why Object.prototype.toString return [object Object] [closed]

Answer 1

See answers from @Leo and @Joel Gregory for an explanation from the spec. You can display an objects' contents using JSON.stringify, e.g.:

const log = (...args) => document.querySelector("pre").textContent += `${args.join("\n")}\n`;
const obj = { name: 'John' }

log(obj.toString());
log(JSON.stringify(obj));

<pre></pre>

Answer 2

From Mozilla https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/toString:

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.

Answer 3

According to ECMAScript Language Specification:

15.2.4.2 Object.prototype.toString ( ) When the toString method is called, the following steps are taken:

1. If the this value is undefined, return "[object Undefined]".
2. If the this value is null, return "[object Null]".
3. Let O be the result of calling ToObject passing the this value as the argument.
4. Let class be the value of the [[Class]] internal property of O.
5. Return the String value that is the result of concatenating the three Strings "[object ", class, and "]".

The language is designed like this. You'd have to ask Brendan Eich, or TC39, I guess.