Generator sort

Sorting on multiple criteria with generator functions

Background

With the Array.sort() method in javascript, it is possible to sort an array of anything based on a sorting function. For example, if you have an array of objects like this:

let characters = [
    {
        firstName: 'Luke',
        lastName: 'Skywalker',
    },
    {
        firstName: 'Han',
        lastName: 'Solo'
    }
    {
        firstName: 'Anakin',
        lastName: 'Skywalker'
    }
]

And you want to sort them based on lastName first and then firstName, you have to write a sort-function like this:

characters.sort( (a,b) => {
    if(a.lastName > b.lastName) {
        return 1
    }
    else if(a.lastName < b.lastName) {
        return -1
    }
    else {
        if(a.firstName > b.firstName) {
            return 1
        }
        else if(a.firstName < b.firstName) {
            return -1
        }
        else {
            return 0
        }
    }
})

Writing these sorting functions is not particularly difficult, but they can become quite long and rarely look elegant.

Sorting with generator functions

With the sort-generator module, the same sort can be written as

import {sortFunction, compareStrings} from 'generator-sort'

characters.sort(sortFunction(function*(oA, oB) {
    yield compareStrings(oA.lastName, oB.lastName)
    yield compareStrings(oA.firstName, oB.lastName)
}))

The sortFunction function takes a generator function as its only argument. The generator function will be called for each pair of object that shall be compared. The function shall yield numbers. If it yields a positive number, oA is larger than oB. If it yields a negative number, oB is larger than oA. If it yields 0, it will be asked to yield another number until it yields a non-zero number or is done (i.e. returns).

This makes it easy to write the comparison as a series of yield statements.

Typescript

The module is written in typescript and comes with complete typings, but can be used in plain javascript as well.