@browndragon/switch-type

Install & use with $ npm i @browndragon/switch-type and then

import switchType from '@browndragon/switch-type';
switchType(
    undefined, 
    {default(x, ...params) {console.log(`Default ${x} & params ${params}!`);}},
    'someParam1', 'someParam2'
);
// Prints 'Default undefined & params someParam1,someParam2!'

Why switchType?

I got very fed up with the mess that is javascript type introspection, so I wrote my own.

switchType(unknownObject, handler) will do type analysis on the unknown object and then invoke the correctly named method on handler with the object.

The methods it will call are (roughly in order, omit irrelevants):

• undefined (will also try null)
• null (will also try undefined)
• boolean (a value)
• bigint (a value)
• number (a value)
• string (a value and an iterable)
• function (a value)

Then we start considering (non-null) objects;

• array (an iterable): Anything that is Array.isArray.
• map (an associative and an iterable) -- uses instanceof Map (as do the other specialized types).
• set (an iterable)
• iterable: Anything with a Symbol.iterator on it (including collections & arrays & strings).
• regExp: An instanceof RegExp.
• literal: an object whose prototype is literally Object (as created by Object.create and the {} syntax). Also associative.
• associative: An object which @browndragon/obj can get/set fields on. A Map or an object literal.

Finally, objects will try to call:

• object: Any non-null js typeof(x)=='object'.

Finally-finally, anything will try to call:

• default: Anything that wasn't otherwise matched.

switchType returns the result of the called method or else undefined.

What about other "root types"?

To make this work, I've invented several new root types which "feel like" javascript basic types. To me.

• The values are mostly unobjectionable.
• Everyone agrees the undefined/null/class Object situation is messy; this is just a semi-opinionated, semi-flexible way to cut through that. I don't consider undefined or null to be values in this library.
• Object is the root of the object hierarchy, but is also used directly as a container.
  • As a container type, it's called a literal.
  • As the root of the object hierarchy, it's customized into a few builtin es6 types (Map, Set, Array, RegExp). You can add more with withHandler (which returns a new instance with the additional handlers) or addHandler (which modifies the handler in-place, possibly including the globally imported one!).