Advanced switch case

Switch case library for advanced switch-case use.

For example, say you are given two numbers and you need to do this:

• if both are prime, return the larger prime number
• if only one of them is prime, return the difference of the two numbers
• if neither are prime, return the sum of the two numbers

Getting this done using native switch-case can be tricky and/or cumbersome.

Using this library, you can do this:

import { switchCase, otherwise } from "adv-switch-case";

const isPrime = (a) => {
  // assume that this function returns true if a is prime,
  // and if not, false
};

const getLarger = (a, b) => (a > b ? a : b);
const getDifference = (a, b) => Math.abs(a - b);

const result = switchCase(
  [(a, b) => isPrime(a) && isPrime(b), getLarger],
  [(a, b) => isPrime(a) || isPrime(b), getDifference],
  [otherwise, (a, b) => a + b]
)(a, b);

// result(10, 3) => 7
// result(10, 10) => 20
// result(2, 7) => 7

Install

npm:

npm install adv-switch-case

yarn:

yarn add adv-switch-case

Use

In your js:

import { switchCase, otherwise } from "adv-switch-case";
// or
// const { switchCase, otherwise } = require('adv-switch-case')

And then use it like so:

const condition1 = (arg1, arg2, ...) = > {
  // do something and return a boolean
}
const condition2 = (arg1, arg2, ...) = > {
  // do something and return a boolean
}

const result1 = (arg1, arg2, ...) = > {
  // do something with the args and return anything
}
const result2 = (arg1, arg2, ...) = > {
  // do something with the args and return anything
}
const result3 = (arg1, arg2, ...) = > {
  // do something with the args and return anything
}

const result = switchCase(
  [condition1, result1],
  [condition2, result2],
  [otherwise, result3]
)(arg1, arg2, ...)

All conditionX and resultX should be a function.

conditionX should be functions that return a boolean value. resultX can be functions that return any value.

Both conditionX and resultX functions will take the same arguments as that passed to the switchCase (ie, (arg1, arg2, ...)).

Just like how you'd have a default case in switch - case, you have to ensure that you add the otherwise condition to the end. This makes sure your case switching is complete.

What happens if no case matches?

If you miss adding an otherwise condition, it's possible your switchCase will throw an error when no other condition is satisified.

This will throw an exception.

So make sure you always include an otherwise condition to the end of your switchCase.

Why is everything a function?

I supposed one could ask: why not something like...

switchCase(
  [3, doSomethingWithArgs],
  [4, doSomeOtherThing],
  [otherwise, doNothing]
)(3); // should run `doSomethingWithArgs(3)`

If your task is to just compare literal values directly, then adv-switch-case is an overkill. You can accomplish the task with a simple switch case from native JS.

The other benefit of making everything a function is that you could have a potentially expensive computation in your condition and it wont run unless preceding conditions have returned false.

What's otherwise?

otherwise is just a function that returns true. It's a shortcut to writing () => true in your switchCase.

Typescript

adv-switch-case comes with a fair bit of TS support.

You can explicitly type your switchCase to let TS check for argument and return types.

For example, let's use the example from the intro.

const result = switchCase(
  [(a, b) => isPrime(a) && isPrime(b), getLarger],
  [(a, b) => isPrime(a) || isPrime(b), getDifference],
  [otherwise, (a, b) => a + b]
)(a, b);

can be typed as:

const result = switchCase(
  [(a: number, b: number) => isPrime(a) && isPrime(b), getLarger],
  [(a: number, b: number) => isPrime(a) || isPrime(b), getDifference],
  [otherwise, (a: number, b: number) => a + b]
)(2, 3);

// typeof result = number
// typeof condition functions inside switchCase will be
// (...args: number[]) => boolean
// and typeof result functions inside switchCase will be
// (...args: number[]) => number

Any arguments to the condition function we use inside switchCase will be type-checked for (a: number, b: number) because the arguments we passed are (2, 3)

result will automatically type to 'number' because the output of the result functions are numbers.

Sometimes, the results of different conditions could be different types. In such cases, explicitly type switchCase like so:

const result = switchCase<number[], number | string>(
  [(a: number, b: number) => isPrime(a) && isPrime(b), getLarger],
  [(a: number, b: number) => isPrime(a) || isPrime(b), getDifference],
  [otherwise, "Neither is a prime."]
)(2, 3);

That is:

switchCase<ArgsArray, ResultTypes>(...)
type ArgsArray = array of sum type of your argument types
type ResultTypes = sum type of your results for all conditions