function.md

Function and closure

Declaration

There are three kind of syntax to define a function:

1. Function declaration syntax

   function add(x: number, y: number): number {
       return x + y;
   }

   • if this is in global scope
     • This defines a function with name add, and it can be directly called through the name add.
   • if this is inside another function scope
     • This defines a function with a mangled name which can not be directly invoked, the compiler will implicitly create a closure with the given name add in the outer scope,

2. Function expression syntax

   let add = function(x: number, y: number): number {
       return x + y;
   }

   This defines an anonymous function, and assigned to variable add as a closure

3. Arrow function syntax

   let add = (x: number, y: number): number => {
       return x + y;
   }
   let add1 = (x: number, y: number): number => x + y;

   This defines two anonymous functions, and assigned to variable add and add1 as closure

Parameters

Optional parameter

function add(x: number, y: number, z?: number): number {
    if (z) {
        // type narrowing not supported, currently must explicitly cast the type
        let z_value: number = z;
        return x + y + z_value;
    } else {
        return x + y;
    }
}

Default parameter

function add(x: number, y: number, z: number = 0): number {
    return x + y + z;
}

Note: Default parameter is not supported in class static method and closure

Rest parameter

function add(x: number, y: number, ...z: number[]): number {
    let sum = x + y;
    for (let i = 0; i < z.length; i++) {
        sum += z[i];
    }
    return sum;
}

Function type and type alias

type Add = (x: number, y: number) => number;

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

let add1: Add = function(x: number, y: number): number {
    return x + y;
}

Limitations

• Function overload is not supported

  function add(x: number, y: number): number;
  function add(x: string, y: string): string;
  function add(x: any, y: any): any {
      return x + y;
  }