Sum of unique numbers

Cassidy Williams has an awesome newsletter that includes, among other things, interview questions. Here is a interesting one:

Given an array of numbers, add all of the values together but only if the number doesn't repeat a digit.

Example:

uniqueSum([1, 2, 3]); // 6
uniqueSum([11, 22, 33]); // 0
uniqueSum([101, 2, 3]); // 5

When solving this type of questions, my first thought is always to think about each step individually. I need to first remove from the array all the numbers that repeat an number, and then sum the rest. So an .filter followed by an .reduce is the perfect match for what we need.

function uniqueSum(array: number[]) {
    return array.filter(hasNoRepeatingDigit).reduce((a, b) => a + b))
}

Now we need to create a function that can check if the number has any repeating number. My first instinct is to transform the number into an array and then use split to get an array.

String(12345).split(""); // [ '1', '2', '3', '4', '5' ]

You can iterate over that array to check if any number repeats, but you can also check if all the values are unique but transforming into an Set and checking if the lenght stays the same.

new Set(String(112).split("")).size == String(112).split("").size; // false
new Set(String(123).split("")).size == String(123).split("").size; // true

So the function can be written as:

function hasNoRepeatingDigit(number) {
  const arrayOfNumber = String(number).split("");
  // Note that we use `size` for Set and `length` for arrays
  return new Set(arrayOfNumber).size == arrayOfNumber.length;
}

You can check the full code in the stackblitz bellow, which also includes some testing using vitest:

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