We can mark a variable to store only functions using Function type in TypeScript.

## Setting Function Type

Here is how we set `Function`

type:

`let a: Function;`

Here is a valid assignment:

`a = function () {};`

Here is an invalid assignment:

`a = 10;`

Now TypeScript compiler says:

`Type 'number' is not assignable to type 'Function'.`

## Function Type with more specifics

Here is a Function type that accepts two numbers and return a number.

`let a: (num1: number, num2: number) => number;`

Now `a`

can accept only a function that has two number parameters and that returns a number type. No other function formats are accepted.