Define Callback Function Type in TypeScript

Functions accept callback function as argument. Using TypeScript we can define the type of that function.

Here is a function that accepts another function as callback:

function parent(callback) {
  //....
  callback(message, statusCode);
}

As shown in above code, our assumption is that, message is a string and statusCode is a number. So our callback function accepts a string and a number and returns nothing.

Here is how we can define the type using TypeScript:

function parent(callback: (a: number, b: string) => void) {
  //...
}