contains duplicates function won't return true?

Question

Why doesn't "return true" work here? Instead the function returns undefined...

var containsDuplicate = function(nums) {
    let newArray = nums;
    let p = newArray.pop();
    if (newArray.includes(p)) {
        return true;
    }
    else if (newArray) {
        containsDuplicate(newArray);
    }
    else {
        return false;
    }
};

EDIT:

I'm calling the function with something like this:

console.log(containsDuplicate([1,2,3,9,9,0]))

If you want to remove duplicates, look at this answer: https://stackoverflow.com/a/9229821/10768127. It's a much easier method — Aniket G, Mar 13 '19 at 03:03
...and the inner "containsDuplicate" does not return its value? — Mario Vernari, Mar 13 '19 at 03:04
Thanks, but I know how to remove duplicates. With a Set for example. I'm specifically interested in this odd bug. — , Mar 13 '19 at 03:04
Thanks! It was indeed the missing return before the recursion... — , Mar 13 '19 at 03:19

score 1 · Answer 1 · answered Mar 13 '19 at 03:08

You're missing a return statement before the recursive function.

var containsDuplicate = function(nums) {
    let newArray = nums;
    let p = newArray.pop();
    if (newArray.includes(p)) {
        return true;
    }
    else if (newArray) {
        return containsDuplicate(newArray);
    }
    else {
        return false;
    }
};

score 0 · Answer 2 · answered Mar 13 '19 at 03:04

You should step through it with a browser debugger, but my opinion based on experience is that you may be setting newarray to null, which would happen if nums isn't declared correctly. If newarray is null, the if check will throw an exception and not evaluate.

score 0 · Answer 3 · answered Mar 13 '19 at 03:09

Because block with containsDuplicate(newArray); return nothing. Also function so bad because newArray.includes(p). Use hashmap aka Object:

var keys = {}
for (let i = 0, c = arr.length; i < c; i++) {
   let v = arr[i];
   if (!keys[v]) {
       keys[v] = true;
   } else {
       return true;
   }
}
return false;

score 0 · Answer 4 · answered Mar 13 '19 at 03:13

You need to return the recursive call

var containsDuplicate = function(nums) {
  let newArray = nums;
  let p = newArray.pop();
  if (newArray.includes(p)) {
    return true;
  } else if (newArray) {
    return containsDuplicate(newArray);
  } else {
    return false;
  }
};

console.log(containsDuplicate([1, 2, 3, 9, 9, 0]))

For Remove Duplicate use Array#filter

console.log(containsDuplicate([1, 2, 3, 9, 9, 0]))


function containsDuplicate(arr){
 return arr.filter((a,b)=> arr.indexOf(a)==b)
}

contains duplicates function won't return true?

4 Answers4