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Too smart for your own good, converting is cheating

I have a friend who is a web developer, occasionally he will ask me javascript questions, today I got.

"quick quiz! I want to concatenate 2 numbers together, eg. 1 and 0 = 10."

So I gave it a go.

const firstNumber = 1;
const secondNumber = 0;

function concatNumbers(firstNumber, secondNumber) {
  return Number.parseInt(`${firstNumber}${secondNumber}`, 10);
}

console.log(concatNumbers(firstNumber, secondNumber)); // 10
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I asked, "Do you want the return value a number or a string?"

He said, "no! but you shouldn't convert the type. Get the result using standard multiplication and addition."

"Why?"

"Converting is cheating, it's like using a side effect of concatenating strings to do a calculation."

I responded, "Sounds like something a back end developer would say, type coercion in javascript behaves consistently, but not in a logical way. and it's not a bad thing if you know what you are doing."

I gave it a go...

const firstNumber = 1;
const secondNumber = 0;

function concatNumbers(arr) {
  return arr.reverse().reduce((acc, item, index) => {
    return (acc += item * Math.pow(10, index));
  }, 0);
}

console.log(concatNumbers([firstNumber, secondNumber])); // 10

He responded "Wow!" I thought; I have clearly impressed him "Can you explain what going on here?"

This is something I often do, I can end up with a complex solution to simple problems, leaving behind me code that is hard to maintain and potentially confusing to others.

I want to write code that is easy to maintain and understand. I am very aware it's easier to write complex code than simple code.

He asked his wife, who works in finance and specialises in pure maths/excel stuff, and she came back with this

const firstNumber = 1;
const secondNumber = 0;

function concatNumbers(firstNumber, secondNumber) {
  return firstNumber * 10 + secondNumber;
}

console.log(concatNumbers(firstNumber, secondNumber)); // 10

It made me laugh. I was impressed by how simple the solution is. And I prefer his wife's solution.

This is why I like TDD, his wife's solution meets the test criteria, and if there is a problem, we will need another test case.

Granted his wife's version has limitations, but the function completes the initial requirements.

This also demonstrates one of the issues I have with the reduce() function in javascript, it's very easy to force a problem into it and leave yourself with code that is hard to explain and maintain

Bonus Points

  • Can you think of another way to solve this problem, I would be keen to see
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