Randomly Select Item from Array Based on Frequency

index.js

// Define our array of object we want to randomly select from
const fallingItems = 
[{
    id: 1,
    value: 150,
    image: 'rare.png',
    velocity: 5000,
    size: 60
},
{
    id: 2,
    value: 25,
    image: 'average.png',
    velocity: 2000
    size: 110
},
{
    id: 3,
    value: 2,
    image: 'common.png',
    velocity: 900,
    size: 50
}];
// Define the freqeuncy we want each of the fallingItems to be occur at, 
// relative to one another
const frequencyOfFallingItems = [2, 4, 8];

// Randomly get falling item based on frequency array
const itemToQueue =
    arrayItemArbitraryProbabilityDistribution(fallingItems, frequencyOfFallingItems);

math.js

// Utility function to find index of {random} in prefix[start..end]
function findRandomInPrefixArray(prefix, random, start, end) {
    let mid;
    while (start < end) {
        mid = Math.floor((start + end) / 2);
        if (random > prefix[mid]) {
            start = mid + 1;
        } else {
            end = mid;
        }
    }
    return (prefix[start] >= random) ? start : -1;
}

// Returns a random item from array[]
// according to distribution array
// defined by frequency[].
export const arrayItemArbitraryProbabilityDistribution = (array, frequency) => {
    const size = array.length;
    // Create and fill prefix array
    let prefix = [];
    prefix[0] = frequency[0];
    for (let i = 1; i < size; ++i) {
        prefix[i] = prefix[i - 1] + frequency[i];
    }

    // prefix[n-1] is sum of all frequencies. 
    // Generate a random number with  
    // value from 1 to this sum  
    const random = ((Math.floor((Math.random() * 323567))) % prefix[size - 1]) + 1;
    // Find find index of {random} in prefix array
    const index = findRandomInPrefixArray(prefix, random, 0, size - 1);
    return array[index];
}