Function Type in TypeScript

JavaScript can have variables that store functions. In this case, TypeScript allows us to specify that the variable is of type "function". The type of a function is a combination of the parameter types and the return type. This combination is called the signature of the function.

To specify a function type for a variable, you need to list the parameters and their types in parentheses, and after the arrow => specify the return type. Let's look at an example. Let's declare a variable as containing a function:

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

Let's write a function of a given type into this variable:

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

Specify the function type for the variable:

let func = function(text: string): void {
	alert(text);
};