Require Parameters for JavaScript Functions

By  on  

JavaScript is notorious for being "loose", something that some developers love but other developers loathe.  I hear most of those complaints from server side developers, who want string typing and syntax.  While I like strict coding standards, I also like that JavaScript lets me quickly prototype without having to cross the I's and dot the T's.  Until recently you couldn't define default parameter values for functions in JavaScript, but now you can!

When I posted last week about Six Tiny but Awesome ES6 Features, an awesome reader (cmwd) pointed out that you can not only set default function parameter values but you can throw errors when a given parameter isn't provided to a function:

const isRequired = () => { throw new Error('param is required'); };

const hello = (name = isRequired()) => { console.log(`hello ${name}`) };

// This will throw an error because no name is provided
hello();

// This will also throw an error
hello(undefined);

// These are good!
hello(null);
hello('David');

I love this tip -- it shows how with each addition to JavaScript we can stretch the language to do interesting things.  How practical it is to throw errors in production is up to you but this is an awesome ability during development.  Happy coding!

Recent Features

  • By
    6 Things You Didn’t Know About Firefox OS

    Firefox OS is all over the tech news and for good reason:  Mozilla's finally given web developers the platform that they need to create apps the way they've been creating them for years -- with CSS, HTML, and JavaScript.  Firefox OS has been rapidly improving...

  • By
    fetch API

    One of the worst kept secrets about AJAX on the web is that the underlying API for it, XMLHttpRequest, wasn't really made for what we've been using it for.  We've done well to create elegant APIs around XHR but we know we can do better.  Our effort to...

Incredible Demos

  • By
    5 More HTML5 APIs You Didn’t Know Existed

    The HTML5 revolution has provided us some awesome JavaScript and HTML APIs.  Some are APIs we knew we've needed for years, others are cutting edge mobile and desktop helpers.  Regardless of API strength or purpose, anything to help us better do our job is a...

  • By
    HTML5 Input Types Alternative

    As you may know, HTML5 has introduced several new input types: number, date, color, range, etc. The question is: should you start using these controls or not? As much as I want to say "Yes", I think they are not yet ready for any real life...

Discussion

  1. decksterr

    Would it still show an exception for say hello(undefined) ? or hello(null) ?

    Just curious, not having set up any ES6 environment yet …

  2. Yes for undefined, no for null (“Hello null”)

  3. Augusto Borges de Moura

    With Method Parameter Decorators proposal, you can even do function (@Required arg) {...}

  4. One trick to do with required params is using TypeError instead of “plain” Error. If you try to call one of the builtin functions without the right amount of params, you’ll get a TypeError, so makes sense to have custom code do that too :)

  5. Adam van den Hoven

    its a little verbose, but you could pass the parameter name into isRequired to get better error messages.

  6. Interesting! And apparently arguments is already available to the default function parameter: https://jsfiddle.net/2447ksz7/

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!