unary operator prefix execution in recursion is not working as expected

Question

I am trying to count number of combinations in given range and length of combinations. Here is following code.

function generateCombination() {
  const perm = [];
  const permLength = 2;
  const numberRange = 8;
  let total = 0;

  const getCombinations = (result, permLength, numberRange) => {
    if (result.length === permLength) {
      //console.log('result: ', result);
      return 1;
    }
  
    for (var i = 0; i < numberRange; i++) {
      if (result.indexOf(i) === -1) {
        result.push(i);
        total = total + getCombinations(result, permLength, numberRange);
        result.pop();
      }
    }

    return 0;
  }
  
  getCombinations(perm, permLength, numberRange);
  console.log("total: ", total); // expected value "total:  56"
}

generateCombination();

the console log for total variable always print 0. but following code works as expected with little change in for loop code. I couldn't understand how the prefix is works here (somex = somex + fn()). Can someone please help here?

// working solution
function generateCombination() {
  const perm = [];
  const permLength = 2;
  const numberRange = 8;
  let total = 0;

  const getCombinations = (result, permLength, numberRange) => {
    if (result.length === permLength) {
      //console.log('result: ', result);
      return 1;
    }
  
    for (var i = 0; i < numberRange; i++) {
      if (result.indexOf(i) === -1) {
        result.push(i);
        if (getCombinations(result, permLength, numberRange)) {
           total += 1;
        }
        result.pop();
      }
    }

    return 0;
  }
  
  getCombinations(perm, permLength, numberRange);
  console.log("total: ", total); // expected value is "total: 56" and working here
}

generateCombination();

My question is, I don't understand, why solution 1 (top one) is not working as expected (printing total as 0 rather than 56 )?

Thanks

Answer 1

You could move total into getCombinations and return this value on exit.

function generateCombination() {
  const perm = [];
  const permLength = 2;
  const numberRange = 8;

  const getCombinations = (result, permLength, numberRange) => {
    if (result.length === permLength) return 1;

    let total = 0;
  
    for (let i = 0; i < numberRange; i++) {
      if (result.indexOf(i) === -1) {
        result.push(i);
        total += getCombinations(result, permLength, numberRange);
        result.pop();
      }
    }

    return total;
  }
  
  console.log("total: ", getCombinations(perm, permLength, numberRange)); // expected value "total:  56"
}

generateCombination();