chooser

A small, but fully featured, library that aims to replace switch case logic (mainly to aid with Functional Programming), with a simple declarative API for setting up a predefined map of cases, and choosing a result from it based on input.

Features

• Map cases to results.
• Map multiple cases to the same result.
• Customize the equality function used to produce a match, either at case declaration, or at the time of input.
• Refer from one case to another by ref-ing its index.
• Refer from one case to another by use-ing its when value.
• Leverage the lazy nature of functions to compute a result only when needed, and not at the time of the case declaration.
• If a function is intended to be returned as is, it can be put under the eager key.
• Written in TypeScript and bundled with Rollup, featuring:
  • A CommonJS module for node.
  • An ES Module for node.

Installation

$ npm install chooser

Usage

const { chooser } = require('chooser');

const choose = chooser([
  { when: [1, 2], then: 'one' },
  { when: 3, then: 'two' },
  { when: 4, ref: 0 },
  { when: 5, use: 3 },
]);

choose(1); // => 'one'
choose(2); // => 'one'
choose(3); // => 'two'
choose(4); // => 'one'
choose(5); // => 'two'

Terminology

Choices

The choices parameter should be either a plain js object with keys (for cases) and values (for results), or an array of choice objects adhering to the choice schema.

Choice

The choice objects are objects that adhere to the following schema:

type Choice = {
  when: any;
  then?: any;
  ref?: number;
  use?: any;
  eager?: Function;
};

Examples

Basic example:

const { chooser } = require('chooser');

const choose = chooser([
  { when: [1, 2], then: 'one' },
  { when: 3, then: 'two' },
  { when: 4, ref: 0 },
  { when: 5, use: 3 },
]);

choose(1); // => 'one'
choose(2); // => 'one'
choose(3); // => 'two'
choose(4); // => 'one'
choose(5); // => 'two'

Leveraging lazy evaluation:

const { chooser } = require('chooser');

const choose = chooser([
  { when: 1, then: () => 'one' },
  { when: 2, then: () => 'two' },
]);

choose(1); // => 'one'
choose(2); // => 'two'

Overriding lazy evaluation to return the function as is:

const { chooser } = require('chooser');

const fn = () => 'one';

const choose = chooser([
  { when: 1, then: fn },
  { when: 2, eager: fn },
]);

choose(1); // => 'one'
choose(2); // => Function: fn

Using a plain object:

const { chooser } = require('chooser');

const choose = chooser({
  1: 'one',
  2: 'two',
});

choose(1); // => 'one'
choose('1'); // => 'one'
choose(2); // => 'two'
choose('2'); // => 'two'

Note: When using a plain object without specifying an equalityFn (see below), the input is automatically converted to a string.

Using use shorthand in plain objects:

const choose = chooser({
  1: 'one',
  2: 'two',
  3: '{#2}',
});

choose(1); // => 'one'
choose(2); // => 'two'
choose(3); // => 'two'

Using ref shorthand in plain objects:

const { chooser } = require('chooser');

const choose = chooser({
  1: 'one',
  2: 'two',
  3: '{$0}',
});

choose(1); // => 'one'
choose(2); // => 'two'
choose(3); // => 'one'

Note: A caveat of using a plain object as choices is that in the case of numbered keys, the keys are automatically sorted by the language. This can lead to wrong refs.

Demonstrating the sorted keys problem:

const { chooser } = require('chooser');

const choose = chooser({
  2: 'two',
  1: 'one',
  3: '{$0}',
});

choose(3); // => 'one' instead of the usually expected 'two'

Providing a custom equalityFn at case declaration:

const { chooser } = require('chooser');

const equalityFn = (input, when) => input.nested.value == when;

const choose = chooser(
  [
    { when: 1, then: 'one' },
    { when: 2, then: 'two' },
  ],
  undefined,
  equalityFn,
);

const obj1 = { nested: { value: 1 } };
const obj2 = { nested: { value: 2 } };

choose(obj1); // => 'one'
choose(obj2); // => 'two'

Providing a custom equalityFn at the time of input:

const equalityFnPerson = (input, when) => input.person.age == when;
const equalityFnDog = (input, when) => input.dog.age == when;

const choose = chooser([
  { when: 28, then: 'I am 28 years old!' },
  { when: 2, then: 'Woof woof!' },
]);

const data = {
  person: { age: 28 },
  dog: { age: 2 },
};

choose(data, equalityFnPerson); // => 'I am 28 years old!'
choose(data, equalityFnDog); // => 'Woof woof!'

Specifying a defaultValue:

const { chooser } = require('chooser');

const choose = chooser(
  [
    { when: 1, then: 'one' },
    { when: 2, then: 'two' },
  ],
  'default string',
);

choose(3); // => 'default string'

Closing Notes

Thank you for using this library. I hope it helps you write better, more declarative code, without any duplication or resorting to functions with switch cases or mapper functions.