In recursion, why is this the output

December 10, 2021

just started learning javascript and I came across this code while learning recursion, could someone please break this down for me on why the output is [1,2,3,4,5]?

function rangeOfNumbers(startNum, endNum) {

  if (endNum - startNum === 0) {
    return [startNum];
  } else {
    var numbers = rangeOfNumbers(startNum, endNum - 1);
    numbers.push(endNum);
    return numbers;
  }
}
console.log(rangeOfNumbers(1,5))

I can’t seem to grasp this line of code "var numbers = rangeOfNumbers(startNum, endNum – 1);"

All I know is that it calls itself again while subtracting 1 on endNum each call and pushes it into the numbers array

>Solution :

I wrote some comments to the lines for you to graps the functional purpose of each, if you need a deeper understanding on what happens, try yousing the debug tool of your IDE (vscode for example).

There you will see you will step into the else statement, call the function recursively, jump again into the else and recursion until you hit the end defined in the first if, then you are done with calling recursion and everz recursive state will call the next line where you push the number and then return to the next higher recursion state.

So the runthrough with 1,3:

3 > 1, => recurse to 2>1, => recurse to 1=1, break recursion, push 1, step out, push 2, step out, push 3, then you are done

function rangeOfNumbers(startNum, endNum) {

  if (endNum - startNum === 0) { // This statement checks if the recursion has hit its end when endNum === startNum
    return [startNum]; // This is the point when the recursion is stopped
  } else {
    var numbers = rangeOfNumbers(startNum, endNum - 1); // If the recursion is not finished yet, the function will call itself with the reduced endNum
    numbers.push(endNum); // This is first called in the deepest recursion, so the first one to be added is 1, then it steps back to the recursion state one above and in this case adds the 2 to the array
    return numbers; // Finally we return to be done with the function
  }
}
console.log(rangeOfNumbers(1,5)) // This is the start, the function is called with the 2 parameters 1 and 5