How to get image mask in canvas?

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// Get canvas and context
var canvas = document.getElementById('myCanvas');
var ctx = canvas.getContext("2d");

// Create the mask image
var maskImg = new Image();
maskImg.src = 'mask.png';

// Create the main image
var mainImg = new Image();
mainImg.src = 'mainImg.png';

// When both images have loaded
maskImg.onload = function() {
  mainImg.onload = function() {
    // Draw the mask image on the canvas
    ctx.drawImage(maskImg, 0, 0, canvas.width, canvas.height);
    
    // Set global composite operation to 'source-in' to apply the mask
    ctx.globalCompositeOperation = 'source-in';
    
    // Draw the main image on the canvas, clipped by the mask
    ctx.drawImage(mainImg, 0, 0, canvas.width, canvas.height);
    
    // Reset global composite operation
    ctx.globalCompositeOperation = 'source-over';
  };
};