CanvasRenderingContext2D: arc() メソッド

arc(x, y, radius, startAngle, endAngle)
arc(x, y, radius, startAngle, endAngle, counterclockwise)

<canvas></canvas>

const canvas = document.querySelector("canvas");
const ctx = canvas.getContext("2d");
ctx.beginPath();
ctx.arc(100, 75, 50, 0, 2 * Math.PI);
ctx.stroke();

<canvas width="150" height="200"></canvas>

const canvas = document.querySelector("canvas");
const ctx = canvas.getContext("2d");
// Draw shapes
for (let i = 0; i <= 3; i++) {
  for (let j = 0; j <= 2; j++) {
    ctx.beginPath();
    let x = 25 + j * 50; // x 座標
    let y = 25 + i * 50; // y 座標
    let radius = 20; // 弧の半径
    let startAngle = 0; // 弧の開始位置
    let endAngle = Math.PI + (Math.PI * j) / 2; // 弧の終了位置
    let counterclockwise = i % 2 === 1; // 弧を描く方向
    ctx.arc(x, y, radius, startAngle, endAngle, counterclockwise);
    if (i > 1) {
      ctx.fill();
    } else {
      ctx.stroke();
    }
  }
}