With : `i << 2`

    const data = context.getImageData(x, y, width, height).data;
    const pixels = [];
    
    for (let i = 0, dx = 0; dx < data.length; i++, dx = i << 2) {
        if (data[dx+3] <= 8)
            console.log("transparent x= " + i);
    }