Javascript: Generate a random number within a range using crypto.getRandomValues

The easiest way is probably by rejection sampling (see http://en.wikipedia.org/wiki/Rejection_sampling). For example, assuming that max - min is less than 256:

function getRandomInt(min, max) {       
    // Create byte array and fill with 1 random number
    var byteArray = new Uint8Array(1);
    window.crypto.getRandomValues(byteArray);

    var range = max - min + 1;
    var max_range = 256;
    if (byteArray[0] >= Math.floor(max_range / range) * range)
        return getRandomInt(min, max);
    return min + (byteArray[0] % range);
}

IMHO, the easiest way to generate a random number in a [min..max] range with window.crypto.getRandomValues() is described here.

An ECMAScript 2015-syntax code, in case the link is TL;TR:

function getRandomIntInclusive(min, max) {
    const randomBuffer = new Uint32Array(1);

    window.crypto.getRandomValues(randomBuffer);

    let randomNumber = randomBuffer[0] / (0xffffffff + 1);

    min = Math.ceil(min);
    max = Math.floor(max);
    return Math.floor(randomNumber * (max - min + 1)) + min;
}

Many of these answers are going to produce biased results. Here's an unbiased solution.

function random(min, max) {
    const range = max - min + 1
    const bytes_needed = Math.ceil(Math.log2(range) / 8)
    const cutoff = Math.floor((256 ** bytes_needed) / range) * range
    const bytes = new Uint8Array(bytes_needed)
    let value
    do {
        crypto.getRandomValues(bytes)
        value = bytes.reduce((acc, x, n) => acc + x * 256 ** n, 0)
    } while (value >= cutoff)
    return min + value % range
}