ES6 允许扩展特殊对象。所以可以从函数继承。这样的对象可以作为函数调用，但我如何实现这样的调用的逻辑呢？
class Smth extends Function {
constructor (x) {
// What should be done here
super();
}
}
(new Smth(256))() // to get 256 at this call?
类的任何方法都通过this. 但是当它作为函数调用时，this指的是window. 当它作为函数调用时，如何获取对类实例的引用？
PS：同样的问题在俄语中。
该super调用将调用Function构造函数，该构造函数需要一个代码字符串。如果你想访问你的实例数据，你可以硬编码它：
class Smth extends Function {
constructor(x) {
super("return "+JSON.stringify(x)+";");
}
}
但这并不令人满意。我们想使用闭包。
让返回的函数成为可以访问实例变量的闭包是可能的，但并不容易。好消息是，super如果您不想调用，则不必调用 - 您仍然可以return从 ES6 类构造函数中获取任意对象。在这种情况下，我们会做
class Smth extends Function {
constructor(x) {
// refer to `smth` instead of `this`
function smth() { return x; };
Object.setPrototypeOf(smth, Smth.prototype);
return smth;
}
}
但是我们可以做得更好，并将这个东西抽象出来Smth：
class ExtensibleFunction extends Function {
constructor(f) {
return Object.setPrototypeOf(f, new.target.prototype);
}
}
class Smth extends ExtensibleFunction {
constructor(x) {
super(function() { return x; }); // closure
// console.log(this); // function() { return x; }
// console.log(this.prototype); // {constructor: …}
}
}
class Anth extends ExtensibleFunction {
constructor(x) {
super(() => { return this.x; }); // arrow function, no prototype object created
this.x = x;
}
}
class Evth extends ExtensibleFunction {
constructor(x) {
super(function f() { return f.x; }); // named function
this.x = x;
}
}
诚然，这在继承链中创建了额外的间接级别，但这不一定是坏事（您可以扩展它而不是 native Function）。如果你想避免它，请使用
function ExtensibleFunction(f) {
return Object.setPrototypeOf(f, new.target.prototype);
}
ExtensibleFunction.prototype = Function.prototype;
但请注意，Smth不会动态继承静态Function属性。
这是一种创建可调用对象的方法，这些对象可以正确引用其对象成员，并保持正确的继承，而不会弄乱原型。
class ExFunc extends Function {
constructor() {
super('...args', 'return this.__self__.__call__(...args)')
var self = this.bind(this)
this.__self__ = self
return self
}
// Example `__call__` method.
__call__(a, b, c) {
return [a, b, c];
}
}
扩展这个类并添加一个__call__方法，更多内容如下...
// This is an approach to creating callable objects
// that correctly reference their own object and object members,
// without messing with prototypes.
// A Class that extends Function so we can create
// objects that also behave like functions, i.e. callable objects.
class ExFunc extends Function {
constructor() {
super('...args', 'return this.__self__.__call__(...args)');
// Here we create a function dynamically using `super`, which calls
// the `Function` constructor which we are inheriting from. Our aim is to create
// a `Function` object that, when called, will pass the call along to an internal
// method `__call__`, to appear as though the object is callable. Our problem is
// that the code inside our function can't find the `__call__` method, because it
// has no reference to itself, the `this` object we just created.
// The `this` reference inside a function is called its context. We need to give
// our new `Function` object a `this` context of itself, so that it can access
// the `__call__` method and any other properties/methods attached to it.
// We can do this with `bind`:
var self = this.bind(this);
// We've wrapped our function object `this` in a bound function object, that
// provides a fixed context to the function, in this case itself.
this.__self__ = self;
// Now we have a new wrinkle, our function has a context of our `this` object but
// we are going to return the bound function from our constructor instead of the
// original `this`, so that it is callable. But the bound function is a wrapper
// around our original `this`, so anything we add to it won't be seen by the
// code running inside our function. An easy fix is to add a reference to the
// new `this` stored in `self` to the old `this` as `__self__`. Now our functions
// context can find the bound version of itself by following `this.__self__`.
self.person = 'Hank'
return self;
}
// An example property to demonstrate member access.
get venture() {
return this.person;
}
// Override this method in subclasses of ExFunc to take whatever arguments
// you want and perform whatever logic you like. It will be called whenever
// you use the obj as a function.
__call__(a, b, c) {
return [this.venture, a, b, c];
}
}
// A subclass of ExFunc with an overridden __call__ method.
class DaFunc extends ExFunc {
constructor() {
super()
this.a = 'a1'
this.b = 'b2'
this.person = 'Dean'
}
ab() {
return this.a + this.b
}
__call__(ans) {
return [this.ab(), this.venture, ans];
}
}
// Create objects from ExFunc and its subclass.
var callable1 = new ExFunc();
var callable2 = new DaFunc();
// Inheritance is correctly maintained.
console.log('\nInheritance maintained:');
console.log(callable2 instanceof Function); // true
console.log(callable2 instanceof ExFunc); // true
console.log(callable2 instanceof DaFunc); // true
// Test ExFunc and its subclass objects by calling them like functions.
console.log('\nCallable objects:');
console.log( callable1(1, 2, 3) ); // [ 'Hank', 1, 2, 3 ]
console.log( callable2(42) ); // [ 'a1b2', Dean', 42 ]
// Test property and method access
console.log(callable2.a, callable2.b, callable2.ab())bind：function.bind()工作起来很像function.call()，并且它们共享相似的方法签名：
fn.call(this, arg1, arg2, arg3, ...);更多关于mdn
fn.bind(this, arg1, arg2, arg3, ...);更多关于mdn
在这两个中，第一个参数重新定义了this函数内部的上下文。其他参数也可以绑定到一个值。但是 wherecall立即使用绑定值调用函数，bind返回一个“异国情调”的函数对象，该对象透明地包装原始的，this以及任何预设的参数。
所以当你定义一个函数时bind，它的一些参数：
var foo = function(a, b) {
console.log(this);
return a * b;
}
foo = foo.bind(['hello'], 2);
您只使用剩余的参数调用绑定函数，它的上下文是预设的，在本例中为['hello'].
// We pass in arg `b` only because arg `a` is already set.
foo(2); // returns 4, logs `['hello']`
不幸的是，这并不完全有效，因为它现在返回的是一个函数对象而不是一个类，所以看起来这实际上不能在不修改原型的情况下完成。瘸。
基本上问题是没有办法为构造函数设置this值。Function真正做到这一点的唯一方法是.bind事后使用该方法，但这对类不是很友好。
我们可以在辅助基类中执行此操作，但是this在初始调用之后才可用super，因此有点棘手。
'use strict';
class ClassFunction extends function() {
const func = Function.apply(null, arguments);
let bound;
return function() {
if (!bound) {
bound = arguments[0];
return;
}
return func.apply(bound, arguments);
}
} {
constructor(...args) {
(super(...args))(this);
}
}
class Smth extends ClassFunction {
constructor(x) {
super('return this.x');
this.x = x;
}
}
console.log((new Smth(90))());（示例需要现代浏览器或node --harmony.）
基本上，基本函数extends 将使用类似于的自定义函数ClassFunction包装构造函数调用，但允许稍后在第一次调用时绑定。然后在构造函数本身中，它调用返回的函数，现在是绑定函数，传递完成自定义绑定函数的设置。Function.bindClassFunctionsuperthis
(super(...))(this);
这一切都相当复杂，但它确实避免了改变原型，出于优化原因，这被认为是错误的形式，并且可能在浏览器控制台中生成警告。
我从 Bergi 的回答中得到了建议，并将其包装到NPM 模块中。
var CallableInstance = require('callable-instance');
class ExampleClass extends CallableInstance {
constructor() {
// CallableInstance accepts the name of the property to use as the callable
// method.
super('instanceMethod');
}
instanceMethod() {
console.log("instanceMethod called!");
}
}
var test = new ExampleClass();
// Invoke the method normally
test.instanceMethod();
// Call the instance itself, redirects to instanceMethod
test();
// The instance is actually a closure bound to itself and can be used like a
// normal function.
test.apply(null, [ 1, 2, 3 ]);
概括Oriol 的答案：
class Smth extends Function {
constructor(x) {
super();
this.x = x;
return new Proxy(this, {
apply: (target, that, args) => target.__call__(...args)
});
}
__call__(v) {
return this.x * v;
}
}
这是我制定的解决方案，可以满足我扩展功能的所有需求，并且为我提供了很好的服务。这种技术的好处是：
ExtensibleFunction时，代码是扩展任何 ES6 类的惯用代码（不，与假装的构造函数或代理混在一起）。instanceof/.constructor返回预期值。.bind() .apply()并且.call()所有功能都按预期进行。这是通过重写这些方法来改变“内部”函数的上下文而不是ExtensibleFunction（或它的子类）实例来完成的。.bind()返回函数构造函数的新实例（无论是它ExtensibleFunction还是子类）。它用于Object.assign()确保存储在绑定函数上的属性与原始函数的属性一致。Symbol，它可以被模块或 IIFE（或任何其他私有化引用的常用技术）混淆。事不宜迟，代码：
// The Symbol that becomes the key to the "inner" function
const EFN_KEY = Symbol('ExtensibleFunctionKey');
// Here it is, the `ExtensibleFunction`!!!
class ExtensibleFunction extends Function {
// Just pass in your function.
constructor (fn) {
// This essentially calls Function() making this function look like:
// `function (EFN_KEY, ...args) { return this[EFN_KEY](...args); }`
// `EFN_KEY` is passed in because this function will escape the closure
super('EFN_KEY, ...args','return this[EFN_KEY](...args)');
// Create a new function from `this` that binds to `this` as the context
// and `EFN_KEY` as the first argument.
let ret = Function.prototype.bind.apply(this, [this, EFN_KEY]);
// For both the original and bound funcitons, we need to set the `[EFN_KEY]`
// property to the "inner" function. This is done with a getter to avoid
// potential overwrites/enumeration
Object.defineProperty(this, EFN_KEY, {get: ()=>fn});
Object.defineProperty(ret, EFN_KEY, {get: ()=>fn});
// Return the bound function
return ret;
}
// We'll make `bind()` work just like it does normally
bind (...args) {
// We don't want to bind `this` because `this` doesn't have the execution context
// It's the "inner" function that has the execution context.
let fn = this[EFN_KEY].bind(...args);
// Now we want to return a new instance of `this.constructor` with the newly bound
// "inner" function. We also use `Object.assign` so the instance properties of `this`
// are copied to the bound function.
return Object.assign(new this.constructor(fn), this);
}
// Pretty much the same as `bind()`
apply (...args) {
// Self explanatory
return this[EFN_KEY].apply(...args);
}
// Definitely the same as `apply()`
call (...args) {
return this[EFN_KEY].call(...args);
}
}
/**
* Below is just a bunch of code that tests many scenarios.
* If you run this snippet and check your console (provided all ES6 features
* and console.table are available in your browser [Chrome, Firefox?, Edge?])
* you should get a fancy printout of the test results.
*/
// Just a couple constants so I don't have to type my strings out twice (or thrice).
const CONSTRUCTED_PROPERTY_VALUE = `Hi, I'm a property set during construction`;
const ADDITIONAL_PROPERTY_VALUE = `Hi, I'm a property added after construction`;
// Lets extend our `ExtensibleFunction` into an `ExtendedFunction`
class ExtendedFunction extends ExtensibleFunction {
constructor (fn, ...args) {
// Just use `super()` like any other class
// You don't need to pass ...args here, but if you used them
// in the super class, you might want to.
super(fn, ...args);
// Just use `this` like any other class. No more messing with fake return values!
let [constructedPropertyValue, ...rest] = args;
this.constructedProperty = constructedPropertyValue;
}
}
// An instance of the extended function that can test both context and arguments
// It would work with arrow functions as well, but that would make testing `this` impossible.
// We pass in CONSTRUCTED_PROPERTY_VALUE just to prove that arguments can be passed
// into the constructor and used as normal
let fn = new ExtendedFunction(function (x) {
// Add `this.y` to `x`
// If either value isn't a number, coax it to one, else it's `0`
return (this.y>>0) + (x>>0)
}, CONSTRUCTED_PROPERTY_VALUE);
// Add an additional property outside of the constructor
// to see if it works as expected
fn.additionalProperty = ADDITIONAL_PROPERTY_VALUE;
// Queue up my tests in a handy array of functions
// All of these should return true if it works
let tests = [
()=> fn instanceof Function, // true
()=> fn instanceof ExtensibleFunction, // true
()=> fn instanceof ExtendedFunction, // true
()=> fn.bind() instanceof Function, // true
()=> fn.bind() instanceof ExtensibleFunction, // true
()=> fn.bind() instanceof ExtendedFunction, // true
()=> fn.constructedProperty == CONSTRUCTED_PROPERTY_VALUE, // true
()=> fn.additionalProperty == ADDITIONAL_PROPERTY_VALUE, // true
()=> fn.constructor == ExtendedFunction, // true
()=> fn.constructedProperty == fn.bind().constructedProperty, // true
()=> fn.additionalProperty == fn.bind().additionalProperty, // true
()=> fn() == 0, // true
()=> fn(10) == 10, // true
()=> fn.apply({y:10}, [10]) == 20, // true
()=> fn.call({y:10}, 20) == 30, // true
()=> fn.bind({y:30})(10) == 40, // true
];
// Turn the tests / results into a printable object
let table = tests.map((test)=>(
{test: test+'', result: test()}
));
// Print the test and result in a fancy table in the console.
// F12 much?
console.table(table);因为我有心情，所以我想我会在 npm 上为此发布一个包。
首先，我用 来解决arguments.callee，但这很糟糕。
我预计它会在全局严格模式下中断，但似乎即使在那里也能正常工作。
class Smth extends Function {
constructor (x) {
super('return arguments.callee.x');
this.x = x;
}
}
(new Smth(90))()
这是一个不好的方法，因为使用arguments.callee，将代码作为字符串传递并强制其以非严格模式执行。但是出现了覆盖的想法apply。
var global = (1,eval)("this");
class Smth extends Function {
constructor(x) {
super('return arguments.callee.apply(this, arguments)');
this.x = x;
}
apply(me, [y]) {
me = me !== global && me || this;
return me.x + y;
}
}
和测试，表明我能够以不同的方式将其作为函数运行：
var f = new Smth(100);
[
f instanceof Smth,
f(1),
f.call(f, 2),
f.apply(f, [3]),
f.call(null, 4),
f.apply(null, [5]),
Function.prototype.apply.call(f, f, [6]),
Function.prototype.apply.call(f, null, [7]),
f.bind(f)(8),
f.bind(null)(9),
(new Smth(200)).call(new Smth(300), 1),
(new Smth(200)).apply(new Smth(300), [2]),
isNaN(f.apply(window, [1])) === isNaN(f.call(window, 1)),
isNaN(f.apply(window, [1])) === isNaN(Function.prototype.apply.call(f, window, [1])),
] == "true,101,102,103,104,105,106,107,108,109,301,302,true,true"
版本与
super('return arguments.callee.apply(arguments.callee, arguments)');
实际上包含bind功能：
(new Smth(200)).call(new Smth(300), 1) === 201
版本与
super('return arguments.callee.apply(this===(1,eval)("this") ? null : this, arguments)');
...
me = me || this;
使call和不一致apply：window
isNaN(f.apply(window, [1])) === isNaN(f.call(window, 1)),
isNaN(f.apply(window, [1])) === isNaN(Function.prototype.apply.call(f, window, [1])),
所以支票应该移到apply：
super('return arguments.callee.apply(this, arguments)');
...
me = me !== global && me || this;
有一个利用 JavaScript 函数功能的简单解决方案：将“逻辑”作为函数参数传递给类的构造函数，将该类的方法分配给该函数，然后从构造函数返回该函数作为结果：
class Funk
{
constructor (f)
{ let proto = Funk.prototype;
let methodNames = Object.getOwnPropertyNames (proto);
methodNames.map (k => f[k] = this[k]);
return f;
}
methodX () {return 3}
}
let myFunk = new Funk (x => x + 1);
let two = myFunk(1); // == 2
let three = myFunk.methodX(); // == 3
以上是在 Node.js 8 上测试的。
上面示例的一个缺点是它不支持从超类链继承的方法。为了支持这一点，只需将“Object .getOwnPropertyNames(...)”替换为还返回继承方法名称的内容。我相信如何做到这一点在 Stack Overflow 上的其他一些问题答案中进行了解释:-)。顺便提一句。如果 ES7 添加一个方法来生成继承方法的名称，那就太好了；-)。
如果您需要支持继承的方法，一种可能性是向上述类添加一个静态方法，该方法返回所有继承的和本地方法名称。然后从构造函数中调用它。如果您随后扩展该类 Funk，您也将继承该静态方法。