@types/which-boxed-primitive/index.d.ts

TypeScript definitions for which-boxed-primitive

export = whichBoxedPrimitive;

/**
 * Which kind of boxed JS primitive is this? This function works cross-realm/iframe,
 * does not depend on `instanceof` or mutable properties, and works despite ES6 `Symbol.toStringTag`.
 *
 * @example
 * import assert = require('node:assert');
 * import whichBoxedPrimitive = require('which-boxed-primitive');
 *
 * // 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(BigInt(42)), null);
 * assert.equal(whichBoxedPrimitive(Object(BigInt(42))), '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(() => {}), undefined);
 * assert.equal(whichBoxedPrimitive(function* () {}), undefined);
 * assert.equal(whichBoxedPrimitive((x: number) => x * x), undefined);
 * assert.equal(whichBoxedPrimitive([]), undefined);
 */
declare function whichBoxedPrimitive<T>(value: T): WhichBoxed<T>;

type WhichBoxed<T> = T extends string | number | boolean | symbol | bigint | null | undefined ? null
    : T extends String ? "String"
    : T extends Number ? "Number"
    : T extends Boolean ? "Boolean"
    : T extends Symbol ? "Symbol"
    : T extends BigInt ? "BigInt"
    : undefined;