Function Relationship with "this"

Function and this

TypeScript has the capability of letting the author of a function specify the type of this. Without it, even with the arrow function, this may be of type any. The lack of a type on this often occurs if you are using an object literal with a function that returns a function that uses this. The function in the object literal is called a function expression and returns any type when the value is accessed with this.

However, we know that the type means the object literal, hence we can specify to TypeScript the type of this one. The syntax is to use this followed by a colon symbol, followed by the expected type as the first parameter of the function signature.

In the following example, line 12 returns any when you may expect a string since the type on line 2 is an array of string.