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 think you shouldn't be using Uint8Array to store your data - it simply doesn't work like a Uint8ClampedArray. For example:

a = new Uint8Array(1);
b = new Uint8ClampedArray(1);
a[0] = 256;
b[0] = 256;

a[0] === 0;    // true
b[0] === 255;  // true

I would just use a regular array (if that's an option) and clamp the values when you store them into it, or when you read them to generate your string like this:

s += String.fromCharCode(Math.max(0, Math.min(255, array[i])));

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

I'm not seeing those time differences in browser or in NodeJs

foo = new Uint8Array(256 * 1024**2); // 256MiB
console.time(label = 'Copy into Uint8ClampedArray');
bar = new Uint8ClampedArray(foo);
console.timeEnd(label);

foo = new Uint8Array(256 * 1024**2); // 256MiB
console.time(label = 'Clone, then use ArrayBuffer');
bar = new Uint8ClampedArray(new Uint8Array(foo).buffer);
console.timeEnd(label);

$ node -v && node test.js
v17.2.0
Copy into Uint8ClampedArray: 174.049ms
Clone, then use ArrayBuffer: 193.819ms

$ node -v && node test.js
v17.2.0
Copy into Uint8ClampedArray: 170.152ms
Clone, then use ArrayBuffer: 192.41ms

$ node -v && node test.js
v17.2.0
Copy into Uint8ClampedArray: 168.555ms
Clone, then use ArrayBuffer: 209.525ms