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