[Javascript Crocks] Understand the Maybe Data Type

In this lesson, we’ll get started with the Maybe type. We’ll look at the underlying Just and Nothing types and create a simple utility function to create a Maybe from a value. Then we’ll see how we can apply functions to values and skip invocation for values that might cause errors or unexpected results.

Most of time, we will meet this kind of problem in our code:

const {inc} = require('./utils');

const res = inc(2); // 3
const res1 = inc('2'); // 21
const res2 = inc(undefined); // NaN

We expect the function 'inc' can help to increase the value by 1, but if we pass the string type of undefined, we won't get the result we want.

Of course, you can update your inc function like that:

// from
const inc = n => n + 1;

// to
const inc = n => typeof n === 'number' ? n + 1: 0;

module.exports = {
    inc
};

But it only works if you have full control of your code, if you are using a thrid-party lib, then it doesn't work.

Install:

npm i -S crocks

By import Maybe from crocks lib:

const Maybe = require('crocks/Maybe');

You two APIs to use:

Maybe.Just(2) // Just 2 
Maybe.Nothing() // Nothing

Update our code:

// utils.js
const inc = n => n + 1;

module.exports = {
    inc
};


// index.js
const {inc} = require('./utils');
const Maybe = require('crocks/Maybe');

const safeNum = n => typeof n === 'number' ? Maybe.Just(n) : Maybe.Nothing();
const input = safeNum(2); // Just 3
// const input = safeNum('2'); // Nothing
// const input = safeNum(undefined); // Nothing
const result = input
    .map(inc);

console.log(result); // Just 3

Now we get 'Just 3' as a result, but we want the actual value and also apply a default value 0:

const safeNum = n => typeof n === 'number' ? Maybe.Just(n) : Maybe.Nothing();
const input = safeNum(2); // 3
// const input = safeNum('2'); // 0
// const input = safeNum(undefined); // 0

const result = input
    .map(inc)
    .option(0) // unwrap the value and give a default value;

console.log(result);