Array with random unique integer numbers (0 to array.length - 1)

First populate a sequential array, then sort by returning a random result in the compareFunction.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort

let arrLen = 7, resArr = []
for(let i=0; i < arrLen; i++){
  resArr.push(i)
}

resArr.sort((a,b) => {
  let ranBool = Math.floor(Math.random() * 2)
  //returns 0 or -1, which either leaves the integers in place or sorts the second one before the first 
  return ranBool - 1
})

console.log(resArr)

• In my solution we create an array of numbers in order [1, 2, 3] call arrayOfNumbers
• We then create an empty array call randomArray
• We forEach loop through the array we created in first step
• We generate a random number base on the length of the array and decreasing it throughout the forEach loop
• We then push a random number from the arrayOfNumbers to randomArray
• At the same time (loop) after pushing a random number from arrayOfNumbers we use array methods splice to remove it from arrayOfNumbers (so we can't longer use it)
• We short the length by one so we can only generate random number in the length of the arrayOfNumbers array. So next time we call arrayOfNumbers[random] we'll have a unique number from this array.

   function randomNumbers(howMany) {
  arrayOfNumbers = [];

  for (let i = 0; i < howMany; i++) {
    arrayOfNumbers.push(i);
  };

  const arr = [...arrayOfNumbers];
  let length = arr.length
  const randomArr = [];

  arr.forEach((num, i) => {
    const random = Math.floor(Math.random() * length);

    randomArr.push(arrayOfNumbers[random]);
    arrayOfNumbers.splice(arrayOfNumbers.indexOf(arrayOfNumbers[random]), 1);
    length--

  });

  return randomArr;
};

console.log(randomNumbers(101));