Object.prototype.toString()

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

A string representing the object.

JavaScript calls the toString method to convert an object to a primitive value. You rarely need to invoke the toString method yourself; JavaScript automatically invokes it when encountering an object where a primitive value is expected.

This method is called in priority by string conversion, but numeric conversion and primitive conversion call valueOf() in priority. However, because the base valueOf() method returns an object, the toString() method is usually called in the end, unless the object overrides valueOf(). For example, +[1] returns 1, because its toString() method returns "1", which is then converted to a number.

All objects that inherit from Object.prototype (that is, all except null-prototype objects) inherit the toString() method. When you create a custom object, you can override toString() to call a custom method, so that your custom object can be converted to a string value. Alternatively, you can add a @@toPrimitive method, which allows even more control over the conversion process, and will always be preferred over valueOf or toString for any type conversion.

To use the base Object.prototype.toString() with an object that has it overridden (or to invoke it on null or undefined), 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 arr = [1, 2, 3];

arr.toString(); // "1,2,3"
Object.prototype.toString.call(arr); // "[object Array]"

Object.prototype.toString() returns "[object Type]", where Type is the object type. If the object has a Symbol.toStringTag property whose value is a string, that value will be used as the Type. Many built-in objects, including Map and Symbol, have a Symbol.toStringTag. Some objects predating ES6 do not have Symbol.toStringTag, but have a special tag nonetheless. They include (the tag is the same as the type name given below):

• Array
• Function (anything whose typeof returns "function")
• Error
• Boolean
• Number
• String
• Date
• RegExp

The arguments object returns "[object Arguments]". Everything else, including user-defined classes, unless with a custom Symbol.toStringTag, will return "[object Object]".

Object.prototype.toString() invoked on null and undefined returns [object Null] and [object Undefined], respectively.

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

The following code defines a Dog class.

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

If you call the toString() method, either explicitly or implicitly, on an instance of Dog, it returns the default value inherited from Object:

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

theDog.toString(); // "[object Object]"
`${theDog}`; // "[object Object]"

The following code overrides the default toString() method. This method generates a string containing the name, breed, color, and sex of the object.

class Dog {
  constructor(name, breed, color, sex) {
    this.name = name;
    this.breed = breed;
    this.color = color;
    this.sex = sex;
  }
  toString() {
    return `Dog ${this.name} is a ${this.sex} ${this.color} ${this.breed}`;
  }
}

With the preceding code in place, any time an instance of Dog is used in a string context, JavaScript automatically calls the toString() method.

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

`${theDog}`; // "Dog Gabby is a female chocolate Lab"

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

const toString = Object.prototype.toString;

toString.call(new Date()); // [object Date]
toString.call(new String()); // [object String]
// Math has its Symbol.toStringTag
toString.call(Math); // [object Math]

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]

• Polyfill of Object.prototype.toString with Symbol.toStringTag support in core-js
• Object.prototype.valueOf()
• Number.prototype.toString()
• Symbol.toPrimitive
• Symbol.toStringTag