TypeScript Total Functions

A collection of total functions to replace TypeScript's built-in partial functions.

Intended to be used with strictNullChecks enabled.

Installation

# yarn
yarn add total-functions

# npm
npm install total-functions

The Functions

get (type-safe array index operator)

The array index operator is not well-typed in TypeScript:

const a: object[] = [];
const b = a[0]; // b has type object, not object | undefined as you might expect
b.toString(); // boom

const record = { foo: "foo" } as Record<string, string>;
const bar = record["bar"]; // bar has type string, not string | undefined
bar.toUpperCase(); // boom

get is a safe alternative:

import { get } from "total-functions";

const b = get(a, 0); // b has type object | undefined

const bar = get(record, "bar"); // bar has type string | undefined

Note that get will exclude undefined from the return type when there is enough type information to be confident that the result cannot be undefined. See the object and tuple examples below for examples where undefined is not included in the return type.

More usage examples:

// tuple
const xs = [1, 2, 3] as const;
const x1 = get(xs, 1); // 2
const x100 = get(xs, 100); // undefined
const xMinus1 = get(xs, -1); // undefined
xs.map(x => x /* 1 | 2 | 3 */);

// array
const ys = [1, 2, 3];
const y1 = get(ys, 1); // number | undefined
const y100 = get(ys, 100); // number | undefined
ys.map(y => y /* number */);

// sparse array
const zs = [1, , 2, 3];
const z1 = get(zs, 1); // number | undefined
const z100 = get(zs, 100); // number | undefined
zs.map(z => z /* number | undefined */);

// readonly array
const as = [1, 2, 3] as ReadonlyArray<1 | 2 | 3>;
const a1 = get(as, 1); // 1 | 2 | 3 | undefined
const a100 = get(as, 100); // 1 | 2 | 3 | undefined

// record
const record = { 1: "asdf" } as Record<number, string>;
const record1 = get(record, 1); // string | undefined
const record100 = get(record, 100); // string | undefined

// object
const obj = { 1: "asdf" };
const obj1 = get(obj, 1); // string
const obj100 = get(obj, 100); // doesn't compile

// const object
const constObj = { 1: "asdf" } as const;
const constObj1 = get(constObj, 1); // "asdf"
const constObj100 = get(constObj, 100); // doesn't compile

ESLint

There's also a corresponding ESLint rule to ban the unsafe array index operator.

See https://github.com/danielnixon/eslint-plugin-total-functions

See Also

• https://github.com/danielnixon/readonly-types
• https://github.com/jonaskello/eslint-plugin-functional