Convert a number (integer) to a human-readable string.

humanReadableInteger.js

// `reverse` reverses a string. `reverse` is not Unicode-
// safe.
function reverseString(str) {
	return str.split("").reverse().join("")
}

// `humanReadableInteger` converts a number to a human-
// readable string. `humanReadableInteger` is not Unicode-
// safe.
//
// 1. Convert the number to a string.
// 2. Reverse the string.
// 3. Concatenate a comma per three digits (not at the end of the line).
// 4. Re-reverse the string.
function humanReadableInteger(int) {
	let str = int.toString()
	str = reverseString(str)                               
	str = str.replace(/\d{3}(?!$)/g, match => match + ",") 
	str = reverseString(str)                               
	return str
}

console.log(humanReadableInteger(1))          //             1
console.log(humanReadableInteger(12))         //            12
console.log(humanReadableInteger(123))        //           123
console.log(humanReadableInteger(1234))       //         1,234
console.log(humanReadableInteger(12345))      //        12,345
console.log(humanReadableInteger(123456))     //       123,456
console.log(humanReadableInteger(1234567))    //     1,234,567
console.log(humanReadableInteger(12345678))   //    12,345,678
console.log(humanReadableInteger(123456789))  //   123,456,789
console.log(humanReadableInteger(1234567890)) // 1,234,567,890