Error in a simple JS code

Hello everyone.

I’ve been doing fcc courses when I arrived at the lasts JS challenges.

The point is to write a simple function that will go trough a contact list to find a property based via the first name of that contact.

The contact list is stored in an object. Here’s what the code looks like when I started :

var contacts = [
        "firstName": "Akira",
        "lastName": "Laine",
        "number": "0543236543",
        "likes": ["Pizza", "Coding", "Brownie Points"]
        "firstName": "Harry",
        "lastName": "Potter",
        "number": "0994372684",
        "likes": ["Hogwarts", "Magic", "Hagrid"]
        "firstName": "Sherlock",
        "lastName": "Holmes",
        "number": "0487345643",
        "likes": ["Intriguing Cases", "Violin"]
        "firstName": "Kristian",
        "lastName": "Vos",
        "number": "unknown",
        "likes": ["Javascript", "Gaming", "Foxes"]

function lookUpProfile(firstName,prop){
// Only change code below this line

// Only change code above this line

// Change these values to test your function

I tried to use a while function that would go trough the contact list and stop as soon as it find a match between the argument “firstName” and the first name of one of the contact. Here’s what my code looks like :

 var i = 0;
  while (contacts[i].firstName != firstName|| i>=contacts.length){
  return contacts[i][prop];

It’s all working so far, and everything work, but I stumbled upon a mistake that, it seems, comes from nowhere :

If I type a name that’s not on the contact list, such as ‘lookUpProfile(“Bob”,“number”);’ the console will display “Type Error : Cannot read property ‘firstName’ of undefined”. However, if I’m not mistaken, since I told the code to stop looking if " i " exeeds the array length, it should normally stop at 3 and simply return the property of the 4th contact, regardless of the name I type in the function.

I really do not understand what I did wrong, I know that I do not need to solve this for succeeding in the challenge, but it really bugs me and I can’t go forward before figuring where the hell this error comes from, since I tought I put a correct barrier to overlooping the contact’s array length.

Thanks a bunch to those who had the courage to read the whole thing. See ya !

edit : well, it was just a stupid mistake of mine, sorry !


if I’m not mistaken, since I told the code to stop looking if " i " exeeds the array length,
you are correct on the concept but not in the implementation:
your code should look like

 while (contacts[i].firstName != firstName || i < contacts.length ){

this means i should never exceed the array length. The way you put it stands for the contrary: i should always exceed the array length:

Also, you should consider this solution:

var firstName = 'Sherlock'
var prop = 'likes'
var matches = contacts.filter(function(e) {
   return e.firstName === firstName;
if(!matches.lenght) return false;

return matches[0][prop]

You can read more about Array.filter in:

I also recommend to take a look to:

They both are great when managing arrays of objects