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Node.js Object TypeTags

String representations of object in Node.js

This package contains a list of default string representations of objects in Node.js, as returned from Object.prototype.toString.call(value) — see the full list here.

📦 typetags

NPM

npm install typetags

Yarn

yarn add typetags

Check the reference here

Usage ⚡️

Typetags

The TypeTags object contains a list of strings, each representing an object type. You can use it to check if an object's typetag matches the default tag of its data type.

import { TypeTags } from 'typetags'

const o = { foo: 'bar' }

if (o.toString() !== TypeTags.Object) {
  doSomething()
}

Helpers

The getTag function can be used to retrieve an object's typetag. Behind the curtains, all it does is to call Object.prototype.toString over any value and return the result.

const { getTag } = require('typetags')

const o = { foo: 'bar' }

const tag = getTag(o)

console.log(tag) // → '[object Object]'

Predicates

There are two kinds of predicate functions to help us with assertions: generics and specifics.

Generics

Used for overall checks. For example, to check if an object has a default typetag:

const { isDefaultTag } = require('typetags')

const o = Object.create(null)

isDefaultTag(o) // → false

Specifics

Allow us to check for specific tags:

const { isArrayTag, getTag } = require('typetags')

const arr = [1, 2, 3]
const tag = arr.toString() // → 1,2,3

isArrayTag(tag) // → false

// however
console.log(getTag(arr)) // → '[object Array]'

Check this page for a reference list of all predicates

Bundles

esm

import { isDefaultTag } from 'typetags'

cjs

const { isDefaultTag } = require('typetags')

umd

<script src="dist/umd/typetags.min.js"></script>

Using typetags to detect object class

Every object has a toString() method that is automatically called when the object is to be represented as a text value or when an object is referred to in a manner in which a string is expected.

By default, the toString() method is inherited by every object descended from Object. If this method is not overridden in a custom object, toString() returns "[object type]", where type is the object type. The following code illustrates this:

const o = new Object()
o.toString() // → [object Object]

^{See Mozilla MDN Web Docs}

Although using toString() in this way is unreliable, as objects can change the behavior of Object.prototype.toString() this library might be helpful when you need to compare objects' tags inside a procedure call or if you just need a quick reference.

const { TypeTags, getTag } = require('typetags')

function isArray (value) {
  return typeof value === 'object' && getTag(value) === TypeTags.Array
}