry 9 | custom: # Replace with a single custom sponsorship URL 10 | -------------------------------------------------------------------------------- /.github/workflows/release.yml: -------------------------------------------------------------------------------- 1 | name: Release 2 | on: 3 | push: 4 | branches: 5 | - master 6 | 7 | jobs: 8 | test: 9 | name: Test 10 | runs-on: ubuntu-latest 11 | steps: 12 | - uses: actions/checkout@v3 13 | - uses: actions/setup-node@v3 14 | with: 15 | node-version: 18 16 | cache: npm 17 | - run: npm ci --prefer-offline 18 | - run: npm test 19 | 20 | release: 21 | name: Release 22 | needs: Test 23 | runs-on: ubuntu-latest 24 | steps: 25 | - uses: actions/checkout@v3 26 | - run: npm ci --prefer-offline 27 | - uses: cycjimmy/semantic-release-action@v3 28 | id: semantic # Need an `id` for output variables 29 | with: 30 | branch: master 31 | extra_plugins: | 32 | @semantic-release/changelog 33 | @semantic-release/git 34 | env: 35 | GITHUB_TOKEN: ${{ secrets.GH_TOKEN }} 36 | NPM_TOKEN: ${{ secrets.NPM_TOKEN }} 37 | -------------------------------------------------------------------------------- /.github/workflows/test.yml: -------------------------------------------------------------------------------- 1 | name: Test 2 | on: 3 | pull_request: 4 | branches: 5 | - master 6 | 7 | jobs: 8 | test: 9 | name: Test 10 | runs-on: ubuntu-latest 11 | steps: 12 | - uses: actions/checkout@v3 13 | - uses: actions/setup-node@v3 14 | with: 15 | node-version: 18 16 | - run: npm ci --prefer-offline 17 | - run: npm test 18 | -------------------------------------------------------------------------------- /.gitignore: -------------------------------------------------------------------------------- 1 | node_modules 2 | coverage 3 | -------------------------------------------------------------------------------- /CHANGELOG.md: -------------------------------------------------------------------------------- 1 | # [2.2.0](https://github.com/bennypowers/lazy-image/compare/v2.1.2...v2.2.0) (2020-02-18) 2 | 3 | 4 | ### Features 5 | 6 | * add loadend event when src image finished loading ([4b9bc5d](https://github.com/bennypowers/lazy-image/commit/4b9bc5ddbb96baae3dca7ca80066cff2cdb62243)) 7 | 8 | ## [2.1.2](https://github.com/bennypowers/lazy-image/compare/v2.1.1...v2.1.2) (2020-01-28) 9 | 10 | 11 | ### Bug Fixes 12 | 13 | * bump version ([895bb4c](https://github.com/bennypowers/lazy-image/commit/895bb4c135ec70908c0ecc0846ce52dcad72c865)) 14 | -------------------------------------------------------------------------------- /LICENSE: -------------------------------------------------------------------------------- 1 | GNU GENERAL PUBLIC LICENSE 2 | Version 3, 29 June 2007 3 | 4 | Copyright (C) 2007 Free Software Foundation, Inc. 5 | Everyone is permitted to copy and distribute verbatim copies 6 | of this license document, but changing it is not allowed. 7 | 8 | Preamble 9 | 10 | The GNU General Public License is a free, copyleft license for 11 | software and other kinds of works. 12 | 13 | The licenses for most software and other practical works are designed 14 | to take away your freedom to share and change the works. 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If the Program does not specify a version number of the 576 | GNU General Public License, you may choose any version ever published 577 | by the Free Software Foundation. 578 | 579 | If the Program specifies that a proxy can decide which future 580 | versions of the GNU General Public License can be used, that proxy's 581 | public statement of acceptance of a version permanently authorizes you 582 | to choose that version for the Program. 583 | 584 | Later license versions may give you additional or different 585 | permissions. However, no additional obligations are imposed on any 586 | author or copyright holder as a result of your choosing to follow a 587 | later version. 588 | 589 | 15. Disclaimer of Warranty. 590 | 591 | THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY 592 | APPLICABLE LAW. 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Interpretation of Sections 15 and 16. 613 | 614 | If the disclaimer of warranty and limitation of liability provided 615 | above cannot be given local legal effect according to their terms, 616 | reviewing courts shall apply local law that most closely approximates 617 | an absolute waiver of all civil liability in connection with the 618 | Program, unless a warranty or assumption of liability accompanies a 619 | copy of the Program in return for a fee. 620 | 621 | END OF TERMS AND CONDITIONS 622 | 623 | How to Apply These Terms to Your New Programs 624 | 625 | If you develop a new program, and you want it to be of the greatest 626 | possible use to the public, the best way to achieve this is to make it 627 | free software which everyone can redistribute and change under these terms. 628 | 629 | To do so, attach the following notices to the program. It is safest 630 | to attach them to the start of each source file to most effectively 631 | state the exclusion of warranty; and each file should have at least 632 | the "copyright" line and a pointer to where the full notice is found. 633 | 634 | 635 | Copyright (C) 636 | 637 | This program is free software: you can redistribute it and/or modify 638 | it under the terms of the GNU General Public License as published by 639 | the Free Software Foundation, either version 3 of the License, or 640 | (at your option) any later version. 641 | 642 | This program is distributed in the hope that it will be useful, 643 | but WITHOUT ANY WARRANTY; without even the implied warranty of 644 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 645 | GNU General Public License for more details. 646 | 647 | You should have received a copy of the GNU General Public License 648 | along with this program. If not, see . 649 | 650 | Also add information on how to contact you by electronic and paper mail. 651 | 652 | If the program does terminal interaction, make it output a short 653 | notice like this when it starts in an interactive mode: 654 | 655 | Copyright (C) 656 | This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'. 657 | This is free software, and you are welcome to redistribute it 658 | under certain conditions; type `show c' for details. 659 | 660 | The hypothetical commands `show w' and `show c' should show the appropriate 661 | parts of the General Public License. Of course, your program's commands 662 | might be different; for a GUI interface, you would use an "about box". 663 | 664 | You should also get your employer (if you work as a programmer) or school, 665 | if any, to sign a "copyright disclaimer" for the program, if necessary. 666 | For more information on this, and how to apply and follow the GNU GPL, see 667 | . 668 | 669 | The GNU General Public License does not permit incorporating your program 670 | into proprietary programs. If your program is a subroutine library, you 671 | may consider it more useful to permit linking proprietary applications with 672 | the library. If this is what you want to do, use the GNU Lesser General 673 | Public License instead of this License. But first, please read 674 | . 675 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | [![Published on webcomponents.org](https://img.shields.io/badge/webcomponents.org-published-blue.svg)](https://www.webcomponents.org/element/@power-elements/lazy-image) 2 | [![npm (scoped)](https://img.shields.io/npm/v/@cycle/core.svg)](https://www.npmjs.com/package/@power-elements/lazy-image) 3 | [![Build Status](https://travis-ci.org/bennypowers/lazy-image.svg?branch=master)](https://travis-ci.org/bennypowers/lazy-image) 4 | [![Contact me on Codementor](https://cdn.codementor.io/badges/contact_me_github.svg)](https://www.codementor.io/bennyp?utm_source=github&utm_medium=button&utm_term=bennyp&utm_campaign=github) 5 | 6 | # 7 | 8 | [Lazily load](https://web.dev/lazy-loading-images/) your images! 9 | 10 | ## 🚛 Get it! 11 | ``` 12 | npm i -S @power-elements/lazy-image 13 | ``` 14 | 15 | ## 📦 Load it! 16 | ```html 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 28 | ``` 29 | 30 | ## 💪 Use it! 31 | ```html 32 | 33 | 34 | 35 | ``` 36 | 37 | The optional placeholder could be any element. Inline SVG, Pure CSS graphics, or an `` would work best. 38 | 39 | ## 💄 Style it! 40 | You should give your `` elements some specific dimensions, since it absolutely positions its shadow children. In most cases, you should set the wrapping element as well as the `--lazy-image-` custom properties to the known display dimensions of your image. 41 | 42 | ```html 43 | 54 | 55 | 56 | ``` 57 | 58 | `` exposes a set of custom properties for your customizing delight: 59 | 60 | Property|Purpose|Default 61 | -----|-----|----- 62 | `--lazy-image-width`|Width of the internal image and placeholder elements|100% 63 | `--lazy-image-height`|Height of the internal image and placeholder elements|100% 64 | `--lazy-image-fit`|[`object-fit`](https://developer.mozilla.org/en-US/docs/Web/CSS/object-fit) property of the internal image and placeholder elements|contain 65 | `--lazy-image-fade-duration`|Duration of the fade in from the placeholder to the image. Set to 0 to disable fading.|0.3s 66 | `--lazy-image-fade-easing`|[`ease`](https://developer.mozilla.org/en-US/docs/Web/CSS/transition-timing-function) property of the opacity transition for the image and placeholder|ease 67 | 68 | ### Browser support 69 | `lazy-image` manages the loading of your images via an Intersection Observer. In browsers where an Intersection Observer is not present, your images will be loaded immediately much like standard `` elements. Conditionally delivering the [IntersectionObserver polyfill](https://github.com/w3c/IntersectionObserver/tree/master/polyfill) along with your `lazy-image`s to your users will ensure that all users experience the benefits of loading images lazily. Stay lazy, friend! 70 | -------------------------------------------------------------------------------- /examples/gluon-lazy-image.js: -------------------------------------------------------------------------------- 1 | import { GluonElement, html } from 'https://unpkg.com/@gluon/gluon/gluon.js?module'; 2 | 3 | const isIntersecting = ({ isIntersecting }) => isIntersecting; 4 | 5 | class GluonLazyImage extends GluonElement { 6 | static get is() { 7 | return 'gluon-lazy-image'; 8 | } 9 | 10 | get template() { 11 | return html` 12 | 41 | 42 |
43 | 44 |
45 | 46 | ${this.alt} 52 | `; 53 | } 54 | 55 | static get observedAttributes() { 56 | return ['alt', 'src']; 57 | } 58 | 59 | /** 60 | * Implement the vanilla `attributeChangedCallback` 61 | * to observe and sync attributes. 62 | */ 63 | attributeChangedCallback(name, _oldVal, newVal) { 64 | switch (name) { 65 | case 'alt': return this.alt = newVal 66 | case 'src': return this.src = newVal 67 | } 68 | } 69 | 70 | /** 71 | * Whether the element is on screen. 72 | * Note how we coerce the value, 73 | * this is how you do typed properties with Gluon. 74 | * @type {Boolean} 75 | */ 76 | get intersecting() { 77 | return !!this.__intersecting; 78 | } 79 | 80 | set intersecting(value) { 81 | this.__intersecting = !!value; 82 | this.render(); 83 | } 84 | 85 | /** 86 | * Image alt-text. 87 | * @type {String} 88 | */ 89 | get alt() { 90 | return this.getAttribute('alt'); 91 | } 92 | 93 | set alt(value) { 94 | if (this.alt != value) this.setAttribute('alt', value); 95 | this.render(); 96 | } 97 | 98 | /** 99 | * Image URI. 100 | * @type {String} 101 | */ 102 | get src() { 103 | return this.getAttribute('src'); 104 | } 105 | 106 | set src(value) { 107 | if (this.src != value) this.setAttribute('src', value); 108 | this.render(); 109 | } 110 | 111 | /** 112 | * Whether the image has loaded. 113 | * @type {Boolean} 114 | */ 115 | get loaded() { 116 | return this.hasAttribute('loaded'); 117 | } 118 | 119 | set loaded(value) { 120 | value ? this.setAttribute('loaded', '') : this.removeAttribute('loaded'); 121 | this.render(); 122 | } 123 | 124 | constructor() { 125 | super(); 126 | this.observerCallback = this.observerCallback.bind(this); 127 | this.intersecting = false; 128 | this.loading = false; 129 | } 130 | 131 | connectedCallback() { 132 | super.connectedCallback(); 133 | this.setAttribute('role', 'presentation'); 134 | this.initIntersectionObserver(); 135 | } 136 | 137 | disconnectedCallback() { 138 | super.disconnectedCallback(); 139 | this.disconnectObserver(); 140 | } 141 | 142 | /** 143 | * Sets the `intersecting` property when the element is on screen. 144 | * @param {[IntersectionObserverEntry]} entries 145 | * @protected 146 | */ 147 | observerCallback(entries) { 148 | if (entries.some(isIntersecting)) this.intersecting = true; 149 | } 150 | 151 | /** 152 | * Sets the `loaded` property when the image is finished loading. 153 | * @protected 154 | */ 155 | onLoad() { 156 | this.loaded = true; 157 | // Dispatch an event that supports Polymer two-way binding. 158 | this.dispatchEvent(new CustomEvent('loaded-changed', { 159 | bubbles: true, 160 | composed: true, 161 | detail: { value: true }, 162 | })); 163 | } 164 | 165 | /** 166 | * Initializes the IntersectionObserver when the element instantiates. 167 | * @protected 168 | */ 169 | initIntersectionObserver() { 170 | // if IntersectionObserver is unavailable, simply load the image. 171 | if (!('IntersectionObserver' in window)) return this.intersecting = true; 172 | // Short-circuit if observer has already initialized. 173 | if (this.observer) return; 174 | // Start loading the image 10px before it appears on screen 175 | const rootMargin = '10px'; 176 | this.observer = 177 | new IntersectionObserver(this.observerCallback, { rootMargin }); 178 | this.observer.observe(this); 179 | } 180 | 181 | /** 182 | * Disconnects and unloads the IntersectionObserver. 183 | * @protected 184 | */ 185 | disconnectObserver() { 186 | this.observer.disconnect(); 187 | this.observer = null; 188 | delete this.observer; 189 | } 190 | } 191 | 192 | 193 | customElements.define(GluonLazyImage.is, GluonLazyImage); 194 | -------------------------------------------------------------------------------- /examples/hybrids-lazy-image.js: -------------------------------------------------------------------------------- 1 | import { html, dispatch } from 'hybrids'; 2 | 3 | const style = html` 4 | 34 | `; 35 | 36 | const constant = x => () => x; 37 | const passThroughSetter = (_, v) => v; 38 | const isIntersecting = ({ isIntersecting }) => isIntersecting; 39 | const intersect = (options) => { 40 | if (!('IntersectionObserver' in window)) return constant(true); 41 | return { 42 | connect: (host, propName) => { 43 | const observerCallback = entries => 44 | (host[propName] = entries.some(isIntersecting)); 45 | const observer = new IntersectionObserver(observerCallback, options); 46 | const disconnect = () => observer.disconnect(); 47 | observer.observe(host); 48 | return disconnect; 49 | } 50 | } 51 | } 52 | 53 | const bubbles = true; 54 | const composed = true; 55 | const detail = { value: true }; 56 | const onLoad = host => { 57 | host.loaded = true; 58 | // Dispatch an event that supports Polymer two-way binding. 59 | dispatch(host, 'loaded-changed', { bubbles, composed, detail }) 60 | }; 61 | 62 | const render = ({ alt, src, intersecting, loaded }) => html` 63 | ${style} 64 |
67 | 68 |
69 | 70 | ${alt} 77 | `; 78 | 79 | export const LazyImage = { 80 | src: '', 81 | alt: '', 82 | loaded: false, 83 | intersecting: intersect({ rootMargin: '10px' }), 84 | render, 85 | } 86 | -------------------------------------------------------------------------------- /examples/lit-lazy-image.js: -------------------------------------------------------------------------------- 1 | import { LitElement, css, html } from 'lit'; 2 | 3 | const isIntersecting = ({ isIntersecting }) => isIntersecting; 4 | 5 | class LitLazyImage extends LitElement { 6 | static get is() { 7 | return 'lit-lazy-image'; 8 | } 9 | 10 | static get properties() { 11 | return { 12 | alt: { type: String }, 13 | intersecting: { type: Boolean }, 14 | src: { type: String }, 15 | loaded: { type: Boolean, reflect: true }, 16 | } 17 | } 18 | 19 | static get styles() { 20 | return css` 21 | :host { 22 | position: relative; 23 | } 24 | 25 | #image, 26 | #placeholder ::slotted(*) { 27 | position: absolute; 28 | top: 0; 29 | left: 0; 30 | transition: 31 | opacity 32 | var(--lazy-image-fade-duration, 0.3s) 33 | var(--lazy-image-fade-easing, ease); 34 | object-fit: var(--lazy-image-fit, contain); 35 | width: var(--lazy-image-width, 100%); 36 | height: var(--lazy-image-height, 100%); 37 | } 38 | 39 | #placeholder ::slotted(*), 40 | :host([loaded]) #image { 41 | opacity: 1; 42 | } 43 | 44 | #image, 45 | :host([loaded]) #placeholder ::slotted(*) { 46 | opacity: 0; 47 | } 48 | `; 49 | } 50 | 51 | render() { 52 | return html` 53 |
54 | 55 |
56 | 57 | ${this.alt} 62 | `; 63 | } 64 | 65 | constructor() { 66 | super(); 67 | /** @type{IntersectionObserver} */ 68 | this.#io; 69 | /** Image URI. */ 70 | this.src = ''; 71 | /** Image alt-text. */ 72 | this.alt = ''; 73 | /** Whether the element is on screen. */ 74 | this.intersecting = false; 75 | /** Whether the image has loaded. */ 76 | this.loading = false; 77 | } 78 | 79 | connectedCallback() { 80 | super.connectedCallback(); 81 | // Remove the wrapping `` element from the a11y tree. 82 | this.setAttribute('role', 'presentation'); 83 | // if IntersectionObserver is available, initialize it. 84 | this.#initIO(); 85 | } 86 | 87 | /** Disconnects and unloads the IntersectionObserver. */ 88 | disconnectedCallback() { 89 | super.disconnectedCallback(); 90 | this.#io.disconnect(); 91 | this.#io = null; 92 | delete this.#io; 93 | } 94 | 95 | /** 96 | * Sets the `intersecting` property when the element is on screen. 97 | * @param {IntersectionObserverEntry[]} entries 98 | */ 99 | #observerCallback(entries) { 100 | if (entries.some(isIntersecting)) this.intersecting = true; 101 | } 102 | 103 | /** Sets the `loaded` property when the image is finished loading. */ 104 | #onLoad(event) { 105 | this.loaded = true; 106 | // Dispatch and event that supports Polymer two-way binding. 107 | const bubbles = true; 108 | const composed = true; 109 | const detail = { value: true }; 110 | this.dispatchEvent(new CustomEvent('loaded-changed', { 111 | bubbles, 112 | composed, 113 | detail 114 | })); 115 | } 116 | 117 | /** Initializes the IntersectionObserver when the element instantiates. */ 118 | #initIO() { 119 | // if IntersectionObserver is unavailable, simply load the image. 120 | if (!('IntersectionObserver' in window)) return this.intersecting = true; 121 | // Short-circuit if observer has already initialized. 122 | if (this.#io) return; 123 | // Start loading the image 10px before it appears on screen 124 | const rootMargin = '10px'; 125 | this.#io = 126 | new IntersectionObserver(x => this.#observerCallback(x), { rootMargin }); 127 | this.#io.observe(this); 128 | } 129 | } 130 | 131 | customElements.define(LitLazyImage.is, LitLazyImage); 132 | -------------------------------------------------------------------------------- /examples/polymer-lazy-image.js: -------------------------------------------------------------------------------- 1 | import { PolymerElement, html } from '@polymer/polymer'; 2 | 3 | const isIntersecting = ({ isIntersecting }) => isIntersecting; 4 | 5 | class PolymerLazyImage extends PolymerElement { 6 | static get template() { 7 | return html` 8 | 37 | 38 |
39 | 40 |
41 | 42 | 48 | `; 49 | } 50 | 51 | static get is() { 52 | return 'polymer-lazy-image'; 53 | } 54 | 55 | static get properties() { 56 | return { 57 | /** Image alt-text. */ 58 | alt: String, 59 | 60 | /** Whether the element is on screen. */ 61 | intersecting: Boolean, 62 | 63 | /** Image URI. */ 64 | src: String, 65 | 66 | /** 67 | * Whether the image has loaded. 68 | * @type {Boolean} 69 | */ 70 | loaded: { 71 | type: Boolean, 72 | reflectToAttribute: true, 73 | value: false, 74 | }, 75 | }; 76 | } 77 | 78 | constructor() { 79 | super(); 80 | this.observerCallback = this.observerCallback.bind(this); 81 | } 82 | 83 | connectedCallback() { 84 | super.connectedCallback(); 85 | // Remove the wrapping `` element from the a11y tree. 86 | this.setAttribute('role', 'presentation'); 87 | // if IntersectionObserver is available, initialize it. 88 | if ('IntersectionObserver' in window) this.initIntersectionObserver(); 89 | // if IntersectionObserver is unavailable, simply load the image. 90 | else this.intersecting = true; 91 | } 92 | 93 | disconnectedCallback() { 94 | super.disconnectedCallback(); 95 | this.disconnectObserver(); 96 | } 97 | 98 | /** 99 | * Loads the img when IntersectionObserver fires. 100 | * @param {Boolean} intersecting 101 | * @param {String} src 102 | * @return {String} 103 | */ 104 | computeSrc(intersecting, src) { 105 | return intersecting ? src : undefined; 106 | } 107 | 108 | /** 109 | * "true" when intersecting, "false" otherwise. 110 | * @protected 111 | */ 112 | computePlaceholderAriaHidden(intersecting) { 113 | return String(intersecting); 114 | } 115 | 116 | /** 117 | * "false" when intersecting, "true" otherwise. 118 | * @protected 119 | */ 120 | computeImageAriaHidden(intersecting) { 121 | return String(!intersecting); 122 | } 123 | 124 | /** 125 | * Sets the `loaded` property when the image is finished loading. 126 | * @protected 127 | */ 128 | onLoad() { 129 | this.loaded = true; 130 | } 131 | 132 | /** 133 | * Sets the `intersecting` property when the element is on screen. 134 | * @param {[IntersectionObserverEntry]} entries 135 | * @protected 136 | */ 137 | observerCallback(entries) { 138 | if (entries.some(isIntersecting)) this.intersecting = true; 139 | } 140 | 141 | /** 142 | * Initializes the IntersectionObserver when the element instantiates. 143 | * @protected 144 | */ 145 | initIntersectionObserver() { 146 | if (this.observer) return; 147 | // Start loading the image 10px before it appears on screen 148 | const rootMargin = '10px'; 149 | this.observer = 150 | new IntersectionObserver(this.observerCallback, { rootMargin }); 151 | this.observer.observe(this); 152 | } 153 | 154 | /** 155 | * Disconnects and unloads the IntersectionObserver. 156 | * @protected 157 | */ 158 | disconnectObserver() { 159 | this.observer.disconnect(); 160 | this.observer = null; 161 | delete this.observer; 162 | } 163 | } 164 | 165 | customElements.define(PolymerLazyImage.is, PolymerLazyImage); 166 | -------------------------------------------------------------------------------- /index.html: -------------------------------------------------------------------------------- 1 | 2 | 3 | 4 | 5 | Lazy-image 6 | 7 | 8 | 9 | 17 | 18 | 19 | 20 |

<lazy-image>

21 |

I'm a tall element! Scroll down to see the image fade in

22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | -------------------------------------------------------------------------------- /lazy-image.js: -------------------------------------------------------------------------------- 1 | const isIntersecting = ({isIntersecting}) => isIntersecting; 2 | 3 | const tagName = 'lazy-image'; 4 | const template = document.createElement('template'); 5 | template.innerHTML = ` 6 | 36 |
37 | 38 |
39 | 40 | `; 41 | 42 | /* istanbul ignore next */ 43 | if (window.ShadyCSS) window.ShadyCSS.prepareTemplate(template, tagName); 44 | 45 | class LazyImage extends HTMLElement { 46 | /** 47 | * Guards against loops when reflecting observed attributes. 48 | * @param {String} name Attribute name 49 | * @param {any} value 50 | * @protected 51 | */ 52 | safeSetAttribute(name, value) { 53 | if (this.getAttribute(name) !== value) this.setAttribute(name, value); 54 | } 55 | 56 | static get observedAttributes() { 57 | return ['src', 'alt']; 58 | } 59 | 60 | /** 61 | * Image URI. 62 | * @type {String} 63 | */ 64 | set src(value) { 65 | this.safeSetAttribute('src', value); 66 | if (this.intersecting) this.loadImage(); 67 | } 68 | 69 | get src() { 70 | return this.getAttribute('src'); 71 | } 72 | 73 | /** 74 | * Image alt-text. 75 | * @type {String} 76 | */ 77 | set alt(value) { 78 | this.safeSetAttribute('alt', value); 79 | this.shadowImage.alt = value; 80 | } 81 | 82 | get alt() { 83 | return this.getAttribute('alt'); 84 | } 85 | 86 | /** 87 | * Whether the element is on screen. 88 | * @type {Boolean} 89 | */ 90 | get intersecting() { 91 | return this.hasAttribute('intersecting'); 92 | } 93 | 94 | set intersecting(v) {} 95 | 96 | constructor() { 97 | super(); 98 | this.observerCallback = this.observerCallback.bind(this); 99 | this.loadImage = this.loadImage.bind(this); 100 | this.onLoad = this.onLoad.bind(this); 101 | this.onError = this.onError.bind(this); 102 | this.attachShadow({mode: 'open'}); 103 | this.shadowRoot.appendChild(template.content.cloneNode(true)); 104 | this.shadowImage = this.shadowRoot.getElementById('image'); 105 | this.shadowImage.onload = this.onLoad; 106 | this.shadowImage.onerror = this.onError; 107 | this.shadowPlaceholder = this.shadowRoot.getElementById('placeholder'); 108 | } 109 | 110 | connectedCallback() { 111 | this.setAttribute('role', 'presentation'); 112 | this.src = this.getAttribute('src'); 113 | this.alt = this.getAttribute('alt'); 114 | this.placeholder = this.getAttribute('placeholder'); 115 | this.updateShadyStyles(); 116 | if ('IntersectionObserver' in window) this.initIntersectionObserver(); 117 | else this.loadImage(); 118 | } 119 | 120 | attributeChangedCallback(name, oldVal, newVal) { 121 | this[name] = newVal; 122 | } 123 | 124 | disconnectedCallback() { 125 | this.disconnectObserver(); 126 | } 127 | 128 | /** 129 | * Sets the intersecting attribute and reload styles if the polyfill is at play. 130 | */ 131 | loadImage() { 132 | this.setAttribute('intersecting', ''); 133 | this.disconnectObserver(); 134 | this.shadowImage.src = this.src; 135 | } 136 | 137 | onLoad(event) { 138 | this.dispatchEvent(new CustomEvent('loadend', {detail: {success: true}})); 139 | this.shadowImage.removeAttribute('aria-hidden'); 140 | this.shadowPlaceholder.setAttribute('aria-hidden', 'true'); 141 | this.disconnectObserver(); 142 | this.updateShadyStyles(); 143 | } 144 | 145 | onError(event) { 146 | this.dispatchEvent(new CustomEvent('loadend', {detail: {success: false}})); 147 | } 148 | 149 | /** 150 | * When the polyfill is at play, ensure that styles are updated. 151 | * @protected 152 | */ 153 | updateShadyStyles() { 154 | /* istanbul ignore next */ 155 | if(window.ShadyCSS) window.ShadyCSS.styleElement(this); 156 | } 157 | 158 | /** 159 | * Sets the `intersecting` property when the element is on screen. 160 | * @param {[IntersectionObserverEntry]} entries 161 | * @protected 162 | */ 163 | observerCallback(entries) { 164 | if (entries.some(isIntersecting)) this.loadImage(); 165 | } 166 | 167 | /** 168 | * Initializes the IntersectionObserver when the element instantiates. 169 | * @protected 170 | */ 171 | initIntersectionObserver() { 172 | /* istanbul ignore if */ 173 | if (this.observer) return; 174 | const rootMargin = '10px'; 175 | this.observer = 176 | new IntersectionObserver(this.observerCallback, { rootMargin }); 177 | this.observer.observe(this); 178 | } 179 | 180 | 181 | /** 182 | * Disconnects and unloads the IntersectionObserver. 183 | * @protected 184 | */ 185 | disconnectObserver() { 186 | if (!this.observer) return; 187 | this.observer.disconnect(); 188 | this.observer = null; 189 | delete this.observer; 190 | } 191 | } 192 | 193 | customElements.define(tagName, LazyImage); 194 | -------------------------------------------------------------------------------- /lazy-image.test.js: -------------------------------------------------------------------------------- 1 | import './lazy-image.js' 2 | 3 | import { expect, fixture, aTimeout } from '@open-wc/testing'; 4 | 5 | // eslint-disable-next-line max-len 6 | const src = 'data:image/gif;base64,R0lGODlhEAAQAMQAAORHHOVSKudfOulrSOp3WOyDZu6QdvCchPGolfO0o/XBs/fNwfjZ0frl3/zy7////wAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACH5BAkAABAALAAAAAAQABAAAAVVICSOZGlCQAosJ6mu7fiyZeKqNKToQGDsM8hBADgUXoGAiqhSvp5QAnQKGIgUhwFUYLCVDFCrKUE1lBavAViFIDlTImbKC5Gm2hB0SlBCBMQiB0UjIQA7'; 7 | 8 | let element; 9 | 10 | afterEach(function() { 11 | element = undefined; 12 | }) 13 | 14 | describe('', function() { 15 | beforeEach(async function() { 16 | element = await fixture(''); 17 | }) 18 | it('has an img element in shadow root', function() { 19 | expect(element.shadowImage).to.be.an.instanceof(HTMLImageElement); 20 | }); 21 | it('reflects src property', function() { 22 | element.src = 'foo' 23 | expect(element.getAttribute("src")).to.equal('foo') 24 | }); 25 | it('has read only intercting prop', function() { 26 | const init = element.intersecting; 27 | element.intersecting = Math.random(); 28 | expect(element.intersecting).to.equal(init); 29 | }) 30 | }); 31 | 32 | describe('', function() { 33 | beforeEach(async function() { 34 | element = await fixture(''); 35 | }) 36 | it('returns alt dom property', function() { 37 | expect(element.alt).to.equal('alt') 38 | }); 39 | }); 40 | 41 | describe('', function() { 42 | if ("IntersectionObserver" in window) { 43 | describe('"IntersectionObserver" supported', function() { 44 | beforeEach(async function() { element = await fixture(``)}); 45 | it('initializes an IntersectionObserver', function() { 46 | expect(element.observer).to.be.an.instanceof(IntersectionObserver); 47 | }); 48 | 49 | it('does not set img src', function() { 50 | expect(element.shadowImage.src).to.not.be.ok; 51 | }); 52 | 53 | it('does not set intersecting attr', async function() { 54 | expect(element.hasAttribute('intersecting')).to.be.false; 55 | }); 56 | 57 | describe("when image scrolls into view", function() { 58 | beforeEach(async function() { 59 | element.style.left = '100px'; 60 | await aTimeout(100); 61 | }); 62 | 63 | it('Loads image', async function() { 64 | expect(element.shadowImage.src).to.equal(src); 65 | }); 66 | 67 | it('sets intersecting attr', async function() { 68 | expect(element.hasAttribute('intersecting')).to.be.true; 69 | }); 70 | }) 71 | }); 72 | } else { 73 | describe('"IntersectionObserver" not supported', function() { 74 | beforeEach(async function() { element = await fixture(``)}); 75 | it('sets img src immediately', function() { 76 | expect(element.shadowImage.src).to.equal(src); 77 | }); 78 | }); 79 | } 80 | }); 81 | -------------------------------------------------------------------------------- /logo.png: -------------------------------------------------------------------------------- https://raw.githubusercontent.com/bennypowers/lazy-image/6cd7b09774508b068e2a06917c294e17a92ec031/logo.png -------------------------------------------------------------------------------- /package.json: -------------------------------------------------------------------------------- 1 | { 2 | "name": "@power-elements/lazy-image", 3 | "version": "2.2.0", 4 | "description": "Lazy loads images with IntersectionObserver", 5 | "main": "lazy-image.js", 6 | "module": "lazy-image.js", 7 | "scripts": { 8 | "test": "wtr --coverage --playwright --node-resolve --files lazy-image.test.js" 9 | }, 10 | "repository": { 11 | "type": "git", 12 | "url": "git+https://github.com/bennypowers/lazy-image.git" 13 | }, 14 | "logo": { 15 | "file": "./logo.png" 16 | }, 17 | "keywords": [ 18 | "custom element", 19 | "lazy loading", 20 | "performance" 21 | ], 22 | "author": "Benny Powers", 23 | "license": "ISC", 24 | "bugs": { 25 | "url": "https://github.com/bennypowers/lazy-image/issues" 26 | }, 27 | "homepage": "https://github.com/bennypowers/lazy-image#readme", 28 | "devDependencies": { 29 | "@open-wc/testing": "^3.1.7", 30 | "@web/test-runner": "^0.15.0", 31 | "@web/test-runner-playwright": "^0.9.0", 32 | "lit-html": "^2.5.0", 33 | "sinon": "^15.0.1" 34 | }, 35 | "files": [ 36 | "README.md", 37 | "CHANGELOG.md", 38 | "lazy-image.js", 39 | "index.html", 40 | "LICENSE", 41 | "examples/*" 42 | ], 43 | "release": { 44 | "branches": [ 45 | "master" 46 | ], 47 | "plugins": [ 48 | "@semantic-release/commit-analyzer", 49 | "@semantic-release/github", 50 | "@semantic-release/npm", 51 | "@semantic-release/release-notes-generator", 52 | "@semantic-release/changelog", 53 | "@semantic-release/git" 54 | ] 55 | } 56 | } 57 | --------------------------------------------------------------------------------