An Easier Way to Enforce Required Parameters in ES6

Expands on Handling required parameters in ECMAScript 6 by Axel Rauschmayer.

The idea (which is credited to Allen Wirfs-Brock) is, in essence, to use default parameter values to call a function which throws an Error if the parameter is missing:

const throwIfMissing () => { throw new Error('Missing parameter') }

const foo = (mustBeProvided = throwIfMissing()) => {}

foo() // ==> Error: Missing parameter

This idea is great. However, the Error message does not tell you which parameter is missing, which could be quite helpful if the function had multiple parameters or an option object with multiple options, for example. To give more accurate Error feedback, we could refactor the code as follows:

const throwIfMissing = p => { throw new Error(`Missing parameter: ${p}`) }

const foo = (mustBeProvided = throwIfMissing('mustBeProvided')) => {}

foo() // ==> Error: Missing parameter: mustBeProvided

Now, the code for such a seemingly simple task, enforcing that a parameter is required, has become quite long, however: minus the parameter name, throwIfMissing('') is 18 characters long.

Since this is such a fundamental and often used task, it makes sense to make it as short and easy-to-type as possible.

If we use template literal syntax foo´string´ for calling the function instead of the normal round brackets syntax foo('string'), we can already reduce the number of characters by 2.

const throwIfMissing = p => { throw new Error(`Missing parameter: ${p}`) }

const foo = (mustBeProvided = throwIfMissing`mustBeProvided`) => {}

foo() // ==> Error: Missing parameter: mustBeProvided

Nevertheless, it is still 16 characters long, excluding the parameter name, of course. Now, the only option left to reduce the number of characters needed for this simple task is to rename the throwIfMissing function.

There are quite a few shorter alternatives which I like, for example:

• throwIfNo (e.g. throwIfNo´password´)
• enforce (e.g. enforce´password´)
• ensure (e.g. ensure´password´)

My favorite, however, is just x. Why an x?

• Well, the x represents a check mark: [x].
• It's also super short. Now, we're from 18 characters down to 3 (plus the parameter name).

Before:

const throwIfMissing () => { throw new Error('Missing parameter') }

const foo = (mustBeProvided = throwIfMissing()) => {}

foo() // ==> Error: Missing parameter

After:

const x = p => { throw new Error(`Missing parameter: ${p}`) }

const foo = (mustBeProvided = x`mustBeProvided`) => {}

foo() // ==> Error: Missing parameter: mustBeProvided

Simple "real world" example using throw-if-missing:

const x = require('throw-if-missing')

const login = ({ username = x`username`, password = x`password` } = {}) => {}

login({ username: 'C-3PO' }) // ==> Error: Missing password