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which-boxed-primitive

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which-boxed-primitive

Which kind of boxed JS primitive is this?


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Package description

What is which-boxed-primitive?

The `which-boxed-primitive` npm package is designed to help developers identify the type of boxed primitives (objects that encapsulate primitive values) in JavaScript. This can be particularly useful when working with code that needs to distinguish between different types of object wrappers around primitive values, such as `Number`, `String`, `Boolean`, `Symbol`, `BigInt`, etc.

What are which-boxed-primitive's main functionalities?

Identifying boxed primitives

This feature allows developers to pass an object to the `whichBoxedPrimitive` function and receive a string indicating the type of boxed primitive, or `undefined` if the object is not a boxed primitive. This can be useful for type checking and ensuring that certain operations are only performed on the expected types of objects.

"use strict";\nconst whichBoxedPrimitive = require('which-boxed-primitive');\n\nconsole.log(whichBoxedPrimitive(new String('hello'))); // 'String'\nconsole.log(whichBoxedPrimitive(new Number(123))); // 'Number'\nconsole.log(whichBoxedPrimitive(new Boolean(true))); // 'Boolean'\nconsole.log(whichBoxedPrimitive(Object(Symbol('sym')))); // 'Symbol'\nconsole.log(whichBoxedPrimitive(Object(BigInt(10)))); // 'BigInt'\nconsole.log(whichBoxedPrimitive([])); // undefined (not a boxed primitive)"

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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

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);

Tests

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

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Last updated on 14 Dec 2020

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