which-boxed-primitive

Which kind of boxed JS primitive is this? This module works cross-realm/iframe, does not depend on instanceof or mutable properties, and works despite ES6 Symbol.toStringTag.

Example

1var whichBoxedPrimitive = require('which-boxed-primitive');
2var assert = require('assert');
3
4// unboxed primitives return `null`
5// boxed primitives return the builtin constructor name
6
7assert.equal(whichBoxedPrimitive(undefined), null);
8assert.equal(whichBoxedPrimitive(null), null);
9
10assert.equal(whichBoxedPrimitive(false), null);
11assert.equal(whichBoxedPrimitive(true), null);
12assert.equal(whichBoxedPrimitive(new Boolean(false)), 'Boolean');
13assert.equal(whichBoxedPrimitive(new Boolean(true)), 'Boolean');
14
15assert.equal(whichBoxedPrimitive(42), null);
16assert.equal(whichBoxedPrimitive(NaN), null);
17assert.equal(whichBoxedPrimitive(Infinity), null);
18assert.equal(whichBoxedPrimitive(new Number(42)), 'Number');
19assert.equal(whichBoxedPrimitive(new Number(NaN)), 'Number');
20assert.equal(whichBoxedPrimitive(new Number(Infinity)), 'Number');
21
22assert.equal(whichBoxedPrimitive(''), null);
23assert.equal(whichBoxedPrimitive('foo'), null);
24assert.equal(whichBoxedPrimitive(new String('')), 'String');
25assert.equal(whichBoxedPrimitive(new String('foo')), 'String');
26
27assert.equal(whichBoxedPrimitive(Symbol()), null);
28assert.equal(whichBoxedPrimitive(Object(Symbol()), 'Symbol');
29
30assert.equal(whichBoxedPrimitive(42n), null);
31assert.equal(whichBoxedPrimitive(Object(42n), 'BigInt');
32
33// non-boxed-primitive objects return `undefined`
34assert.equal(whichBoxedPrimitive([]), undefined);
35assert.equal(whichBoxedPrimitive({}), undefined);
36assert.equal(whichBoxedPrimitive(/a/g), undefined);
37assert.equal(whichBoxedPrimitive(new RegExp('a', 'g')), undefined);
38assert.equal(whichBoxedPrimitive(new Date()), undefined);
39assert.equal(whichBoxedPrimitive(function () {}), undefined);
40assert.equal(whichBoxedPrimitive(function* () {}), undefined);
41assert.equal(whichBoxedPrimitive(x => x * x), undefined);
42assert.equal(whichBoxedPrimitive([]), undefined);
43

Tests

Simply clone the repo, npm install, and run npm test