Question

we kown for..in loop does not care if the property is the attribute of object’s prototype-chain .so it will output the attribute all.
and let us do a test

var o={};
for(var i in o){
    console.log(o[i]);//  output  none
}

a object(created by this way:{}) is from Object,
but when we run the code,the Object prototype-chain method (like toString )is not outputed
why the method (toString) in prototype-chain of Object is not outputed ????

Answer 1

toString is not enumerable:

("toString" in {}) === true
({}).propertyIsEnumerable("toString") === false

You can create your own non-enumerable properties like so:

var object = {};

Object.defineProperty(object, "property", { enumerable: false, value: 5 });

_{Technically you can omit the enumerable: false, since that is the default.}

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This is indicated in the spec in the blanket section 15 clause:

In every case, the length property of a built-in Function object described in this clause has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }. Every other property described in this clause has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified.