JS recursion returns undefined

Question

I would like to get back the result as a return value from the recursion, but I get 'undefined'. I read through all the related solutions (see ***) here, but somehow my example doesn't work. If I'm using a global variable then I can get the result otherwise the return values is 'undefined'.

https://stackblitz.com/edit/typescript-1b2ktw

export interface Needle {
  needles: Needle[];
  text: string;
}

export const Needles: Needle[] = [
  {
    text: 'root',
    needles: [
      {
        text: 'a',
        needles: [
          { text: 'a1', needles: null },
          { text: 'a2', needles: null },
        ],
      },
      { text: 'b', needles: null },
    ],
  },
];

let result;

function findNeedle(needles: Needle[], key: string, value: string): any {
  needles.forEach((x) => {
    if (x[key] === value) {
      // return x as Needle; // doesnt work
      result = x; // works just ugly
    }
    if (x.needles) {
      needles = x.needles;
      // return findNeedle(needles, key, value); solution *** like this doesnt work also
      findNeedle(needles, key, value);
    }
  });
}

console.clear();
let x = findNeedle(Needles, 'text', 'a1');
console.log('x:', x, 'result', result);
--- CONSOLE
Console was cleared
x:undefined, result {text: "a1", needles: null}

How can I fix this to get back the result in return value ?

Thanks in advance.

Csaba

findNeedle doesn’t return anything, so why would you expect return findNeedle(…); or let x = findNeedle(…); to do anything useful? Also, please see What does return keyword mean inside forEach function?. — Sebastian Simon
– Sebastian Simon, Commented Jan 9, 2022 at 21:41
you are right, when I checked the related question here, many guy mentioned : write return before the recursion. It was just a attempt, just a try. — user647314
– user647314, Commented Jan 9, 2022 at 22:03

trincot · Accepted Answer · 2022-01-09 21:55:24Z

1

Don't use forEach, but a for..of loop. That way you can return something without being trapped within a callback function.

function findNeedle(needles: Needle[], key: string, value: string): any {
  for (let x of needles) {
    if (x[key] === value) return x;
    if (x.needles) {
      result = findNeedle(x.needles, key, value);
      if (result) return result;
    }
  }
}

answered Jan 9, 2022 at 21:55

trincot

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3 Comments

user647314 Over a year ago

Thank you, it works, can you tell me why the first return (if (x[key] === value) return x;) is not enough ?

trincot Over a year ago

The first return will only have checked the current object, but nested objects need to be checked also, and that is why findNeedle needs to be called recursively on the nested object.

user647314 Over a year ago

Oh, I got it, since I can get the result from nested calls thats why not enough the first if. Thank you.

Nathan Wiles · Accepted Answer · 2022-01-09 21:42:57Z

0

x is undefined because findNeedle() doesn't return a result, it assigns one instead to the result variable.

answered Jan 9, 2022 at 21:42

Nathan Wiles

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1 Comment

user647314 Over a year ago

yes, but there is a commented line : // return x as Needle; // doesnt work. I just want to prove it works but ugly

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