Looking at the examples for Uint8ClampedArray and Uint8Array, it looks like the difference is how values are treated when assigned.

If you are trying to set one element to a clamped array to any value outside of the range 0-255, it will simply default to 0 or 255 (depending on whether the value is smaller or larger). A normal Uint8Array array just takes the first 8 bit of the value.

Examples:

var x = new Uint8ClampedArray([17, -45.3]);
console.log(x[0]); // 17
console.log(x[1]); // 0
console.log(x.length); // 2

var x = new Uint8Array([17, -45.3]);
console.log(x[0]); // 17
console.log(x[1]); // 211
console.log(x.length); // 2

You can convert a typed array to a regular array by using Array.prototype.slice

var typedArray = new Uint8ClampedArray([1, 2, 3, 4]);
var normalArray = Array.prototype.slice.call(typedArray);

Also if using ES6 you may be able to use Array.from instead:

var normalArray = Array.from(typedArray);

See MDN - JavaScript typed arrays

I ended up using canvas.toDataURL instead of trying to convert this myself.