Preloading images with JavaScript

Question

Is the function I wrote below enough to preload images in most  if not all  browsers commonly used today   function preloadImage url        var img new Image        img src url      I have an array of image URLs that I loop over and call the preloadImage function for each URL

User · Answer

The browser will work best using the link tag in the head   export function preloadImages  imageSources  string     void     imageSources      forEach i   gt          const linkEl   document createElement  link          linkEl setAttribute  rel    preload          linkEl setAttribute  href   i         linkEl setAttribute  as    image          document head appendChild linkEl

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I can confirm that the approach in the question is sufficient to trigger the images to be downloaded and cached  unless you have forbidden the browser from doing so via your response headers  in  at least    Chrome 74 Safari 12 Firefox 66 Edge 17   To test this  I made a small webapp with several endpoints that each sleep for 10 seconds before serving a picture of a kitten  Then I added two webpages  one of which contained a  lt script gt  tag in which each of the kittens is preloaded using the preloadImage function from the question  and the other of which includes all the kittens on the page using  lt img gt  tags   In all the browsers above  I found that if I visited the preloader page first  waited a while  and then went to the page with the  lt img gt  tags  my kittens rendered instantly  This demonstrates that the preloader successfully loaded the kittens into the cache in all browsers tested   You can see or try out the application I used to test this at https   github com ExplodingCabbage preloadImage-test   Note in particular that this technique works in the browsers above even if the number of images being looped over exceeds the number of parallel requests that the browser is willing to make at a time  contrary to what Robin s answer suggests  The rate at which your images preload will of course be limited by how many parallel requests the browser is willing to send  but it will eventually request each image URL you call preloadImage   on

User · Answer

In my case it was useful to add a callback to your function for onload event   function preloadImage url  callback        var img new Image        img src url      img onload   callback      And then wrap it for case of an array of URLs to images to be preloaded with callback on all is done  https   jsfiddle net 4r0Luoy7   function preloadImages urls  allImagesLoadedCallback       var loadedCounter   0    var toBeLoadedNumber   urls length    urls forEach function url       preloadImage url  function            loadedCounter                console log  Number of loaded images      loadedCounter         if loadedCounter    toBeLoadedNumber           allImagesLoadedCallback                            function preloadImage url  anImageLoadedCallback         var img   new Image          img onload   anImageLoadedCallback        img src   url            Let s call it  preloadImages          upload wikimedia org wikipedia commons d da Internet2 jpg        www csee umbc edu wp-content uploads 2011 08 www jpg     function        console log  All images were loaded

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I recommend you use a try catch to prevent some possible issues   OOP       var preloadImage   function  url            try               var  img   new Image                 img src   url            catch  e              Standard       function preloadImage  url            try               var  img   new Image                 img src   url            catch  e              Also  while I love DOM  old stupid browsers may have problems with you using DOM  so avoid it altogether IMHO contrary to freedev s contribution  Image   has better support in old trash browsers

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This approach is a little more elaborate  Here you store all preloaded images in a container  may be a div  And after you could show the images or move it within the DOM to the correct position   function preloadImg containerId  imgUrl  imageId        var i   document createElement  img       or new Image       i id   imageId      i onload   function              var container   document getElementById containerId            container appendChild this              i src   imgUrl      Try it here  I have also added few comments

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Solution for ECMAScript 2017 compliant browsers  Note  this will also work if you are using a transpiler like Babel    use strict    function imageLoaded src  alt             return new Promise function resolve            const image   document createElement  img             image setAttribute  alt   alt           image setAttribute  src   src            image addEventListener  load   function                 resolve image                          async function runExample         console log  Fetching my cat s image            const myCat   await imageLoaded  https   placekitten com 500         console log  My cat s image is ready  Now is the time to load my dog s image            const myDog   await imageLoaded  https   placedog net 500         console log  Whoa  This is now the time to enable my galery          document body appendChild myCat       document body appendChild myDog      runExample      You could also have waited for all images to load   async function runExample         const  myCat  myDog              await imageLoaded  https   placekitten com 500            await imageLoaded  https   placedog net 500               document body appendChild myCat       document body appendChild myDog       Or use Promise all to load them in parallel   async function runExample         const  myCat  myDog    await Promise all           imageLoaded  https   placekitten com 500            imageLoaded  https   placedog net 500                document body appendChild myCat       document body appendChild myDog       More about Promises   More about  Async  functions   More about the destructuring assignment   More about ECMAScript 2015   More about ECMAScript 2017

User · Answer

Yes  This should work on all major browsers

User · Answer

Working solution as of 2020 Most answers on this post no longer work -  atleast on Firefox  Here s my solution  var cache   document createElement  quot CACHE quot    cache style    quot position absolute z-index -1000 opacity 0  quot   document body appendChild cache   function preloadImage url        var img   new Image        img src   url      img style    quot position absolute quot       cache appendChild img      Usage  preloadImage  quot example com yourimage png quot     Obviously  lt cache gt  is not a  quot defined quot  element  so you could use a  lt div gt  if you wanted to  Use this in your CSS  instead of applying the style attribute  cache       position  absolute      z-index  -1000      opacity  0     cache image       position  absolute     If you have tested this  please leave a comment  Notes   Do NOT apply display  none  to cache - this will not load the image  Don t resize the image element  as this will also affect the quality of the loaded image when you come to use it  Setting position  absolute to the image is necessary  as the image elements will eventually make it s way outside of the viewport - causing them to not load  and affect performance    UPDATE While above solution works  here s a small update I made to structure it nicely   This also now accepts multiple images in one function  var cache   document createElement  quot CACHE quot    document body appendChild cache   function preloadImage         for  var i 0  i lt arguments length  i              var img   new Image            img src   arguments i           var parent   arguments i  split  quot   quot   1      Set to index of folder name         if     cache    parent    length    0                var ele   document createElement  quot DIV quot                ele id   parent              cache appendChild ele                        cache    parent    0  appendChild img           console log parent            preloadImage       quot assets office 58 png quot        quot assets leftbutton 124 png quot        quot assets leftbutton 125 png quot        quot assets leftbutton 130 png quot        quot assets leftbutton 122 png quot        quot assets leftbutton 124 png quot      Preview   Notes   Try not to keep too many images preloaded at the same time  this can cause major performance issues  - I got around this by hiding images  which I knew wasn t going to be visible during certain events  Then  of course  show them again when I needed it

User · Answer

Yes this will work  however browsers will limit between 4-8  the actual calls and thus not cache preload all desired images   A better way to do this is to call onload before using the image like so   function  imageUrls  index          var img   new Image         img onload   function              console log  isCached      isCached imageUrls index              DoSomething         img src   imageUrls index     function isCached imgUrl        var img   new Image        img src   imgUrl      return img complete     img  width   img  height   gt  0

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const preloadImage   src   gt     new Promise r   gt        const image   new Image       image onload   r     image onerror   r     image src   src           Preload an image await preloadImage  https   picsum photos 100 100       Preload a bunch of images in parallel  await Promise all images map x   gt  preloadImage x src

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Try this I think this is better   var images       function preload         for  var i   0  i  lt  arguments length  i              images i    new Image            images i  src   preload arguments i              -- usage --   preload       http   domain tld gallery image-001 jpg        http   domain tld gallery image-002 jpg        http   domain tld gallery image-003 jpg      Source  http   perishablepress com 3-ways-preload-images-css-javascript-ajax

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CSS2 Alternative  http   www thecssninja com css even-better-image-preloading-with-css2  body after     content  url img01 jpg  url img02 jpg  url img03 jpg     display  none       CSS3 Alternative  https   perishablepress com preload-images-css3   H T Linh Dam    preload-images     display  none     width  0    height  0    background  url img01 jpg                 url img02 jpg                 url img03 jpg       NOTE  Images in a container with display none might not preload  Perhaps visibility hidden will work better but I have not tested this  Thanks Marco Del Valle for pointing this out

User · Answer

You can move this code to index html for preload images from any url   lt link rel  preload  href  https   via placeholder com 160  as  image  gt

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Here is my approach   var preloadImages   function  srcs  imgs  callback        var img      var remaining   srcs length      for  var i   0  i  lt  srcs length  i              img   new Image          img onload   function                  --remaining              if  remaining  lt   0                    callback                                     img src   srcs i           imgs push img

[javascript] Preloading images with JavaScript

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