type-checked-collections

 

^{Social Media Photo by Julius Drost on Unsplash}

Collections that actually type-check at runtime, not only in IDEs.

``js
import { typedSet, typedMap, typedWeakSet, typedWeakMap } from 'type-checked-collections';

// define typeof or instanceof where
// * typeof is a string with possible unions in it
// * instanceof is a Class or an array of classes (unions)

// Set define the kind of value they can use
/* @type {Set} /
const StringSet = typedSet({typeof: 'string'});

const myStrings = new StringSet(['a', 'b']);
myString.add('c');

// this throws a TypeError
myString.add(1);

// Map and WeakMap define a key/value pair where
// null(ish) key or value means: any
const StringKeyMap = typedMap([
// key
{typeof: 'string | number'},
// value as any
null
]);
`

It is possible to define either keys or values as a combination of typeof and instanceof:

`js
import { typedSet } from 'type-checked-collections';

const TypedSet = typedSet({
// accept typeof string and typeof number
typeof: 'string | number',

// accept only instance of String
instanceof: String,
// ... or ...
// accept instance of String or instance of Number
instanceof: [String, Number],

// optionally it is possible to pass an error handler
onerror(/* @type {string} / message, value) {
throw new TypeError(message);
// value is literally whatever value was used
// and failed either as key or as value
},
});

const ts = new TypedSet([
new String('a'),
'b',
1,
new Number(2)
]);

// throws a TypeError
ts.add({});
ts.add(null);
ts.add(void 0);
ts.add(true);
`

| typeof | runtime check |
| :-------- | :-------------------------------------------- |
| string | typeof value === 'string' |
| boolean | typeof value === 'boolean' |
| number | typeof value === 'number' |
| function | typeof value === 'function' |
| symbol | typeof value === 'symbol' |
| undefined | typeof value === 'undefined' |
| bigint | typeof value === 'bigint' |
| object | typeof value === 'object' && value !== null |
| null | value === null |


The instanceof single or multiple class value is checked via instanceof operator.

Performance

If typeof value is specified, it is the first performed check and it is the fastest check in ECMAScript.

If the instanceof value is defined, an instanceof operator is used to validate further in case previous typeof, if present, failed.

In Map and WeakMap cases, if either the key or the value is null or undefined, the check is a no-op such as () => true.

Last, but not least, if there is only one type to check, either one typeof without unions or one instanceof class only, the check is done directly without looping through the array of possible valid types (as in: best possible perf).

$3

If you are happy with the development experience and you'd like to drop all checks and use just native Set, Map or others, there is a type-checked-collections/dummy export that is a drop-in replacement that results into a no-op, meaning all you get is the native Set, Map` or others constructors resulting in zero runtime overhead.

This could be handy also for perf comparison (in case you believe the issue is caused by this library) or debugging sessions (to have the least amount of code to step into while debugging).