Learn TypeScript 3 by Building Web Applications

An example with generics

Because TypeScript supports generics, it can actually leverage them for its own libraries.

If you look at the lib.es5.d.ts (https://github.com/Microsoft/TypeScript/blob/master/lib/lib.es5.d.ts) file (which is part of TypeScript) and search for Array<T>, you'll stumble upon this definition:

interface Array<T> { 
    ... 
    push(...items: T[]): number; 
    pop(): T; 
    ... 
} 

As you can see, the Array interface accepts a generic parameter (inside the angle brackets: <>) named T. This T is actually just a label used to refer to a type that will be defined/given at runtime. That same label is then used in function definitions.

If we observe the push method, we can see that it accepts an array of T elements. The pop method returns ...