Simple pattern matching for ES6

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Simple pattern matching for ES6 (no transpilation!)

Property matching

const { match } = require('xmatch');

match(obj, [
	({ x }) => console.log('x', x),
	({ y }) => console.log('y', y),
	({ z }) => console.log('z', z),
	// Exhaustive match; will throw `xmatch.UnmatchedPatternError` unless uncommented:
	// other => console.error('Something else', other),
]);

Iterable matching

const { match } = require('xmatch');

match(arr, [
	([]) => 'empty',
	([x]) => `x=${x}`,
	([x, y]) => `x=${x},y=${y}`,
	([x, y, ...{ length }]) => `x=${x},y=${y},rest.length=${length}`,
]);

Custom guards

const { match, guard } = require('xmatch');

match(obj, [
	({ command }) => {
		// When you want to match simple values:
		guard(command === 'ignore');
		/* Do nothing */
	},
	({ command }) => {
		// Or, say, match result of regex:
		let [, name, args] = guard(command.match(/^(\w+):(.*)$/));
		console.log({ name, args });
	},
	({ command }) => {
		throw new Error(`Invalid command: ${command}`);
	},
]);

Shape assertions

const { guard } = require('xmatch');

const { x, y } = guard({ x: 1, y: 2 }); // OK
const { x, y } = guard({ x: 1, z: 2 }); // throws `xmatch.UnmatchedPatternError`

Known issues

• You can't use literals directly in patterns (this is limitation of ES6 syntax, can be fixed as part of https://github.com/tc39/proposal-pattern-matching).
• You can't use default values for parameters. This is limitation of the way matching is implemented, and you'll have to resolve defaults yourself if that's what you want.
• Trying to further destructure or access undefined properties of an object will also trigger the match guard (#1). This is tricky to workaround without changing the syntax, but I'll look into it (and happy to hear any suggestions).
• This uses dynamic metaprogramming via Proxy which might have undesirable performance effect on hot code paths. If your benchmarks suggest it's causing critical performance issues, consider using transpiler plugins instead.
• Proxy is not implemented in pre-ES6 browsers and can't be polyfilled, so use this only if you're okay with the supported target set: https://caniuse.com/#feat=proxy