crypto.getRandomValues是否会返回负值？

Question

Below is code from https://www.google-analytics.com/analytics.js

var a = new Uint32Array(1);
window.crypto.getRandomValues(a);
return a[0] & 2147483647;

As far as I can imagine, a positive value & 2147483647 will always be itself. If it's necessary, can I deduce that the crypto.getRandomValues will return negative values? I cannot find that from MDN.

Answer 1

The Web Cryptography API draft states (emphasis mine):

1. If array is not of an integer type (ie, Int8Array , Uint8Array , Int16Array , Uint16Array , Int32Array , or Uint32Array ), throw a TypeMismatchError and terminate the algorithm.

2. If the byteLength of array is greater than 65536, throw a QuotaExceededError and terminate the algorithm.

3. Overwrite all elements of array with cryptographically random values of the appropriate type .

In other words: if you pass an Int* array as argument, you can expect negative values. If you don't want those, use Uint* (which you're already doing, so you can expect a[0] to be positive).