3 Steps To Resize Images In Javascript (Simple Examples)

Welcome to a quick tutorial and example of how to resize images in Javascript. Yes, resizing images is no longer a server-side thing. We can also do it in Javascript with just a few lines of code.

That covers the quick basics, but let us walk through a few actual examples – Read on!

ⓘ 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.

 

 

TLDR – QUICK SLIDES

Fullscreen Mode – Click Here

 

TABLE OF CONTENTS

 

 

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.

 

 

 

EXAMPLE 1) RESIZE IMAGE & SHOW ON PAGE

1A) THE HTML

<!-- (A) HTML -->
<!-- (A1) ORIGINAL IMAGE -->
<h1>Original Image (300 X 277)</h1>
<img src="corgi.png">
 
<!-- (A2) RESIZE USING CANVAS -->
<h1>Resized Image (50%)</h1>
<canvas id="resized"></canvas>

Nothing much here… All we actually need is an empty <canvas> to draw the resized image.

 

1B) THE JAVASCRIPT

// (B1) NEW IMAGE OBJECT & HTML CANVAS
var img = new Image(),
    canvas = document.getElementById("resized"),
    ctx = canvas.getContext("2d");

// (B2) RESIZE ON IMAGE LOAD
img.onload = () => {
  let width = Math.floor(img.naturalWidth * 0.5),
      height = Math.floor(img.naturalHeight * 0.5);
  canvas.width = width;
  canvas.height = height;
  ctx.drawImage(img, 0, 0, width, height);
};

// (B3) GO!
img.src = "corgi.png";

Yep, that’s all, and this should be pretty straightforward.

• (B1) Create a new Image() object, get the HTML canvas.
• (B2) When the image is fully loaded, we calculate the new dimensions and draw the resized version onto the <canvas>.
• (B3) Set the image source, start loading the image.

 

 

1C) THE DEMO

Original Image (300 X 277)

Resized Image (50%)

 

EXAMPLE 2) RESIZE IMAGE & DOWNLOAD

2A) THE HTML

<!-- (A) DOWNLOAD LINK -->
<a id="dlink"></a>

Since we are offering the resized image as a download, all we need is an empty <a> download link.

 

2B) THE JAVASCRIPT

// (B1) NEW IMAGE OBJECT & CANVAS
var img = new Image(),
    canvas = document.createElement("canvas"),
    ctx = canvas.getContext("2d");

// (B2) RESIZE ON IMAGE LOAD
img.onload = () => {
  // RESIZE IMAGE
  let width = Math.floor(img.naturalWidth * 0.5),
      height = Math.floor(img.naturalHeight * 0.5);
  canvas.width = width;
  canvas.height = height;
  ctx.drawImage(img, 0, 0, width, height);

  // SET DOWNLOAD LINK
  let dlink = document.getElementById("dlink");
  dlink.innerHTML = "Download";
  dlink.setAttribute("download", "resized.png");
  dlink.href = canvas.toDataURL("image/png");
};

// (B3) GO!
img.src = "corgi.png";

Look no further, this is essentially the same process as the previous example. The only difference here is that we turn the canvas into a downloadable link – canvas.toDataURL("image/png").

 

 

2C) THE DEMO

 

 

COMPATIBILITY CHECK

• Canvas – CanIUse
• Draw Image – CanIUse
• Arrow Function – CanIUse
• To Data URL – CanIUse
• Natural Width & Height – CanIUse

These examples should work on all modern browsers.

 

LINKS & REFERENCES

• Canvas API – MDN
• Canvas Element – MDN
• Convert Image to Data URI with JavaScript – TutorialPoint
• Download Canvas As Image – Code Boxx

 

TUTORIAL VIDEO

 

INFOGRAPHIC CHEAT SHEET