TypeScript Functions

TypeScript 4.0

Variadic Tuple Types

Pre 4.0 situation

Consider the following function using Tuples:

function concat(arr1, arr2) {
    return [...arr1, ...arr2];
}

The only way to type this in TypeScript used to be:

function concat(arr1: [], arr2: []): [];
function concat<A>(arr1: [A], arr2: []): [A];
function concat<A, B>(arr1: [A, B], arr2: []): [A, B];
function concat<A, B, C>(arr1: [A, B, C], arr2: []): [A, B, C];
function concat<A, B, C, D>(arr1: [A, B, C, D], arr2: []): [A, B, C, D];
function concat<A, B, C, D, E>(arr1: [A, B, C, D, E], arr2: []): [A, B, C, D, E];
function concat<A, B, C, D, E, F>(arr1: [A, B, C, D, E, F], arr2: []): [A, B, C, D, E, F];)
 
function concat<A2>(arr1: [], arr2: [A2]): [A2];
function concat<A1, A2>(arr1: [A1], arr2: [A2]): [A1, A2];
function concat<A1, B1, A2>(arr1: [A1, B1], arr2: [A2]): [A1, B1, A2];
function concat<A1, B1, C1, A2>(arr1: [A1, B1, C1], arr2: [A2]): [A1, B1, C1, A2];
function concat<A1, B1, C1, D1, A2>(arr1: [A1, B1, C1, D1], arr2: [A2]): [A1, B1, C1, D1, A2];
function concat<A1, B1, C1, D1, E1, A2>(arr1: [A1, B1, C1, D1, E1], arr2: [A2]): [A1, B1, C1, D1, E1, A2];
function concat<A1, B1, C1, D1, E1, F1, A2>(arr1: [A1, B1, C1, D1, E1, F1], arr2: [A2]): [A1, B1, C1, D1, E1, F1, A2];

Post 4.0 situation

1. Tail example

   Consider the following example:

   function tail(arg) {
       const [_, ...result] = arg;
       return result
   }

   TypeScript 4.0 bring 2 changes here.

   1. Changes

      1. Generic spreads in tuple types

         The spreads in tuple type syntax to be generic:

         function tail<T extends any[]>(arr: readonly [any, ...T]) {
             const [_ignored, ...rest] = arr;
             return rest;
         }
          
         const myTuple = [1, 2, 3, 4] as const;
         const myArray = ["hello", "world"];
          
         // type [2, 3, 4]
         const r1 = tail(myTuple);
          
         // type [2, 3, 4, ...string[]]
         const r2 = tail([...myTuple, ...myArray] as const);

      2. Rest elements can occur anywhre in a tuple

         Rest elements can occur anywhere in a tuple, not just at the end:

         type Strings = [string, string];
         type Numbers = [number, number];
          
         // [string, string, number, number, boolean]
         type StrStrNumNumBool = [...Strings, ...Numbers, boolean];

2. Concat example

   Note that in cases when we spread in a type without a known length, the resulting type becomes unbounded as well, and all the following elements factor into the resulting rest element type.

   type Strings = [string, string];
   type Numbers = number[]
    
   // [string, string, ...Array<number | boolean>]
   type Unbounded = [...Strings, ...Numbers, boolean];

   Therefore the concat() example can be typed with:

   type Arr = readonly any[];
    
   function concat<T extends Arr, U extends Arr>(arr1: T, arr2: U): [...T, ...U] {
       return [...arr1, ...arr2];
   }

Examples

Parameter annotations

function tralala(tra: number, la: number) {}

Return type annotation

interface Foo {
    bar: string
}
 
function tralala(): Foo {
    return {bar: 'tralala'}
}

Optional Parameters

function foo(bar: number, optional?: string) {}

function foo(bar: number, optional: string = 'This is an optional string') {}

Overloading

This is for documentation purposes but imho just makes things more confusing.

// Overloads
function padding(all: number);
function padding(topAndBottom: number, leftAndRight: number);
function padding(top: number, right: number, bottom: number, left: number);
// Actual implementation that is a true representation of all the cases the function body needs to handle
function padding(a: number, b?: number, c?: number, d?: number) {
    if (b === undefined && c === undefined && d === undefined) {
        b = c = d = a;
    }
    else if (c === undefined && d === undefined) {
        c = a;
        d = b;
    }
    return {
        top: a,
        right: b,
        bottom: c,
        left: d
    };
}
 
padding(1); // Okay: all
padding(1,1); // Okay: topAndBottom, leftAndRight
padding(1,1,1,1); // Okay: top, right, bottom, left
 
padding(1,1,1); // Error: Not a part of the available overloads

Declaring Functions

Use this to declare function type without providing implementation:

type LongHand = {
    (a: number): number;
};
 
type ShortHand = (a: number) => number;

Overloading is supported as well:

type LongHandAllowsOverloadDeclarations = {
    (a: number): number;
    (a: string): string;
};