如何检索数组的元素作为对象属性

Question

My question is why is this function not passing the test that it should return the element at the index of the array at the key of the passed in object .

The problem is:

Given an object, a key, and a numerical index, "getElementOfArrayProperty" returns the value of the element at the given index of the array located within the given object at the given key.

Notes: * If the array is empty, it should return undefined.

• If the given index is out of range of the array located at the given key, 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.

When I pass the sample object: var obj = { key: ['Jamil', 'Albrey'] }; it returns "jamil" .

My Code:

     function getElementOfArrayProperty(obj, key, index) {

    if (!obj["key"]) return undefined
      return obj["key"][index]
    }

    getElementOfArrayProperty(obj, "key", 0)  // returns "jamil"

Yet it still fails the test

Answer 1

I think Nol is right. The problem is that there is a little confusion with key as a property / parameter and "key" as a string literal. Let's see if this clear the things up:

 var obj = { foo: ['Jamil', 'Albrey'] }; function getElementOfArrayProperty(obj, key, index) { if (!obj[key]) return undefined; return obj[key][index]; } alert( getElementOfArrayProperty(obj, "foo", 0) ); 

EDIT : And if you are wondering why "foo" is between quotes, that's because it is not a variable, it is a string literal with the name of the object's property. If you want to pass a variable as parameter, first you need to create it, like:

var bar = obj["foo"]; // or  
var bar = obj.foo;

And then you can use it in the function invocation:

alert( getElementOfArrayProperty(obj, bar, 0) );

But in this case, you don't need to pass the obj variable, because bar already refers to one property of the obj object:

 var obj = { foo: ['Jamil', 'Albrey'] }; var bar = obj.foo; function getElementOfArrayProperty(key, index) { if (!key) return undefined; return key[index]; } alert(getElementOfArrayProperty(bar, 0)); 

Answer 2

I'm not sure if this is a typo, but in your code example, you are passing in the string "key" to obj like obj["key"] instead of using the parameter key from your method definition. Using obj[key] might improve your results.

Here's what I meant:

  var obj = { key: ['Jamil', 'Albrey'] }; function getElementOfArrayProperty(obj, key, index) { if (!obj[key]) { return undefined; } return obj[key][index]; } function getArrayByKey(obj, key) { return obj[key]; } alert(getElementOfArrayProperty(obj, "key", 0) + "\\n" + getArrayByKey(obj, "key")); 

You'll see that this doesn't return undefined if you run it, instead it returns Jamil just like your example with the hard-coded "key" .

If I look for just obj[key] I still end up getting the whole array object, so I can't seem to reproduce your issue.