Basically, SQL COALESCE in JavaScript; return the first value that is not null-like.

Extant installs from NPM.

npm install extant

Extant is an implementation of SQL's COALESCE that I've used for some time to deal with the fact that JavaScript truthiness will treat '' and 0 as true so the || operator can't always be used to create given or default one-liner.

const { coalesce } = require('extant')

function foo (count) {
    count = coalesce(count, 1)
    for (let i = 0; i < count; i++) {
        console.log('hello')
    }
}

foo()

We use the name "extant" on NPM because we want the first extant argument.

This README.md is also a unit test using the Proof unit test framework. We'll use the Proof okay function to assert out statements in the readme. A Proof unit test generally looks like this.

require('proof')(4, async okay => {
    okay('always okay')
    okay(true, 'okay if true')
    okay(1, 1, 'okay if equal')
    okay({ value: 1 }, { value: 1 }, 'okay if deep strict equal')
})

You can run this unit test yourself to see the output from the various code sections of the readme.

git clone git@github.com:bigeasy/duplicitous.git
cd duplicitous
npm install --no-package-lock --no-save
node test/readme.t.js

The 'extant' module exports a single coalesce function.

const { coalesce } = require('extant')

Note that Extant is SQL's COALESCE. It returns the first non-null-like value, that is the first value that is not == null, which would be null or undefined. If there is no such argument it returns null.

const maybe = null
let maybeNot
okay(coalesce(maybe, maybeNot, 1), 1, 'return first non-null value')
okay(coalesce(maybe, maybeNot), null, 'return null')
okay(coalesce(), null, 'no arguments is also null')

I've used this module for a long time to initialize function arguments.

function increment (value, increment) {
    return value + coalesce(increment, 1)
}

okay(increment(1, 2), 3, 'specify parameter')
okay(increment(1), 2, 'use default parameter')

Although these days I'd probably just use the JavaScript default parameters syntax.

function increment (value, increment = 1) {
    return value + increment
}

okay(increment(1, 2), 3, 'specify default parameter')
okay(increment(1), 2, 'use default default parameter')

And when Node.js 14 goes end-of-life I'll switch to ??, but for now its still useful when initializing from named parameters when you want to preserve the original options array. You can always do this for simple cases.

function increment (value, { increment = 1 } = {}) {
    return value + increment
}

okay(increment(1, { increment: 2 }), 3, 'specify destructed named parameters')
okay(increment(1), 2, 'use default destructed named parameters')

But, if you wanted keep the options object around for some reason coalesce still comes in handy.

function increment (value, options = {}) {
    return value + coalesce(options.increment, 1)
}

okay(increment(1, { increment: 2 }), 3, 'specify named arguments')
okay(increment(1), 2, 'use default named arguments')

Really would take an example where your constructing an object that has a lot of options, or nested options. At that point the destructuring gets too verbose and coalesce is more appropriate.

Another place where it is useful is variadic functions where destructed and default parameters are not an option.

function adjuster (value, ...vargs) {
    const increment = typeof vargs[0] == 'boolean' ? vargs.shift() : true
    const by = coalesce(vargs.shift(), 1)
    return value + (by * (increment ? 1 : -1))
}

okay(adjuster(1, true, 2), 3, 'increment variadic')
okay(adjuster(1, false), 0, 'decrement variadic')