Javascript - understanding recursive inner function

Question

The following code prints the path of the key searched by its value inside an object:

function findKeyPath(obj, keyAttr, keyValue) {
  let path = "";

  (function findKey (obj, keyAttr, keyValue, keyPath) {

    const prevPath = `${keyPath ? keyPath + "." : ""}`;
    if (obj[keyAttr] === keyValue) {

      path = `${prevPath}${keyAttr}`;

    } else if (typeof obj === "object" && JSON.stringify(obj) === JSON.stringify(keyValue)) {

      path = prevPath.slice(0, -1); // remove last dot
    }
    if (typeof obj === "object" && !Array.isArray(obj)) {
      for (const key in obj) {

        if (Array.isArray(obj[key])) {
          for (let j = 0; j < obj[key].length; j++) {

            findKey(obj[key][j], keyAttr, keyValue, `${prevPath}${key}[${j}]`);
          }
        }
        if (obj[key] !== null && typeof obj[key] === "object") {

          findKey(obj[key], keyAttr, keyValue, `${prevPath}${key}`);
        }
      }
    }
  }(obj, keyAttr, keyValue));

  return path;
}

console.log(
  findKeyPath({
    users: [
      { name: "sam", surname: "red", age: 12 },
      { name: "sam", surname: "red", age: 42 },
      { name: "sam", surname: "red", age: 16 }
    ]
  }, "age", 16)
);

... it prints users[2].age.

Trying to taking out the inner function, like this:

function findKey(obj, keyAttr, keyValue, keyPath) {
  const prevPath = `${keyPath ? keyPath + "." : ""}`;

  if (obj[keyAttr] === keyValue) {

    return `${prevPath}${keyAttr}`;

  } else if (typeof obj === "object" && JSON.stringify(obj) === JSON.stringify(keyValue)) {

    return prevPath.slice(0, -1); // remove last dot
  }
  if (typeof obj === "object" && !Array.isArray(obj)) {
    for (const key in obj) {

      if (Array.isArray(obj[key])) {
        for (let j = 0; j < obj[key].length; j++) {

          findKey(obj[key][j], keyAttr, keyValue, `${prevPath}${key}[${j}]`);
        }
      }
      if (obj[key] !== null && typeof obj[key] === "object") {

        findKey(obj[key], keyAttr, keyValue, `${prevPath}${key}`);
      }
    }
  }
}

console.log(
  findKey({
    users: [
      { name: "sam", surname: "red", age: 12 },
      { name: "sam", surname: "red", age: 42 },
      { name: "sam", surname: "red", age: 16 }
    ]
  }, "age", 16)
);

It prints undefined. How can I solve it?

UPDATE, now it works:

function findKey(obj, keyAttr, keyValue, keyPath) {

    const prevPath = `${keyPath ? keyPath + "." : ""}`;

    let path = "";

    if (obj[keyAttr] === keyValue) {
        return `${prevPath}${keyAttr}`;
    } else if (typeof obj === "object" && JSON.stringify(obj) === JSON.stringify(keyValue)) {
        return prevPath.slice(0, -1); // remove last dot
    }

    if (typeof obj === "object" && !Array.isArray(obj)) {
        for (const key in obj) {
            if (Array.isArray(obj[key])) {
                for (let j = 0; j < obj[key].length; j++) {
                    path = path.concat(findKey(obj[key][j], keyAttr, keyValue, `${prevPath}${key}[${j}]`));
                }
            }

            if (obj[key] !== null && typeof obj[key] === "object") {
                return path.concat(findKey(obj[key], keyAttr, keyValue, `${prevPath}${key}`));
            }
        }
    }

    return path;
}

Answer 1

One problem with second code snippet I can think of is, we have path as a variable who is getting updated throughout the findKey function calls, it is a sort of "protected variable" which the findKey function can access and modify. In second code snippet we are not modifying the variable but returning it directly to the caller, which might not return anything for this part of code

if (typeof obj === "object" && !Array.isArray(obj)) {
for (const key in obj) {

  if (Array.isArray(obj[key])) {
    for (let j = 0; j < obj[key].length; j++) {

      findKey(obj[key][j], keyAttr, keyValue, `${prevPath}${key}[${j}]`);
    }
  }
  if (obj[key] !== null && typeof obj[key] === "object") {

    findKey(obj[key], keyAttr, keyValue, `${prevPath}${key}`);
  }
}

Imagine findKey getting called here and returning things as per your expectation but then we are not returning anything from this function, hence the undefined is getting returned. We might need to introduce a variable and store results and return it for this part of code.