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!

Cloudinary

Recent Features

  • By
    CSS Gradients

    With CSS border-radius, I showed you how CSS can bridge the gap between design and development by adding rounded corners to elements.  CSS gradients are another step in that direction.  Now that CSS gradients are supported in Internet Explorer 8+, Firefox, Safari, and Chrome...

  • By
    Introducing MooTools Templated

    One major problem with creating UI components with the MooTools JavaScript framework is that there isn't a great way of allowing customization of template and ease of node creation. As of today, there are two ways of creating: new Element Madness The first way to create UI-driven...

Incredible Demos

  • By
    MooTools Wall Plugin

    One of the more impressive MooTools plugins to hit the Forge recently was The Wall by Marco Dell'Anna.  The Wall creates an endless grid of elements which can be grabbed and dragged, fading in elements as they are encountered.  Let me show...

  • By
    Create a CSS Flipping Animation

    CSS animations are a lot of fun; the beauty of them is that through many simple properties, you can create anything from an elegant fade in to a WTF-Pixar-would-be-proud effect. One CSS effect somewhere in between is the CSS flip effect, whereby there's...

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!