How To Crop Images In Javacsript (Simple Examples)

Welcome to a tutorial on how to crop images in Javascript. Want to lessen the server load by cropping the image on the client-side? Or just curious about how those “online image crop tools” work?

To crop an image in Javascript:

  1. Create an image and canvas.
    • var img = new Image();
    • var canvas = document.createElement("canvas");
    • var ctx = canvas.getContext("2d");
  2. On image load, “crop” by copying a part of the source image onto the canvas – img.onload = () => { ctx.drawImage(img, SX, SY, SWIDTH, SHEIGHT, DX, DY, DWIDTH, DHEIGHT); };
  3. Set the image source to start – img.src = "SOURCE.JPG";

That covers the basics, read on for an actual example!

ⓘ I have included a zip file with all the source code at the start of this tutorial, so you don’t have to copy-paste everything… Or if you just want to dive straight in.

 

 

QUICK SLIDES

 

TABLE OF CONTENTS

Download & Notes Crop Image Useful Bits & Links
The End

 

DOWNLOAD & NOTES

Firstly, here is the download link to the example code as promised.

 

QUICK NOTES

If you spot a bug, feel free to comment below. I try to answer short questions too, but it is one person versus the entire world… If you need answers urgently, please check out my list of websites to get help with programming.

 

EXAMPLE CODE DOWNLOAD

Click here to download all the example source code, I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

 

 

JAVASCRIPT CROP IMAGE

All right, let us now get into the examples of how to crop an image in Javascript.

 

EXAMPLE 1) SIMPLE IMAGE CROP

1-simple.html
<!-- (A) HTML CANVAS -->
<canvas id="demo" width="300" height="300"></canvas>
 
 <script>
// (B) SOURCE IMAGE OBJECT
var img = new Image();
 
// (C) CROP ON LOAD
img.onload = () => {
  // (C1) GET CANVAS + 2D CONTEXT
  let canvas = document.getElementById("demo"),
  ctx = canvas.getContext("2d");
 
  // (C2) CROP BY COPYING PART OF SOURCE IMAGE TO CANVAS
  ctx.drawImage(img,
    // SOURCE X, Y, WIDTH, HEIGHT
    170, 20, 300, 300,
    // DESTINATION X, Y, WIDTH, HEIGHT
    0, 0, 300, 300
  );
};
 
// (D) GO!
img.src = "eggs.jpg";

This is the “full version” of the introduction snippet, I think the confusion mostly lies with the “crop” – We are not actually doing a crop, but copying a part of the source image onto the canvas. Just what are those SX, SY, SWIDTH, SHEIGHT, DX, DY, DWIDTH, DHEIGHT? I will let this explain itself:

 

 

EXAMPLE 2) PICK IMAGE & CROP

2-pick-crop.html
<!-- (A) FILE PICKER -->
<input type="file" id="picker" accept="image/jpeg,image/png,image/webp"/>
 
 <script>
// (B) AUTO START ON SELECT FILE
let picker = document.getElementById("picker");
picker.onchange = () => {
  // (C) CREATE CANVAS
  let canvas = document.createElement("canvas"),
      ctx = canvas.getContext("2d");
  canvas.width = 300;
  canvas.height = 300;

  // (D) CREATE ANCHOR + SELECTED IMAGE URL
  let anchor = document.createElement("a"),
      surl = URL.createObjectURL(picker.files[0]);
 
  // (E) CREATE IMAGE + CROP
  let img = new Image();
  img.onload = () => {
    // (E1) DO YOUR OWN CROP CALCULATIONS...
    ctx.drawImage(img, 170, 20, 300, 300, 0, 0, 300, 300);
 
    // (E2) "FORCE DOWNLOAD"
    anchor.href = canvas.toDataURL("image/png");
    anchor.download = "cropped.png";
    anchor.click(); // MAY NOT WORK - BETTER TO LET USER CLICK
 
    // (E3) CLEAN UP
    anchor.remove();
    canvas.remove();
    img.remove();
    URL.revokeObjectURL(surl);
  };
 
  // (F) GO!
  img.src = surl;
};

This is for you guys who are looking to do the “conventional pick file, crop, and download”. Not going to explain this line-by-line, but it essentially does the same “simple crop” – Except that we have to jump through a lot of hoops to read the selected image file, and to “force a download”.

 

 

USEFUL BITS & LINKS

That’s all for the tutorial, and here is a small section on some extras and links that may be useful to you.

 

COMPATIBILITY CHECKS

The examples should work on all modern browsers.

 

LINKS & REFERENCES

 

INFOGRAPHIC CHEAT SHEET

How To Crop Images In Javascript (Click To Enlarge)

 

THE END

Thank you for reading, and we have come to the end. I hope that it has helped you to better understand, and if you want to share anything with this guide, please feel free to comment below. Good luck and happy coding!

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