How to Build a Signature Pad with JavaScript Canvas

A signature pad is a common UI component for capturing digital signatures in web forms, contracts, and e-signing workflows. Konva makes it easy to build one with smooth line drawing, customizable pen settings, and one-click export to PNG.

Instructions: Draw your signature below. Use the controls to change pen color, clear the pad, or save/export the signature as a PNG image.

Vanilla

import Konva from 'konva';

// --- UI Controls ---
const controls = document.createElement('div');
controls.style.cssText = 'display:flex;gap:8px;align-items:center;margin-bottom:4px;flex-wrap:wrap;';

const colorLabel = document.createElement('label');
colorLabel.textContent = 'Pen Color: ';
const colorInput = document.createElement('input');
colorInput.type = 'color';
colorInput.value = '#000000';
colorLabel.appendChild(colorInput);

const widthLabel = document.createElement('label');
widthLabel.textContent = 'Width: ';
const widthSelect = document.createElement('select');
[2, 3, 4, 5, 6].forEach((w) => {
  const opt = document.createElement('option');
  opt.value = w;
  opt.textContent = w + 'px';
  if (w === 3) opt.selected = true;
  widthSelect.appendChild(opt);
});
widthLabel.appendChild(widthSelect);

const clearBtn = document.createElement('button');
clearBtn.textContent = 'Clear';
const saveBtn = document.createElement('button');
saveBtn.textContent = 'Save as PNG';

controls.appendChild(colorLabel);
controls.appendChild(widthLabel);
controls.appendChild(clearBtn);
controls.appendChild(saveBtn);
const container = document.getElementById('container');
container.parentNode.insertBefore(controls, container);

// --- Stage Setup ---
const width = window.innerWidth;
const height = window.innerHeight - 40;

const stage = new Konva.Stage({
  container: 'container',
  width: width,
  height: height,
});

const bgLayer = new Konva.Layer();
stage.add(bgLayer);

// signature line at bottom
const signLine = new Konva.Line({
  points: [40, height - 60, width - 40, height - 60],
  stroke: '#ccc',
  strokeWidth: 1,
  dash: [6, 4],
});
bgLayer.add(signLine);

const signLabel = new Konva.Text({
  x: 40,
  y: height - 50,
  text: 'Sign here',
  fontSize: 14,
  fill: '#aaa',
  fontFamily: 'Arial',
});
bgLayer.add(signLabel);

const drawLayer = new Konva.Layer();
stage.add(drawLayer);

let isPaint = false;
let lastLine;
let lastPointerPosition;

stage.on('mousedown touchstart', function (e) {
  isPaint = true;
  lastPointerPosition = stage.getPointerPosition();
  lastLine = new Konva.Line({
    stroke: colorInput.value,
    strokeWidth: parseInt(widthSelect.value),
    lineCap: 'round',
    lineJoin: 'round',
    tension: 0.3,
    points: [lastPointerPosition.x, lastPointerPosition.y],
  });
  drawLayer.add(lastLine);
});

stage.on('mouseup touchend', function () {
  isPaint = false;
});

stage.on('mousemove touchmove', function (e) {
  if (!isPaint) return;
  e.evt.preventDefault();
  const pos = stage.getPointerPosition();
  const newPoints = lastLine.points().concat([pos.x, pos.y]);
  lastLine.points(newPoints);
  lastPointerPosition = pos;
});

clearBtn.addEventListener('click', function () {
  drawLayer.destroyChildren();
});

saveBtn.addEventListener('click', function () {
  // temporarily hide background elements for clean export
  bgLayer.hide();
  const dataURL = stage.toDataURL({ pixelRatio: 2 });
  bgLayer.show();

  const link = document.createElement('a');
  link.download = 'signature.png';
  link.href = dataURL;
  document.body.appendChild(link);
  link.click();
  document.body.removeChild(link);
});

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React

import React from 'react';
import { Stage, Layer, Line, Text } from 'react-konva';

const App = () => {
  const [lines, setLines] = React.useState([]);
  const [color, setColor] = React.useState('#000000');
  const [strokeWidth, setStrokeWidth] = React.useState(3);
  const isDrawing = React.useRef(false);
  const stageRef = React.useRef(null);
  const bgLayerRef = React.useRef(null);

  const stageWidth = window.innerWidth;
  const stageHeight = window.innerHeight - 50;

  const handleMouseDown = (e) => {
    isDrawing.current = true;
    const pos = e.target.getStage().getPointerPosition();
    setLines([
      ...lines,
      { color, strokeWidth, points: [pos.x, pos.y] },
    ]);
  };

  const handleMouseMove = (e) => {
    if (!isDrawing.current) return;
    e.evt.preventDefault();
    const stage = e.target.getStage();
    const point = stage.getPointerPosition();
    const lastLine = lines[lines.length - 1];
    lastLine.points = lastLine.points.concat([point.x, point.y]);
    lines.splice(lines.length - 1, 1, lastLine);
    setLines(lines.concat());
  };

  const handleMouseUp = () => {
    isDrawing.current = false;
  };

  const handleClear = () => setLines([]);

  const handleSave = () => {
    const stage = stageRef.current;
    bgLayerRef.current.hide();
    const dataURL = stage.toDataURL({ pixelRatio: 2 });
    bgLayerRef.current.show();
    const link = document.createElement('a');
    link.download = 'signature.png';
    link.href = dataURL;
    document.body.appendChild(link);
    link.click();
    document.body.removeChild(link);
  };

  return (
    <>
      <div style={{ display: 'flex', gap: 8, alignItems: 'center', flexWrap: 'wrap', marginBottom: 4 }}>
        <label>
          Pen Color:{' '}
          <input
            type="color"
            value={color}
            onChange={(e) => setColor(e.target.value)}
          />
        </label>
        <label>
          Width:{' '}
          <select
            value={strokeWidth}
            onChange={(e) => setStrokeWidth(Number(e.target.value))}
          >
            {[2, 3, 4, 5, 6].map((w) => (
              <option key={w} value={w}>{w}px</option>
            ))}
          </select>
        </label>
        <button onClick={handleClear}>Clear</button>
        <button onClick={handleSave}>Save as PNG</button>
      </div>
      <Stage
        ref={stageRef}
        width={stageWidth}
        height={stageHeight}
        onMouseDown={handleMouseDown}
        onMousemove={handleMouseMove}
        onMouseup={handleMouseUp}
        onTouchStart={handleMouseDown}
        onTouchMove={handleMouseMove}
        onTouchEnd={handleMouseUp}
      >
        <Layer ref={bgLayerRef}>
          <Line
            points={[40, stageHeight - 60, stageWidth - 40, stageHeight - 60]}
            stroke="#ccc"
            strokeWidth={1}
            dash={[6, 4]}
          />
          <Text
            x={40}
            y={stageHeight - 50}
            text="Sign here"
            fontSize={14}
            fill="#aaa"
            fontFamily="Arial"
          />
        </Layer>
        <Layer>
          {lines.map((line, i) => (
            <Line
              key={i}
              points={line.points}
              stroke={line.color}
              strokeWidth={line.strokeWidth}
              tension={0.3}
              lineCap="round"
              lineJoin="round"
            />
          ))}
        </Layer>
      </Stage>
    </>
  );
};

export default App;

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Vue

<template>
  <div>
    <div style="display:flex;gap:8px;align-items:center;flex-wrap:wrap;margin-bottom:4px">
      <label>
        Pen Color:
        <input type="color" v-model="color" />
      </label>
      <label>
        Width:
        <select v-model.number="strokeWidth">
          <option v-for="w in [2,3,4,5,6]" :key="w" :value="w">{{ w }}px</option>
        </select>
      </label>
      <button @click="handleClear">Clear</button>
      <button @click="handleSave">Save as PNG</button>
    </div>
    <v-stage
      ref="stageRef"
      :config="stageConfig"
      @mousedown="handleMouseDown"
      @mousemove="handleMouseMove"
      @mouseup="handleMouseUp"
      @touchstart="handleMouseDown"
      @touchmove="handleMouseMove"
      @touchend="handleMouseUp"
    >
      <v-layer ref="bgLayerRef">
        <v-line :config="signLineConfig" />
        <v-text :config="signLabelConfig" />
      </v-layer>
      <v-layer>
        <v-line
          v-for="(line, i) in lines"
          :key="i"
          :config="{
            points: line.points,
            stroke: line.color,
            strokeWidth: line.strokeWidth,
            tension: 0.3,
            lineCap: 'round',
            lineJoin: 'round'
          }"
        />
      </v-layer>
    </v-stage>
  </div>
</template>

<script setup>
import { ref, computed } from 'vue';

const color = ref('#000000');
const strokeWidth = ref(3);
const lines = ref([]);
const isDrawing = ref(false);
const stageRef = ref(null);
const bgLayerRef = ref(null);

const stageWidth = window.innerWidth;
const stageHeight = window.innerHeight - 50;

const stageConfig = { width: stageWidth, height: stageHeight };
const signLineConfig = {
  points: [40, stageHeight - 60, stageWidth - 40, stageHeight - 60],
  stroke: '#ccc', strokeWidth: 1, dash: [6, 4]
};
const signLabelConfig = {
  x: 40, y: stageHeight - 50, text: 'Sign here',
  fontSize: 14, fill: '#aaa', fontFamily: 'Arial'
};

const handleMouseDown = (e) => {
  isDrawing.value = true;
  const pos = e.target.getStage().getPointerPosition();
  lines.value.push({ color: color.value, strokeWidth: strokeWidth.value, points: [pos.x, pos.y] });
};

const handleMouseMove = (e) => {
  if (!isDrawing.value) return;
  e.evt.preventDefault();
  const point = e.target.getStage().getPointerPosition();
  const lastLine = lines.value[lines.value.length - 1];
  lastLine.points = lastLine.points.concat([point.x, point.y]);
  lines.value.splice(lines.value.length - 1, 1, { ...lastLine });
};

const handleMouseUp = () => { isDrawing.value = false; };

const handleClear = () => { lines.value = []; };

const handleSave = () => {
  const stage = stageRef.value.getNode();
  bgLayerRef.value.getNode().hide();
  const dataURL = stage.toDataURL({ pixelRatio: 2 });
  bgLayerRef.value.getNode().show();
  const link = document.createElement('a');
  link.download = 'signature.png';
  link.href = dataURL;
  document.body.appendChild(link);
  link.click();
  document.body.removeChild(link);
};
</script>

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