How to create an image on the fly with Node.js?

Question

I'm using Node.js and Express web framework, I need to create an on-the-fly image .png or .jpg (like captcha), then I have to send that image to the browser.

What is the simplest way to do that? In the image I should write letters/numbers (max 5).

P.S. I don't know what library to use that connects to an online service as the recaptcha module does.

Answer 1

Maybe you could use canvas? There is also an implementation in node.js by Learnboost(TJ). I think this screencast is interesting to look at. As you see from presentation it even renders text in some examples. Also in the npm registry / node modules section I found a lot more interesting links

Answer 2

Here's some simple code using the canvas library:

const
    fs = require("fs"),
    { createCanvas } = require("canvas");

const WIDTH = 100;
const HEIGHT = 50;

const canvas = createCanvas(WIDTH, HEIGHT);
const ctx = canvas.getContext("2d");

ctx.fillStyle = "#222222";
ctx.fillRect(0, 0, WIDTH, HEIGHT);
ctx.fillStyle = "#f2f2f2";
ctx.font = "32px Arial";
ctx.fillText("Hello", 13, 35);

const buffer = canvas.toBuffer("image/png");
fs.writeFileSync("test.png", buffer);

This is the resulting test.png file:

To run it, you must first install the library:

npm i canvas

Instead of saving it to a file, you could, of course, send it as the response of an API call.

For more details on how to draw text using the canvas, see this MDN article.