Crypto: getRandomValues() method

The Crypto.getRandomValues() method lets you get cryptographically strong random values. The array given as the parameter is filled with random numbers (random in its cryptographic meaning).

To guarantee enough performance, implementations are not using a truly random number generator, but they are using a pseudo-random number generator seeded with a value with enough entropy. The pseudo-random number generator algorithm (PRNG) may vary across user agents, but is suitable for cryptographic purposes.

getRandomValues() is the only member of the Crypto interface which can be used from an insecure context.

Syntax

getRandomValues(typedArray)

Parameters

typedArray: An integer-based TypedArray, that is one of: Int8Array, Uint8Array, Uint8ClampedArray, Int16Array, Uint16Array, Int32Array, Uint32Array, BigInt64Array, BigUint64Array (but not Float32Array nor Float64Array). All elements in the array will be overwritten with random numbers.

Return value

The same array passed as typedArray but with its contents replaced with the newly generated random numbers. Note that typedArray is modified in-place, and no copy is made.

Exceptions

QuotaExceededError DOMException: Thrown if the byteLength of typedArray exceeds 65,536.

Usage notes

Don't use getRandomValues() to generate encryption keys. Instead, use the generateKey() method. There are a few reasons for this; for example, getRandomValues() is not guaranteed to be running in a secure context.

There is no minimum degree of entropy mandated by the Web Cryptography specification. User agents are instead urged to provide the best entropy they can when generating random numbers, using a well-defined, efficient pseudorandom number generator built into the user agent itself, but seeded with values taken from an external source of pseudorandom numbers, such as a platform-specific random number function, the Unix /dev/urandom device, or other source of random or pseudorandom data.

Examples

const array = new Uint32Array(10);
self.crypto.getRandomValues(array);

console.log("Your lucky numbers:");
for (const num of array) {
  console.log(num);
}

Specifications

Specification
Web Cryptography API # Crypto-method-getRandomValues

Browser compatibility

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