If ids is an array, then just use join:

ids = [1, 2, 3, 4];
val = ids.join(', ');
// val is now "1, 2, 3, 4"

For this particular use, and to avoid evaluating an extra condition in every iteration, you could start with word before entering the loop, and have the loop make one iteration less. To support times being zero, you'll need to add a condition:

function repeatWord(word, times) {
  let result = times ? word : "";
  for (let i = 1; i < times; i++) {
    result += "," + word;
  }
  return result;
}

console.log(repeatWord('hello', 3));

Label doesn't use value to assign values. It uses innerHTML. Try the below.

document.getElementById('result').innerHTML = result;

As pointed out in comments, it is better to use textContent or innerText options to set value (only for plain text) as they are safer than innerHTML.You can use it as shown below.

document.getElementById('result').textContent = result;

or

document.getElementById('result').innerText = result;

innerText property is not supported by FireFox and it uses the textContent property. Hence, the below method will work across browsers.

var resultDiv = document.getElementById('result');

if (typeof resultDiv.innerText === 'string') {
    resultDiv.innerText = result;
}
else {
    resultDiv.textContent = result;
}

Sources:

1. IE8 label update via javascript issue
2. 'innerText' works in IE, but not in Firefox

You can specify the exact number of decimal digits in your options, which is the second parameter in toLocaleString()

const number = 24242324.5754;

number.toLocaleString('en-US', {
    minimumFractionDigits: 2,
    maximumFractionDigits: 2
})

// result is: 24,242,324.58

See also MDN doc here

minimumFractionDigits

The minimum number of fraction digits to use. Possible values are from 0 to 20; the default for plain number and percent formatting is

maximumFractionDigits

The maximum number of fraction digits to use. Possible values are from 0 to 20; the default for plain number formatting is the larger of minimumFractionDigits and 3

(1234567890).toLocaleString();

Just insert the code to add the comma at the start of the outer looop (i.e. when it loops each set). But include a check for whether there is any previous text in the output, otherwise it'll add a comma before the first entry.

Like this:

const repeatNumbers = function(arr) {
  let numbersRepeated = "";
  for (let i = 0; i < arr.length; i++) {
    if (numbersRepeated != "") numbersRepeated += ",";
    
    for (let j = 0; j < arr[i][1]; j++) {
      numbersRepeated += arr[i][0];
    }
  }
  if (arr.length === 0) {
    return numbersRepeated;
  } else {
    return numbersRepeated;
  }
}

console.log(repeatNumbers([
  [1, 2],
  [2, 3]
]));

Just split into two parts with '.' and format them individually.

function commafy( num ) {
    var str = num.toString().split('.');
    if (str[0].length >= 5) {
        str[0] = str[0].replace(/(\d)(?=(\d{3})+$)/g, '$1,');
    }
    if (str[1] && str[1].length >= 5) {
        str[1] = str[1].replace(/(\d{3})/g, '$1 ');
    }
    return str.join('.');
}

As stated in the comments use toLocaleString()

Simple example

let number = 123456789;
let numberWithComma = number.toLocaleString();
console.log(numberWithComma);

Simply

return str1 + ", " + str2;

If the strings are in an Array, you can use Array.prototype.join method, like this

var strings = ["a", "b", "c"];
console.log(strings.join(", "));

Output

a, b, c