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
    Creating Scrolling Parallax Effects with CSS

    Introduction For quite a long time now websites with the so called "parallax" effect have been really popular. In case you have not heard of this effect, it basically includes different layers of images that are moving in different directions or with different speed. This leads to a...

  • By
    Chris Coyier’s Favorite CodePen Demos II

    Hey everyone! Before we get started, I just want to say it's damn hard to pick this few favorites on CodePen. Not because, as a co-founder of CodePen, I feel like a dad picking which kid he likes best (RUDE). But because there is just so...

Incredible Demos

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!