Adding objects to an existing array of objects - JavaScript

The reason your code isn't working the way you think it should is because of this line:

var contactsLength = contacts.length;

Here, you're assigning the value of contacts.length to the variable contactsLength, but thereafter, contactsLength will not change even if the size of your contacts array changes.

Instead, you should simply refer to the property contacts.length.

Here is a corrected version of your code:

var bob = {
    firstName: "Bob",
    lastName: "Jones",
    phoneNumber: "(650) 777-7777",
    email: "[email protected]"
};

var mary = {
    firstName: "Mary",
    lastName: "Johnson",
    phoneNumber: "(650) 888-8888",
    email: "[email protected]"
};

var contacts = [bob, mary];

function add (firstName, lastName, phoneNumber, email) {
    contacts[contacts.length] = {
        firstName: firstName,
        lastName: lastName,
        phoneNumber: phoneNumber,
        email: email
    };
};


function printPerson(person) {
    console.log(person.firstName + " " + person.lastName);
}

function list() {
    for (var i = 0; i < contacts.length; i++) {
        printPerson(contacts[i]);
    }
}

add("MJ", "Foster", "MJ@gmail", "714-333-5555");

list();

Please note: using the native array method push(), would do exactly the same same thing as contacts[contacts.length] = {...}.

Try to use push method

 function add (firstName, lastName,  phoneNumber, email) { 
var newContact = { 
    firstName: firstName, 
    lastName: lastName, 
    phoneNumber: phoneNumber, 
    email: email 
    }; 
    contacts.push(newContact);
 };

Your Add function works fine, although a better way would be to use a push() method as per @MysterX as keeping a track of array index might not always prove very reliable especially if you are using public variable for tracking it. push() method

Your error is in your printPerson function. Have a closer look on your for loop and change it a bit into this

function list() {
    for (var i = 0; i <= contactsLength; i++) {
        printPerson(contacts[i]);
    }
}

and see if you can spot the error...

Hope this helps.