CoffeeScript的解构赋值

CoffeeScript有个语法叫解构赋值(Destructuring Assignment)，可以将一个对象的不同成员一次性赋值给多个的变量。官网中给了下面一个例子：

futurists =
  sculptor: "Umberto Boccioni"
  painter:  "Vladimir Burliuk"
  poet:
    name:   "F.T. Marinetti"
    address: [
      "Via Roma 42R"
      "Bellagio, Italy 22021"
    ]
{poet: {name, address: [street, city]}} = futurists
alert name + " — " + street
运行结果自然是 "F.T. Marinetti — Via Roma 42R"，因为coffee将其翻译为下面的JS：


var city, futurists, name, street, _ref, _ref1;
futurists = {
  sculptor: "Umberto Boccioni",
  painter: "Vladimir Burliuk",
  poet: {
    name: "F.T. Marinetti",
    address: ["Via Roma 42R", "Bellagio, Italy 22021"]
  }
};
_ref = futurists.poet, name = _ref.name, (_ref1 = _ref.address, street = _ref1[0], city = _ref1[1]);
alert(name + " — " + street);
这个语法跟Erlang的模式匹配有点类似，不同的是，Erlang会严格匹配等号两边，不赋值的要用_作为占位符，否则运行时将抛出异常，而coffee则不会，对于不存在的成员，值为undefined：


{poet: {nameX, address: [streetX, city]}} = futurists
# nameX = "undefined"
# streetX = "F.T. Marinetti"
当然，对一个不存在的成员继续解析还是会抛出异常的：


{poetX:{a}} = futurists
# TypeError: Cannot read property 'a' of undefined
另外，和JS一样，coffee也可以连续赋值:


a=b=100
# a=100
# b=100
假如将上面两种语法组合在一起，会怎样呢？就像下面的代码，最终d=?


a={b:1,c:2,x:3,y:4}
d={b,c}=a
简单分析下： 
赋值语句的结合顺序是从右到左，所以 d={b,c}=a 等价于 d=({b,c}=a) 
我们还知道，赋值表达式的值是其本身，那么 {b,c}=a 的值是什么呢， {b,c} 还是 a ? 
可以尝试下，coffee会将语句 {b,c}=a 转成下面的JS：


b = a.b, c = a.c;
这样看来，d最终的值会是2，这也太奇怪了吧?


还是看一下coffee把 d={b,c}=a 到底翻译成什么吧：


d = (b = a.b, c = a.c, a);
结果在意料之内，d的值不是 2 
结果在意料之外，d的值不是 {b,c}


我一直以为，像 {b,c}=a 这样的表达式，返回值将会是 {b,c} 而不是 a ，然后 d={b,c}=a 可以按过滤器的方式执行。但是，我错了， {b,c}=a 这个表达式的值是 a 
再写几行代码验证一次：


f = (a) -> {b,c}=a
coffee将上面的代码翻译成


f = function(a) {
  var b, c;
  return b = a.b, c = a.c, a;
};
好吧，解析赋值表达式的值确实是等号右边的值。 
至于 {b,c}=a 为何会被译成 b = a.b, c = a.c; 估计是因为该表达式的值不产生副作用，所以coffee把该表达式的值抛弃了。


那么，在JS里，普通赋值表达式的值，是不是也是等号右边的值呢？下面的JS语句，result最终应该等于expr1还是expr2呢？


result = expr1 = expr2
乍眼一看，可能会觉得 result==expr1 且 result==expr2 ，等于哪个都一样。


是的，在大多情况下，这都是成立的。但是，为什么对于上面的 {b,c}=a 这类语句，就不一定成立了呢？


这是因为，不成立的原因是：执行 dest = src 后，dest 不一定等于 src


JS中有类似的情形吗？答案当然是肯定的，看下面的JS代码：


var obj, result;
obj = {};
Object.defineProperties(obj, {
  x: {
    set: function() {},
    get: function() {
      return 10;
    }
  }
});
将obj.x的读和写分离，就能产生类似的效果。


obj.x = 11;
result = obj.x;
console.log(result); // 10
回到正题，继续测试：


result = obj.x = 11;
console.log(result); // 11
好吧，JS的赋值表达式的值也是等于等号右边的值。 
当最右值的读取包含副作用时，会怎样呢？


var obj, r1, r2, r3;
obj = {_x:1};
Object.defineProperties(obj, {
  x: {
    set: function() {},
    get: function() {
      return ++this._x;
    }
  }
});
r1 = r2 = r3 = obj.x
console.log(r1,r2,r3);  // 2 2 2
嗯...我知道，在get方法包含带副作用的行为是不对的，这段代码只是测试而已~ 
结果表明，连续赋值时最右值只计算一次。其实，从AST的执行过程来分析，最右值只计算一次，是合理的。


ECMA文档 是这么描述赋值语句的：


> 11.13.1 Simple Assignment ( = )
> The production AssignmentExpression : LeftHandSideExpression = AssignmentExpression is evaluated as follows:
> 1. Let lref be the result of evaluating LeftHandSideExpression.
> 2. Let rref be the result of evaluating AssignmentExpression.
> 3. Let rval be GetValue(rref).
> 4. Throw a SyntaxError exception if the following conditions are all true:
> * Type(lref) is Reference is true
> * IsStrictReference(lref) is true
> * Type(GetBase(lref)) is Environment Record
> * GetReferencedName(lref) is either "eval" or "arguments"
> 5. Call PutValue(lref, rval).
> 6. Return rval.
> NOTE When an assignment occurs within strict mode code, its  LeftHandSide must not evaluate to an unresolvable 
> reference. If it does a  ReferenceError exception is thrown upon assignment. The  LeftHandSide also may not be a 
> reference to a data property with the attribute value  {[[Writable]]:false}, to an accessor property with the attribute value 
> {[[Set]]:undefined}, nor to a non-existent property of an object whose [[Extensible]] internal property has the value false. In 
> these cases a TypeError exception is thrown.
赋值表达式的返回值是等号右边的值，连续赋值表达式的值自然就是最右值了，而且只会计算一次。

CoffeeScript 实现 ECMAScript Harmony 的提议 解构赋值 语法, 这样从复杂的数组和对象展开数据会更方便一些. 当你把数组或者对象的字面量赋值到一个变量时, CoffeeScript 把等式两边都解开配对, 把右边的值赋值给左边的变量. 最简单的例子, 可以用来并行赋值:

theBait   = 1000
theSwitch = 0

[theBait, theSwitch] = [theSwitch, theBait]

var theBait, theSwitch, _ref;

theBait = 1000;

theSwitch = 0;

_ref = [theSwitch, theBait], theBait = _ref[0], theSwitch = _ref[1];

用来处理函数多返回值也很方便.

weatherReport = (location) ->
  # 发起一个 Ajax 请求获取天气...
  [location, 72, "Mostly Sunny"]

[city, temp, forecast] = weatherReport "Berkeley, CA"

var city, forecast, temp, weatherReport, _ref;

weatherReport = function(location) {
  return [location, 72, "Mostly Sunny"];
};

_ref = weatherReport("Berkeley, CA"), city = _ref[0], temp = _ref[1], forecast = _ref[2];

解构赋值可以用在深度嵌套的数组跟对象上, 取出深度嵌套的属性.

futurists =
  sculptor: "Umberto Boccioni"
  painter:  "Vladimir Burliuk"
  poet:
    name:   "F.T. Marinetti"
    address: [
      "Via Roma 42R"
      "Bellagio, Italy 22021"
    ]

{poet: {name, address: [street, city]}} = futurists

var city, futurists, name, street, _ref, _ref1;

futurists = {
  sculptor: "Umberto Boccioni",
  painter: "Vladimir Burliuk",
  poet: {
    name: "F.T. Marinetti",
    address: ["Via Roma 42R", "Bellagio, Italy 22021"]
  }
};

_ref = futurists.poet, name = _ref.name, (_ref1 = _ref.address, street = _ref1[0], city = _ref1[1]);

解构赋值还可以跟 splats 搭配使用.

tag = "<impossible>"

[open, contents..., close] = tag.split("")

var close, contents, open, tag, _i, _ref,
  __slice = [].slice;

tag = "<impossible>";

_ref = tag.split(""), open = _ref[0], contents = 3 <= _ref.length ? __slice.call(_ref, 1, _i = _ref.length - 1) : (_i = 1, []), close = _ref[_i++];
loadrun: contents.join("")

展开式(expansion)可以用于获取数组结尾的元素, 而不需要对中间过程的数据进行赋值. 它也可以用在函数参数的列表上.

text = "Every literary critic believes he will
        outwit history and have the last word"

[first, ..., last] = text.split " "

var first, last, text, _ref;

text = "Every literary critic believes he will outwit history and have the last word";

_ref = text.split(" "), first = _ref[0], last = _ref[_ref.length - 1];

解构赋值也可以用在 class 的构造器上, 从构造器配置对象赋值到示例属性上.

class Person
  constructor: (options) -> 
    {@name, @age, @height} = options

tim = new Person age: 4

var Person, tim;

Person = (function() {
  function Person(options) {
    this.name = options.name, this.age = options.age, this.height = options.height;
  }

  return Person;

})();

tim = new Person({
  age: 4
});