TypeScript - DataTypes - Function Type

 TypeScript has a function type. Not a function, but an actual datatype representing a function. Now, consider our previous example of a add function.

// function add to check for datatypes of arguments

function add(input1:number,input2:number):number{

var myResult = input1 + input2;

return myResult;

}

also, consider the greet function:

//void type

function greet():void{

console.log("Greetings! Welcome to getCity.ts");

}

Now, if we want to assign both add() and greet function to a single variable. We can do it like this:

let myAdd;

myAdd = add;

console.log("calling myAdd() == "+myAdd(2,3));

myAdd = greet;

console.log("calling myAdd() == ");

myAdd();

Now, if we try to compile this:
$ tsc getCity.ts
$

The compiler did not complain. But, we know that greet() function and add() function are clearly different. This is because they both take different number of input parameters and return a different number of return types.

To be able to set a function type, we need a function type. This is so that, we can combine all add() related functions having the same input parameters and return types. To be able to fix the add() function to a datatype, let us see the following:

// function types

let myAdd: (input1:number, input2:number) => number;

In this case, we are saying, the function myAdd()has two input parameters and both are numbers. Also, we have set its return type to be a number. So, now if we assign both add() and greet() function to myAdd() function and try to compile:

// function types

let myAdd: (input1:number, input2:number) => number;

myAdd = add;

console.log("calling myAdd() == "+myAdd(2,3));

myAdd = greet;

console.log("calling myAdd() == ");

myAdd();

On compiling this time:

$ tsc getCity.ts
getCity.ts:29:1 - error TS2322: Type '() => void' is not assignable to type '(input1: number, input2: number) => number'.
  Type 'void' is not assignable to type 'number'.

29 myAdd = greet;
   ~~~~~

getCity.ts:31:1 - error TS2554: Expected 2 arguments, but got 0.

31 myAdd();
   ~~~~~~~

  getCity.ts:26:13
    26 let myAdd: (input1:number, input2:number) => number;
                   ~~~~~~~~~~~~~
    An argument for 'input1' was not provided.

$

.. the compiler complains.  This is because myAdd() is now a function type. To be able to make the compiler to compile. We can remove associating greet() function with myAdd().

// function types

let myAdd: (input1:number, input2:number) => number;

myAdd = add;

console.log("calling myAdd() == "+myAdd(2,3));

$ tsc getCity.ts
$

Now, the compiler has compiled the code as myAdd() function type is associated with the correct function.