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const context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
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window.onload = function () {
var canvas = document.getElementById('canvasId');
var context = canvas.getContext('2d');
// do some drawing
context.clear();
// do some more drawing
context.setTransform(-1, 0, 0, 1, 200, 200);
// do some drawing with the new transform
context.clear(true);
// draw more, still using the preserved transform
};