SOURCE

function draw() {
    const canvas = document.querySelector('#test')
    const ctx = canvas.getContext('2d')
    // ctx.fillStyle = 'rgb(200, 0, 0)'
    ctx.fillRect(10, 10, 50, 50)

    ctx.fillStyle = 'rgba(0,200,0, 0.5)'
    ctx.fillRect(20, 20, 50, 50)

    ctx.fillStyle = 'rgb(0,0,200)'
    ctx.strokeRect(30, 30, 50, 50)

}

draw()

<canvas id='test' width="150" height="150"></canvas>

canvas{
    border: 1px solid black
}