CanvasRenderingContext2D: lineDashOffset property

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

The CanvasRenderingContext2D.lineDashOffset property of the Canvas 2D API sets the line dash offset, or "phase."

Note: Lines are drawn by calling the stroke() method.

Value

A float specifying the amount of the line dash offset. The default value is 0.0.

Examples

Offsetting a line dash

This example draws two dashed lines. The first has no dash offset. The second has a dash offset of 4.

HTML

html

<canvas id="canvas"></canvas>

JavaScript

const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");

ctx.setLineDash([4, 16]);

// Dashed line with no offset
ctx.beginPath();
ctx.moveTo(0, 50);
ctx.lineTo(300, 50);
ctx.stroke();

// Dashed line with offset of 4
ctx.beginPath();
ctx.strokeStyle = "red";
ctx.lineDashOffset = 4;
ctx.moveTo(0, 100);
ctx.lineTo(300, 100);
ctx.stroke();

Result

The line with a dash offset is drawn in red.

The marching ants effect is an animation technique often found in selection tools of computer graphics programs. It helps the user to distinguish the selection border from the image background by animating the border.

<canvas id="canvas"></canvas>

const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");
let offset = 0;

function draw() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  ctx.setLineDash([4, 2]);
  ctx.lineDashOffset = offset;
  ctx.strokeRect(10, 10, 100, 100);
}

function march() {
  offset++;
  if (offset > 5) {
    offset = 0;
  }
  draw();
  setTimeout(march, 20);
}

march();

Specifications

Specification
HTML Standard # dom-context-2d-linedashoffset-dev

Browser compatibility

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