Object.prototype.toString()
The toString() method returns a string representing the object. This method is meant to be overridden by derived objects for custom type conversion logic.
Syntax
toString()
Parameters
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.
Return value
A string representing the object.
Description
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 Symbol.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):
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.