What does this type declaration mean in Typescript?

Question

I\'m new to Typescript and I\'m reading over someone else\'s code and having trouble with this declaration:

    private somevar: { [s: string]: string };
         


        

Answer 1

It's an indexable type. Looking at this variable definition's type expression:

let myIndexVar: { [key: string]: number; };

• : { ... } means it's an object.
• [key: string]: number; is the index signature of the object.

Within the index signature:

• [key: string] defines the name of the key—key—and the type of the key—string.
• : number; is the type of the value—number.

These types are used like so:

let myIndexVar: { [key: string]: number; } = {};

myIndexVar["key"] = 4; // key is string, value is number

Note that you can give the key any name you like. Giving it a descriptive name is helpful to tell what the key is, though the variable name should also do that: