The different constructors are:

new TypedArray(); // new in ES2017
new TypedArray(length);
new TypedArray(typedArray);
new TypedArray(object);

So when you do

new Uint8Array(255)

It will take it as a length. However you can pass an object (an array) instead which will be iterated to create the uint array:

new Uint8Array([255, 255, /*..*/]);

Uint***Arrays construct non-typed arrays (commented by @zfor, so, for example, push would be undefined) with numbers only (still bytes). The difference is that each constructor array has different byte range in memory. Uint8Array has 1 byte only, then the limit of a number is 255. Uint16Array is 2 bytes long, then the limit is 65535. Uint32Array is 4 bytes long, so the limit is 4294967295.

When constructing a Uint*Array you declare the array length as the first argument:

var arr = new Uint8Array(1);

If you declare a array/buffer/object instead, the constructor still proccess them as a Uint*Array.

var arr = new Uint8Array([10, 257]);
console.log(arr[0]); // 10
console.log(arr[1]); // 1 (same thing: 257 % 256)

Now, see some examples:

arr[0] = 256;
console.log(arr[0]); // 0

arr[0] = 255;
console.log(arr[0]); // 255