Foundations Updated 2026-09 View as Markdown

Objects

Objects hold keyed values, and in TypeScript the set of keys and their types is part of the type.

JavaScript is a functional language, and for object oriented programming it uses both objects and functions, but objects are usually used as a data structure, similar to a dictionary in Python or a map in Java. In this tutorial, we will learn how to use objects as a data structure. The advanced tutorials explain more about object oriented JavaScript.

To initialize an object, use curly braces:

let emptyObject = {};
let personObject = {
    firstName : "John",
    lastName : "Smith"
}

Member addressing#

Members of objects can be addressed using the brackets operator [], very much like arrays, but just like many other object oriented languages, the period . operator can also be used. They are very similar, except for the fact that brackets return a member by using a string, in contrast to the period operator, which requires the member to be a simple word (the word should not contain spaces, start with a number or use illegal characters).

For example, we can continue to fill the person object with more details:

let personObject = {
    firstName : "John",
    lastName : "Smith"
}
personObject.age = 23;
personObject["salary"] = 14000;

Iteration#

Iterating over members of a dictionary is not a trivial task, since iterating over objects can also yield members who don’t actually belong to an object. Therefore, we must use the hasOwnProperty method to check that the member in fact belongs to the object.

for (let member in personObject)
{
    if (personObject.hasOwnProperty(member))
    {
        console.log("the member " + member + " of personObject is " + personObject[member])
    }
}

This will eventually print out

the member firstName of personObject is John
the member lastName of personObject is Smith
the member age of personObject is 23
the member salary of personObject is 14000

Note that methods of objects in JavaScript have a fixed order, like arrays.

for (let member in personObject)
{
    if (personObject.hasOwnProperty(member))
    {
        console.log("the member " + member + " of personObject is " + personObject[member])
    }
}

the member firstName of personObject is John
the member lastName of personObject is Smith
the member age of personObject is 23
the member salary of personObject is 14000