JavaScript: Object.prototype.toString

By Xah Lee. Date: . Last updated: .
Object.prototype.toString()

The input of this function is the value of this Binding.

Return a string representation. The result is usually one of the following:

  • "[object Undefined]"
  • "[object Null]"
  • "[object Array]"
  • "[object String]"
  • "[object Arguments]"
  • "[object Function]"
  • "[object Error]"
  • "[object Boolean]"
  • "[object Number]"
  • "[object Date]"
  • "[object RegExp]"
  • "[object Object]"
  • "[object JSON]"
  • "[object Set]"
  • "[object Map]"
  • "[object GeneratorFunction]"
  • "[object Generator]"
  • (and WeakMap, WeakSet, Proxy, Promise, etc)

If input is a object, and has a Symbol key property Symbol.toStringTag, either own property or inherited, and its value is a string, let's say the value is tag, then the result is "[object tag]"

Usually used like this:

Object.prototype.toString.call(obj)

[see Function.prototype.call]

or

Reflect.apply( Object.prototype.toString , obj, [] )

[see Reflect.apply]

Often used to find out the “type of object”. For example, {} is a data object, but /abc/ is regexp object.

[see Determine Type of Object]

const fGetType = ((x) => Reflect.apply(Object.prototype.toString, x, []));

console.log(
  fGetType({}) === "[object Object]" &&
    fGetType([]) === "[object Array]" &&
    fGetType(/./) === "[object RegExp]" &&
    fGetType(new Date()) === "[object Date]" &&
    fGetType(function f() {}) === "[object Function]" &&
    fGetType((x) => 3) === "[object Function]" &&
    fGetType(1) === "[object Number]" &&
    fGetType("") === "[object String]" &&
    fGetType(true) === "[object Boolean]" &&
    fGetType(new Set()) === "[object Set]" &&
    fGetType(new Map()) === "[object Map]",
);
// true

Example of object that has property key Symbol.toStringTag and value is a string.

let uu = {};
uu[Symbol.toStringTag] = "xyz";
console.log( Object.prototype.toString.call ( uu ) === "[object xyz]"); // true

[see Symbol Tutorial]

[see Symbol Object]

History of Object.prototype.toString

JavaScript language by spec does not have the concept of “different type of objects” at the language level.

The typeof operator on Array, RegExp, Date, JSON, etc all just return "object".

The only exception is function, which typeof a_function returns "function".

[see Value Types]

However, JavaScript has some internal way to determine what type of object it is.

In ECMAScript 5.1 §8#sec-8.6.2 (2011), object has a internal slot denoted “[[Class]]”.

Its value is a string, representing the classification of the object.

This [[Class]] is what we think of the “type of object”.

Object.prototype.toString() return the value of internal slot [[class]], in this string form "[object [[Class]]]"

(ECMAScript 5.1 §15#sec-15.2.4.2)

The possible return values are:

etc

But in ES2015, the internal slot “[[Class]]” is removed from the spec. (probably because to avoid confusion with new ES2015 class feature. [see Class] )

However, the behavior of Object.prototype.toString() is kept for backward compatibility, but how it works has changed.

ECMAScript 2015 §Fundamental Objects#sec-object.prototype.tostring

Now, if the object has a symbol property key Symbol.toStringTag, then that is used as part of the return value. Else, it falls back to ES5 behavior as is.

ECMAScript 2015 added new kinds of objects, such as Set Object and Map Object . New object's prototypes typically have symbol properties Symbol.toStringTag. For example, for set object, the value is the string "Set".

The symbol property Symbol.toStringTag can be set by user.

ECMAScript 2015 added this note:

NOTE Historically, this function was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification as a nominal type tag for various built-in objects. The above definition of toString preserves compatibility for legacy code that uses toString as a test for those specific kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of built-in or program defined objects. In addition, programs can use @@toStringTag in ways that will invalidate the reliability of such legacy type tests.

ECMAScript 2015 §Fundamental Objects#sec-object.prototype.tostring

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