Simple Recursive Javascript Function Returns Undefined

Question

For an assignment I am supposed to write a recursive function that checks any integer for even or odd using N-2. If even returns true else returns false. But it returns undefined whenever a value is large enough to call itself. Please help!

function isEven(num) {
  console.log("top of function num = " + num);// For Debugging
  if (num == 0){
      return true;
  }
  else if (num == 1){
      return false;
  }
  else {
    num -= 2;
    console.log("num = " + num);
    isEven(num);
  }
}
console.log(isEven(0));
// → true
console.log(isEven(1));
// → false
console.log(isEven(8));
// → ??

Console Log Results:

top of function num = 0

true

top of function num = 1

false

top of function num = 8

num = 6

top of function num = 6

num = 4

top of function num = 4

num = 2

top of function num = 2

num = 0

top of function num = 0

undefined

Great, homework teaching you the wrong ways to use a recursive function. Use modulus instead. — Xorifelse
– Xorifelse, Commented Nov 14, 2016 at 12:30
@Xorifelse The point is to become comfortable writing recursive functions, not to write an efficient test of whether a number is even. — David Richerby
– David Richerby, Commented Nov 14, 2016 at 12:45

Robba · Accepted Answer · 2016-11-14 12:16:17Z

4

You have forgotten the return statement before the recursive isEven(num) call.

See the snippet below:

function isEven(num) {
  //console.log("top of function num = " + num);// For Debugging
  if (num == 0){
      return true;
  }
  else if (num == 1){
      return false;
  }
  else {
    num -= 2;
    return isEven(num);
  }
}
console.log('0 is even: ', isEven(0));
// → true
console.log('1 is even: ', isEven(1));
// → false
console.log('8 is even: ', isEven(8));

edited Nov 14, 2016 at 12:16

answered Nov 14, 2016 at 12:09

Robba

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4 Comments

Robba Over a year ago

Oh, @cyber_rookie seems to have beaten me to it. Really unsure why we both got -1'd though, our answer is most certainly correct!

Robba Over a year ago

Excellent! Would you be so kind to mark the answer as the answer?

Jules Manson Over a year ago

Thanks for the reminder and once again thank you so much for the fix.

Robba Over a year ago

Use the snippet button in the button bar when typing an answer :) (Ctrl+M also seems to do it on Windows)

cyber_rookie · Accepted Answer · 2016-11-14 12:08:01Z

0

You can change the following line:

isEven(num);

to

return isEven(num);

answered Nov 14, 2016 at 12:08

cyber_rookie

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