为什么函数返回 undefined 而不是给定的元素？

Question

I'm trying to solve the problem below:

// Given an object and a key, "getNthElementOfProperty" returns the nth element of an array located at the given key.

// SAMPLE DATA:

    var obj = {
      key: [1, 2, 6]
    };
    var output = getNthElementOfProperty(obj, 'key', 1);
    console.log(output); // --> 2

    function getNthElementOfProperty(obj, key, n) {
      if(obj[key] === undefined){
        return undefined
      } else if(Array.isArray(obj[key]) === false){
        return undefined
      // otherwise if array empty
      } else if(obj[key].length === 0){
        return undefined
      }
      for(var e in obj[key]){
        if(e === n){
          return obj[key][e];
        } 
      }
      return undefined;
    }

I have tested and all below conditions have been met:

If the array is empty, it should return undefined.
If n is out of range, it should return undefined.
If the property at the given key is not an array, it should return undefined.
If there is no property at the key, it should return undefined.

The function returns undefined instead of the nth element. I'm not looking for the answer, but instead why it's returning undefined .

: https://i.stack.imgur.com/Wk1dC.png

Answer 1

Just remove the loop. JavaScript returns undefined if the index is not inside the array. You can also get rid of if (array.length === 0) because the index can't be inside an array of length 0.

function getNthElementOfProperty(obj, key, n) {
  if(obj[key] === undefined){
    return undefined
  } else if(Array.isArray(obj[key]) === false){
    return undefined
  }   
  return obj[key][n];
}

If you want to know why your loop didn't work:

for (let i in array) assigns the index as a string to i . The === operator checks for types and sees that '0' !== 0 for example.

Answer 2

Turns out that instead of a for in loop I needed to use a regular for loop.

function getNthElementOfProperty(obj, key, n) {
    if(obj[key] === undefined){
      return undefined
    } else if(Array.isArray(obj[key]) === false){
      return undefined
    } else if(obj[key].length === 0){
      return undefined
    }
    for(var i = 0; i < obj[key].length; i++){
        if(i === n){
            return obj[key][i]
        }
    }
    return undefined;
  };

Answer 3

Simple function with all validation no need for loop, you can use hasOwnProperty instead of obj[key] === undefined

function getNthElementOfProperty(obj, key, n) {
    //validate obj and key and n
    if (!obj || !key || isNaN(n)) {
        return undefined;
        // if key in not in Object
    } else if (!obj.hasOwnProperty(key)) {
        return undefined;
        // if property at key is not a array
    } else if (!Array.isArray(obj[key])) {
        return undefined;
        // if array is empty
    } else if (!obj[key].length) {
        return undefined;
        // if n is greater than Array length 
    } else if (obj[key].length < n) {
        return undefined;
    } else {
        return obj[key][n]
    };
};