Generating a random number that is divisible by n in JavaScript

We are required to write a JavaScript function that takes in a number as the only argument. The function should then return a random generated number which is always divisible by the number provided by the argument.

Approach

To generate a random number divisible by n, we generate a random number within a range, divide it by n, round the result, and multiply back by n. This ensures the final number is always a multiple of n.

Example

The code for this will be:

const num = 21;

// function that generates random numbers divisible by n with a default
// upper limit of 1000000
const specialRandom = (num = 1, limit = 1000000) => {
    // getting a random number
    const random = Math.random() * limit;
    // rounding it off to be divisible by num
    const res = Math.round(random / num) * num;
    return res;
};

console.log(specialRandom(num));

Output

And the output in the console will be:

6006

This output is likely to differ on each run.

How It Works

The function works by:

1. Generating a random decimal between 0 and the limit (1,000,000)
2. Dividing by the target number (n) to get how many "groups" of n fit
3. Rounding to the nearest integer to get whole groups
4. Multiplying back by n to get the final divisible number

Multiple Examples

// Generate numbers divisible by different values
console.log("Divisible by 5:", specialRandom(5));
console.log("Divisible by 10:", specialRandom(10)); 
console.log("Divisible by 25:", specialRandom(25));
console.log("Divisible by 100:", specialRandom(100));

// With custom limit
console.log("Divisible by 7 (limit 500):", specialRandom(7, 500));

Divisible by 5: 245580
Divisible by 10: 567890
Divisible by 25: 123450
Divisible by 100: 456700
Divisible by 7 (limit 500): 287

Conclusion

This method efficiently generates random numbers divisible by any given value by using mathematical operations to ensure the result is always a perfect multiple. The approach scales well with different limits and divisors.