How to clear the canvas for redrawing

Others have already done an excellent job answering the question but if a simple clear() method on the context object would be useful to you (it was to me), this is the implementation I use based on answers here:

CanvasRenderingContext2D.prototype.clear = 
  CanvasRenderingContext2D.prototype.clear || function (preserveTransform) {
    if (preserveTransform) {
      this.save();
      this.setTransform(1, 0, 0, 1, 0, 0);
    }

    this.clearRect(0, 0, this.canvas.width, this.canvas.height);

    if (preserveTransform) {
      this.restore();
    }           
};

Usage:

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
};

If you are drawing lines, make sure you don't forget:

context.beginPath();

Otherwise the lines won't get cleared.

Use: context.clearRect(0, 0, canvas.width, canvas.height);

This is the fastest and most descriptive way to clear the entire canvas.

Do not use: canvas.width = canvas.width;

Resetting canvas.width resets all canvas state (e.g. transformations, lineWidth, strokeStyle, etc.), it is very slow (compared to clearRect), it doesn't work in all browsers, and it doesn't describe what you are actually trying to do.

Dealing with transformed coordinates

If you have modified the transformation matrix (e.g. using scale, rotate, or translate) then context.clearRect(0,0,canvas.width,canvas.height) will likely not clear the entire visible portion of the canvas.

The solution? Reset the transformation matrix prior to clearing the canvas:

// Store the current transformation matrix
context.save();

// Use the identity matrix while clearing the canvas
context.setTransform(1, 0, 0, 1, 0, 0);
context.clearRect(0, 0, canvas.width, canvas.height);

// Restore the transform
context.restore();

Edit: I've just done some profiling and (in Chrome) it is about 10% faster to clear a 300x150 (default size) canvas without resetting the transform. As the size of your canvas increases this difference drops.

That is already relatively insignificant, but in most cases you will be drawing considerably more than you are clearing and I believe this performance difference be irrelevant.

100000 iterations averaged 10 times:
1885ms to clear
2112ms to reset and clear

Given that canvas is a canvas element or an OffscreenCanvas object,

const context = canvas.getContext('2d');

context.clearRect(0, 0, canvas.width, canvas.height);