Question

Here is a question in JavaScript below:

// Tested via Google Chrome console.
var toString = Object.prototype.toString;

"foo".toString(); // "foo"
toString.call("foo"); // [object String]

[].toString(); // ""
toString.call([]); // [object Array]

{}.toString(); // syntax error
toString.call({}); // [object Object]

Why the result of toString is different with toString.call() ?

UPDATED

String.prototype.toString.call("foo"); // "foo"
Object.prototype.toString.call("foo"); // [object String]

Is String.prototype.toString not from the prototype chain like below?

toString in String[not found] –> toString in String.prototype[not found]

                           --> toString in Object.prototype[found]

Answer 1

String.prototype.toString overrides Object.prototype.toString. They are not the same function.

From the specification of String.prototype.toString:

Returns this String value. (Note that, for a String object, the toString method happens to return the same thing as the valueOf method.)

And Object.prototype.toString:

When the toString method is called, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let class be the value of the [[Class]] internal property of O.
3. Return the String value that is the result of concatenating the three Strings “[object “, class, and “]“.

Arrays behave similar, they also override toString():

> [1,2].toString()
  "1,2"