Object.prototype.toString() - JavaScript 编辑

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

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

obj.toString()

Return value

A string representing the object.

Description

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(); // returns [object Object]

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

See Using toString() to detect object class.

Parameters

For Numbers and BigInts toString() takes an optional parameter radix the value of radix must be minimum 2 and maximum 36.

By using radix you can also convert base 10 numbers (like 1,2,3,4,5,.........) to another base numbers, in example below we are converting base 10 number to a base 2 (binary) number.

let baseTenInt = 10;
console.log(baseTenInt.toString(2));
// Expected output is "1010"

and same for big integers

let bigNum = BigInt(20);
console.log(bigNum.toString(2));
// Expected output is "10100"

Some common radix are

• 2 for binary numbers,
• 8 for octal numbers,
• 10 for decimal numbers,
• 16 for hexadecimal numbers.

Examples

Overriding the default toString method

You can create a function to be called in place of the default toString() method. The toString() method takes no arguments and should return a string. The toString() method you create can be any value you want, but it will be most useful if it carries information about the object.

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;
}

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() {
  const ret = 'Dog ' + this.name + ' is a ' + this.sex + ' ' + this.color + ' ' + this.breed;
  return ret;
}

Or, using ES6 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 theDog is used in a string 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 Object.prototype.toString() with every object, 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 (ECMA-262) The definition of 'Object.prototype.toString' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• Object.prototype.toSource()
• Object.prototype.valueOf()
• Number.prototype.toString()
• Symbol.toPrimitive