What sets Function Types apart, and how do they prove beneficial in various scenarios?

Checking out the TypeScript documentation available at: https://www.typescriptlang.org/docs/handbook/functions.html, we come across the concept of "Function Types." An example code snippet illustrating this is:

let myAdd: (x: number, y: number) => number =
    function(x: number, y: number): number { return x+y; };

In contrast, a "non-typed" function would look like this:

let myAdd = function(x: number, y: number): number { return x+y; };

The additional syntax in the first example might seem peculiar. Why is there an arrow-function syntax for specifying the return type? And why are there two parameter specifications?

Answer №1

When the function's content is uncertain at variable declaration, typing the function can be beneficial:

let myOperation: (x: number, y: number) => number;

if ( // some condition ) {
  myOperation = function (x: number, y: number): number { return x + y; };
} else {
  myOperation = function (x: number, y: number): number { return x * y; }
}

It is important to note that in certain contexts such as interfaces, describing the type with or without an arrow notation is feasible:

interface WithoutArrow {
  operation(x: number, y: number): number;
}

interface WithArrow {
  operation: (x: number, y: number) => number;
}

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