Convert Image File Format In Javascript (JPG, PNG, WEBP, GIF)

Welcome to a quick tutorial and example on how to convert the image file format in Javascript – JPG to WEBP, PNG to WEBP, JPG to PNG, and so on. Yes, you have also read correctly. We can convert an image directly in the web browser without uploading it to the server.

It’s kind of roundabout, so read on for the exact example!

 

 

TABLE OF CONTENTS

 

 

 

 

PART 1) THE HTML

<div id="cDemo">
  <label>SELECT FORMAT</label>
  <select id="cFormat">
    <option value="webp">WEBP</option>
    <option value="jpeg">JPG</option>
    <option value="png">PNG</option>
    <option value="gif">GIF</option>
  </select>
 
  <label>CHOOSE FILE</label>
  <input type="file" id="cFile" accept="image/*">
 
  <label>CANVAS</label>
  <canvas id="cCanvas"></canvas>
</div>

Well, don’t think this needs much explanation. Just a simple “form” with 3 “fields” –

• <select id="cFormat"> Image format selector.
• <input type="file" id="cFile"> File picker.
• <canvas id="cCanvas"> The selected image will be drawn here, then “converted” and “force download”.

 

 

PART 2) JAVASCRIPT – INIT

var convert = {
  // (A) PROPERTIES
  hFile : null,   // html file picker
  hFormat : null, // html format select
  hCanvas : null, // html canvas
  reader : null,  // file reader
  ctx : null,     // canvas context
 
  // (B) INIT
  init : () => {
    // (B1) GET HTML ELEMENTS
    convert.hFile = document.getElementById("cFile");
    convert.hFormat = document.getElementById("cFormat");
    convert.hCanvas = document.getElementById("cCanvas");
    convert.ctx = convert.hCanvas.getContext("2d");
 
    // (B2) LISTEN TO FILE PICKER
    convert.hFile.onchange = convert.read;
  },
  // ...
};
window.addEventListener("load", convert.init);

When the page loads, convert.init() will run. This pretty much gets the image format selector, file picker, canvas, and attach “read image file on pick”.

 

PART 3) JAVASCRIPT – READ, MAP, CONVERT, DOWNLOAD

// (C) READ SELECTED IMAGE
read : () => {
  convert.reader = new FileReader();
  convert.reader.onload = convert.draw;
  convert.reader.readAsDataURL(convert.hFile.files[0]);
},
 
// (D) DRAW SELECTED IMAGE ON CANVAS
draw : () => {
  let img = new Image();
  img.onload = () => {
    convert.hCanvas.width = img.width;
    convert.hCanvas.height = img.height;
    convert.ctx.drawImage(img, 0, 0);
    convert.go();
  };
  img.src = convert.reader.result;
},
 
// (E) CONVERT & "FORCE DOWNLOAD"
go : () => {
  let a = document.createElement("a"),
      f = convert.hFormat.value;
  a.href = convert.hCanvas.toDataURL(`image/${f}`);
  if (f=="jpeg") { f = "jpg"; }
  a.download = `converted.${f}`;
  a.click();
  a.remove();
}

Not going to explain line-by-line, very boring… But once you trace through, this “waterfall process” is long-winded but pretty straightforward.

• convert.read() Will run when the user chooses an image file. We create a FileReader to read the selected file.
• convert.draw() When the FileReader is ready, we create an Image object from the selected file and “map” it onto the HTML canvas.
• convert.go() “Convert” the canvas and “force download”. Please take note that programmatically clicking on an anchor link a.click() may not always work. It may be a better idea to attach the link to a button on the page and let the user manually click on it.

 

 

 

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EXAMPLE CODE DOWNLOAD

Click here for the source code on GitHub gist, just click on “download zip” or do a git clone. I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

 

 

COMPATIBILITY CHECKS

• Arrow Functions – CanIUse
• File Reader – CanIUse
• Canvas – CanIUse

This example works on all modern “Grade A” browsers.

 

LINKS & REFERENCES

• Download HTML Canvas As Image – Code Boxx
• How To Read Files In Javascript – Code Boxx
• Example on CodePen – JS File Image Format Converter