浅谈JavaScript原型链

var arr = new Array();
console.log( typeof arr ); // object

var arr = new Array();
var Fn = function() {};
var foo = new Fn();
console.log( arr instanceof Array ); // true
console.log( arr instanceof Object ); // true
console.log( foo instanceof Fn); // true
console.log( foo instanceof Function ); // false
console.log( foo instanceof Object ); // true

console.log( String instanceof String );
console.log( Function instanceof Function );
console.log( Function instanceof Object );
console.log( Object instanceof Function );
console.log( Object instanceof Object );

1. If Type(C) is not Object, throw a TypeError exception.
2. Let instOfHandler be GetMethod(C,@@hasInstance).
3. ReturnIfAbrupt(instOfHandler).
4. If instOfHandler is not undefined, then
a. Return ToBoolean(Call(instOfHandler, C, «O»)).
5. If IsCallable(C) is false, throw a TypeError exception.
6. Return OrdinaryHasInstance(C, O).
NOTE Steps 5 and 6 provide compatibility with previous editions of ECMAScript that did not use a @@hasInstance method to define the instanceof operator semantics. If a function object does not define or inherit @@hasInstance it uses the default instanceof semantics.

1. If IsCallable(C) is false, return false.
2. If C has a [[BoundTargetFunction]] internal slot, then
    a. Let BC be the value of C's [[BoundTargetFunction]] internal slot.
    b. Return InstanceofOperator(O,BC) (see 12.9.4).
3. If Type(O) is not Object, return false.
4. Let P be Get(C, "prototype").
5. ReturnIfAbrupt(P).
6. If Type(P) is not Object, throw a TypeError exception.
7. Repeat
    a. Let O be `O.[[GetPrototypeOf]]()`.
    b. ReturnIfAbrupt(O).
    c. If O is null, return false.
    d. If SameValue(P, O) is true, return true.

function instanceOf( L, R ) { //L 表示左表达式，R 表示右表达式
    var P = R.prototype; // 取 R 的显示原型
    L = L.__proto__; // 取 L 的隐式原型
    while ( true ) { 
        if ( L === null ) return false;
        if ( P === L ) return true; 
        L = L.__proto__; 
    } 
}

obj.__proto__.__proto__ ... = Obj.prototype

var Foo = function() {}
var foo = new Foo;
console.log(foo.__proto__ === Foo.prototype) // true
console.log(Foo.__proto__ === Function.prototype) // true

ObjectL = Object, ObjectR = Object; 

R = ObjectR.prototype = Object.prototype 
L = ObjectL.__proto__ = Function.prototype 

R != L 
// 循环查找 L 是否还有 __proto__ 
L = Function.prototype.__proto__ = Object.prototype 

R == L 
// 返回 true

StringL = String, StringR = String;

R = StringR.prototype = String.prototype
L = StringL.__proto__ = Object.prototype

R != L
// 循环查找 L 是否还有 __proto__ 
L = Object.prototype.__proto__ = null
// 返回 false

'string'.__proto__ === String.prototype // true
'string' instanceof String // false

console.log(typeof 'string'); // string

var s1 = "string";
var s2 = s1.substring(2);

var s1 = new String("some text");
var s2 = s1.substring(2);
s1 = null;