Typescript 中的推断类型 -: vs =

Question

https://www.typescriptlang.org/docs/handbook/functions.html#inferring-the-types

Inferring the types In playing with the example, you may notice that the TypeScript compiler can figure out the type even if you only have types on one side of the equation:

// The parameters 'x' and 'y' have the type number
let myAdd = function (x: number, y: number): number {
  return x + y;
};

// myAdd has the full function type
let myAdd2 : (baseValue: number, increment: number) => number = function (x, y) {
  return x + y;
};

What does it mean when : is written instead of = here:

let myAdd =
let myAdd2:

Answer 1

In TypeScript, : is used to explicitly indicate the type of a variable or parameter.

So we have:

let myAdd2

A variable named myAdd2

 : (baseValue: number, increment: number) => number 

Whose type is a function that takes a number called baseValue and another number called increment , and returns a number.

= function (x, y) {
  return x + y;
};

and whose value is this function here.

In the case of myAdd , the function's type is not explicitly specified. The : (baseValue: number, increment: number) => number part is left out. So its type inferred based on the value being assigned to it.