Here we have a type declaring a point in two dimensional space with the members X and Y of type number. We create a constant variable declaration for this point with the members X and Y, both pointing to the value zero. Using a cons declaration ensures that we cannot assign a different value to this variable. However, in JavaScript, a Cons declaration does not prevent us from assigning different values to the members of the variable. Fortunately, TypeScript gives you the opportunity to add a read only modifier to different members of a type. So if you make X as read only, we can no longer assign

to this property after the declaration. And of course, we can do the same for the Y member to make that as read-only. And of course, marking a property as Read-only gives you access to read the property. You just cannot reassign it. The read only modifier in TypeScript is a compiled time only feature used for error checking and does not require any runtime JavaScript support. That's it for types. Now let's look at JavaScript classes. Here we have a simple JavaScript class called Animal with a member name that can be initialized.

Using the constructor, we create an instance of this class passing in the name sheep. Now at this point, we can read the name member as well as modify it. For example, put a wolf in sheep's clothing. Now, if you want to disallow the assignment of the name property after the constructor, we can use the same read-only modifier before the property name. And now if you try to write to the name property after the constructor, we get a compile time error from TypeScript. Now, just like other class access modifiers, public, private, and projected,

the read only modifier in TypeScript is a compile time only feature used for compile time error checking, and does not require runtime JavaScript support. Now, one final thing worth mentioning is that we can combine read only with other class member access modifiers. For example, public.