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.
var whichBoxedPrimitive = require('which-boxed-primitive'); var assert = require('assert'); // unboxed primitives return `null` // boxed primitives return the builtin constructor name assert.equal(whichBoxedPrimitive(undefined), null); assert.equal(whichBoxedPrimitive(null), null); assert.equal(whichBoxedPrimitive(false), null); assert.equal(whichBoxedPrimitive(true), null); assert.equal(whichBoxedPrimitive(new Boolean(false)), 'Boolean'); assert.equal(whichBoxedPrimitive(new Boolean(true)), 'Boolean'); assert.equal(whichBoxedPrimitive(42), null); assert.equal(whichBoxedPrimitive(NaN), null); assert.equal(whichBoxedPrimitive(Infinity), null); assert.equal(whichBoxedPrimitive(new Number(42)), 'Number'); assert.equal(whichBoxedPrimitive(new Number(NaN)), 'Number'); assert.equal(whichBoxedPrimitive(new Number(Infinity)), 'Number'); assert.equal(whichBoxedPrimitive(''), null); assert.equal(whichBoxedPrimitive('foo'), null); assert.equal(whichBoxedPrimitive(new String('')), 'String'); assert.equal(whichBoxedPrimitive(new String('foo')), 'String'); assert.equal(whichBoxedPrimitive(Symbol()), null); assert.equal(whichBoxedPrimitive(Object(Symbol()), 'Symbol'); assert.equal(whichBoxedPrimitive(42n), null); assert.equal(whichBoxedPrimitive(Object(42n), 'BigInt'); // non-boxed-primitive objects return `undefined` assert.equal(whichBoxedPrimitive([]), undefined); assert.equal(whichBoxedPrimitive({}), undefined); assert.equal(whichBoxedPrimitive(/a/g), undefined); assert.equal(whichBoxedPrimitive(new RegExp('a', 'g')), undefined); assert.equal(whichBoxedPrimitive(new Date()), undefined); assert.equal(whichBoxedPrimitive(function () {}), undefined); assert.equal(whichBoxedPrimitive(function* () {}), undefined); assert.equal(whichBoxedPrimitive(x => x * x), undefined); assert.equal(whichBoxedPrimitive([]), undefined);
Simply clone the repo, npm install
, and run npm test