Failure to understand javascript recursive program

Question

I have written the following program in javascript:

function recursiveSum(a) {
  sum = 0;
  for (i=0;i<a.length; ++i) {
      if (typeof a[i] === "number") {
          sum += a[i];
      } else if (a[i] instanceof Array) {
          sum += recursiveSum(a[i]);
      }
  }
  return sum;
}
function arraySum(a) {

    // i will be an array, containing integers, strings and/or arrays like itself.
    // Sum all the integers you find, anywhere in the nest of arrays.

    return recursiveSum(a);
}

And I can't figure out why the result of arraySum([[1,2,3],4,5]) is 6. Why the elements after the first array are not processed?

Where is your sum var defined? It may not be local to the function. — Étienne Miret
– Étienne Miret, Commented Oct 9, 2013 at 14:26
as soon as you can, type "use strict;" at the beginning of all your js files. It doesn't allow your buggy style global variables without var. — Stephane Rolland
– Stephane Rolland, Commented Oct 9, 2013 at 14:36
I am a beginner in javascript and was writing code somewhat in association to other languages. I forgot to put var in front of the variables and so my variables are global as the answers suggested. — Ivaylo Strandjev
– Ivaylo Strandjev, Commented Oct 9, 2013 at 14:37

epascarello · Accepted Answer · 2013-10-09 14:26:43Z

7

You have a problem with Global Variables. You need to use var, it is not optional.

Both sum and i need to be declared with var.

var sum = 0;
for (var i=0;i<a.length; ++i) {

answered Oct 9, 2013 at 14:26

epascarello

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Comments

T.J. Crowder · Accepted Answer · 2013-10-09 14:33:10Z

4

Your sum and i variables are globals, because you haven't declared them as local to the function. You're falling prey to The Horror of Implicit Globals. It's primarily the i variable that's causing the trouble with your specific input: Since your first entry in a is an array, i gets incremented by the recursive call, and the last two entries in the array are never processed by the outer call. (But if you'd used [1, 2, [3, 4, 5]], the fact the calls share both i and sum would be causing trouble.)

Put var in front of each of them. Also look into using the new strict mode, which would have made that a useful error.

edited Oct 9, 2013 at 14:33

answered Oct 9, 2013 at 14:27

T.J. Crowder

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