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Vanilla Captcha

A highly customizable and easy to use Captcha library. Based on my sveltekit component, decided to make a more generic implementation for all frameworks in vanilla JavaScript Supports running in the server as well as in the browser Server usage uses sharp and svg-builder and Browser usage uses the canvas html element


npm install vanilla-captcha #or bun, yarn, pnpm, etc


Be aware that the server usage can't be used in client side and vice versa, you'll get errors like document is not defined or some sharp nasty errors


//in the server
import generate from "vanilla-captcha";
const getCaptcha = async () => {
  const charAmmount = 5;
  const { answer, captcha } = await generate(charAmmount);

answer is the string and captcha is a Buffer, so you can do whatever you want with it, like returning it in a response as is

return new Response(captcha, { status: 200, headers: { "X-answer": answer } });

then you can use the response.blob() method and URL.createObjectURL()

fetch("/get-captcha-url", {
  method: "GET",
  .then((response) => response.blob())
  .then((blob) => {
    const img = document.createElement("img");
    img.src = URL.createObjectURL(blob);


Browser usage is more straightforward

import generateSync from "vanilla-captcha/dom";
const charAmount = 5;
const { answer, captcha } = generateSync(charAmount);
const img = document.createElement("img");
img.src = captcha; //notice that there is no need to URL.createObjectURL(), because is the canvas.toDataURL() string

Notice the difference in the import statement, browser image generation logic is separated from the server one, be sure to import what you need where you need it. And yes the browser image generation version is not async since it does not use sharp, nor Buffer.


the generate methods (generate and generateSync) can recieve a series of options to customize the image


pretty self-explained options

interface CaptchaSyncOptions {
  width: number; // width of the generated image
  height: number; // height of the generated image
  backgroundColor: string;
  font: string; // font of the generated image as "Bold 30px Arial"
  fontColor: string;
  lineAmount: number; //amount of strikes over the image
  lineColor: string;
  lineWidth: number;

default values for the options parameter in the generateSync method:

width: 345,
height: 96,
backgroundColor: "#FFF",
font: "Bold 30px Arial",
fontColor: "#777",
lineColor: "#777",
lineAmount: 10,
lineWidth: 1


export interface CaptchaOptions {
  width: number;
  height: number;
  backgroundColor: string;
  font: string;
  fontColor: string;
  fontSize: number;
  fontWeight: number;
  lineColor: string;
  lineAmount: number;
  lineWidth: number;

notice this one has to extra properties for fontSize and fontWeight, in this case the font property is only the font-family property like "Ubuntu" for example default values for the options parameter in the generate method:

width: 345,
height: 96,
backgroundColor: "#FFF",
font: "Arial",
fontSize: 30,
fontWeight: 600,
fontColor: "#777",
lineColor: "#777",
lineAmount: 10,
lineWidth: 1

Other utilities

you can also use the image generation method if you want to do it with a string of your own instead of the random string generated by vanilla-captcha like so:

import { generateImage } from "vanilla-captcha";
const const answer = "uRock"
const myCaptcha = generateImage(answer)
const myCaptcha2 = generateImage(answer, { backgroundColor: "#000", fontColor: "#FFF" })
//this generates the buffers for 2 images with the same answer

or in the browser

import { generateImageSync } from "vanilla-captcha/dom";

There is also a validation method for the lazzy ones

import { validate } from "vanilla-captcha/utils";
const userInput = "urock";
const answer = "uRock";
const valid = validate(userInput, answer, { caseSensitive: false });
console.log(valid); //true

method uses caseSensitive by default so you don't need to to pass it

const valid = validate(userInput, answer);
console.log(valid); //false


demo for tryout at:

demo source code at:


MIT licensed

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npm i vanilla-captcha

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  • carlos.vilaseca