Object.prototype.toString()

The toString() method returns a string representing the object.

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Syntax

toString()

Return value

A string representing the object.

Description

An object's toString() method is most commonly invoked when that object undergoes...

Note: This assumes the object does not have a custom implementation of Symbol.toPrimitive. If it does, that method will take priority and be called instead of toString().

While not as common, the method can be invoked directly (for example, myObject.toString()).

By default toString() returns "[object Type]", where Type is the object type.

const o = new Object().toString() // o is "[object Object]";

This method is inherited by every object descended from Object, but can be overridden by either the author or built-in descendant objects (for example, Number.prototype.toString()).

Note: Starting in JavaScript 1.8.5, toString() called on null returns [object Null], and undefined returns [object Undefined], as defined in the 5th Edition of ECMAScript and subsequent Errata.

See Using toString() to detect object class.

Parameters

By default toString() takes no parameters. However, objects that inherit from Object may override it with their own implementation that do take parameters. For example, the toString() methods implemented by Number and BigInt take an optional radix parameter.

Examples

Overriding the default toString method

You can create a function to be called in place of the default toString() method. The toString() function you create must return a primitive. If it returns an object and the method is called implicitly (i.e. during type conversion or coercion), then its result will be ignored and the value of a related method, valueOf(), will be used instead, or a TypeError will be thrown if none of these methods return a primitive.

The following code defines the Dog object type and creates theDog, an object of type Dog:

function Dog(name, breed, color, sex) {
  this.name = name;
  this.breed = breed;
  this.color = color;
  this.sex = sex;
}

const theDog = new Dog('Gabby', 'Lab', 'chocolate', 'female');

If you call the toString() method on this custom object, it returns the default value inherited from Object:

theDog.toString(); // returns [object Object]

The following code creates and assigns dogToString() to override the default toString() method. This function generates a string containing the name, breed, color, and sex of the object, in the form "property = value;".

Dog.prototype.toString = function dogToString() {
  return 'Dog ' + this.name + ' is a ' + this.sex + ' ' + this.color + ' ' + this.breed;
}

Or, using template strings:

Dog.prototype.toString = function dogToString() {
  return `Dog ${this.name} is a ${this.sex} ${this.color} ${this.breed}`;
}

With the preceding code in place, any time toString() is used in a Dog context, JavaScript automatically calls the dogToString() function, which returns the following string:

"Dog Gabby is a female chocolate Lab"

Using toString() to detect object class

toString() can be used with every object and (by default) allows you to get its class.

To use the base Object.prototype.toString() with an object that has had it overridden, you need to call Function.prototype.call() or Function.prototype.apply() on it, passing the object you want to inspect as the first parameter (called thisArg).

const toString = Object.prototype.toString;

toString.call(new Date);    // [object Date]
toString.call(new String);  // [object String]
toString.call(Math);        // [object Math]

// Since JavaScript 1.8.5
toString.call(undefined);   // [object Undefined]
toString.call(null);        // [object Null]

Using toString() in this way is unreliable; objects can change the behavior of Object.prototype.toString() by defining a Symbol.toStringTag property, leading to unexpected results. For example:

const myDate = new Date();
Object.prototype.toString.call(myDate);     // [object Date]

myDate[Symbol.toStringTag] = 'myDate';
Object.prototype.toString.call(myDate);     // [object myDate]

Date.prototype[Symbol.toStringTag] = 'prototype polluted';
Object.prototype.toString.call(new Date()); // [object prototype polluted]

Specifications

Specification
ECMAScript Language Specification
# sec-object.prototype.tostring

Browser compatibility

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See also