The CanvasRenderingContext2D.lineCap property of the Canvas 2D API determines the shape used to draw the end points of lines.

One of the following:

"butt"

The ends of lines are squared off at the endpoints. Default value.

"round"

The ends of lines are rounded.

"square"

The ends of lines are squared off by adding a box with an equal width and half the height of the line's thickness.

This example rounds the end caps of a straight line.

HTML

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

JavaScript

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

ctx.beginPath();
ctx.moveTo(20, 20);
ctx.lineWidth = 15;
ctx.lineCap = "round";
ctx.lineTo(100, 100);
ctx.stroke();

Result

In this example three lines are drawn, each with a different value for the lineCap property. Two guides to see the exact differences between the three are added. Each of these lines starts and ends exactly on these guides.

The line on the left uses the default "butt" option. It's drawn completely flush with the guides. The second is set to use the "round" option. This adds a semicircle to the end that has a radius half the width of the line. The line on the right uses the "square" option. This adds a box with an equal width and half the height of the line thickness.

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

// Draw guides
ctx.strokeStyle = "#09f";
ctx.beginPath();
ctx.moveTo(10, 10);
ctx.lineTo(140, 10);
ctx.moveTo(10, 140);
ctx.lineTo(140, 140);
ctx.stroke();

// Draw lines
ctx.strokeStyle = "black";
["butt", "round", "square"].forEach((lineCap, i) => {
  ctx.lineWidth = 15;
  ctx.lineCap = lineCap;
  ctx.beginPath();
  ctx.moveTo(25 + i * 50, 10);
  ctx.lineTo(25 + i * 50, 140);
  ctx.stroke();
});

Screenshot	Live sample

• In WebKit- and Blink-based Browsers, a non-standard and deprecated method ctx.setLineCap() is implemented in addition to this property.

• The interface defining this property: CanvasRenderingContext2D
• CanvasRenderingContext2D.lineWidth
• CanvasRenderingContext2D.lineJoin
• Applying styles and color