JavaScript 迭代器类

17
你知道有一个JavaScript库能实现集合的通用迭代器类(无论是数组还是某些抽象的Enumerable),并具有一整套功能,例如Google CommonApache Commons吗?
编辑:Enumerable#each不是迭代器类。 我正在寻找一个迭代器,类似于以下写法:
var iterator = new Iterator(myCollection);
for (var element = iterator.next(); iterator.hasNext(); element = iterator.next()) {
    // iterator 
}

编辑:mamoo提醒我们在Mozilla的Javascript 1.7中有迭代器实现。因此,现在的目标是在Javascript 1.5(ECMA 4)中找到这个Iterator函数的实现。

编辑2:为什么在库(和ECMA 5)提供了each方法时要使用迭代器?首先,因为each通常会干扰this,因为回调被call了(这就是Prototype中为什么each接受第二个参数的原因)。其次,因为人们对for(;;)循环比对.each(callback)构造更加熟悉(至少在我的领域是这样)。最后,因为迭代器可以迭代普通对象(请参见JavaScript 1.7)。

编辑3:我接受了npup的答案,但这是我的一番努力:

function Iterator(o, keysOnly) {
    if (!(this instanceof arguments.callee))
      return new arguments.callee(o, keysOnly);
    var index = 0, keys = [];
    if (!o || typeof o != "object") return;
    if ('splice' in o && 'join' in o) {
        while(keys.length < o.length) keys.push(keys.length);
    } else {
        for (p in o) if (o.hasOwnProperty(p)) keys.push(p);
    }
    this.next = function next() {
        if (index < keys.length) {
            var key = keys[index++];
            return keysOnly ? key : [key, o[key]];
        } else throw { name: "StopIteration" };
    };
    this.hasNext = function hasNext() {
        return index < keys.length;
    };
}



var lang = { name: 'JavaScript', birthYear: 1995 };  
var it = Iterator(lang);
while (it.hasNext()) {
    alert(it.next());
}
//alert(it.next()); // A StopIteration exception is thrown  


var langs = ['JavaScript', 'Python', 'C++'];  
var it = Iterator(langs);
while (it.hasNext()) {
    alert(it.next());
}
//alert(it.next()); // A StopIteration exception is thrown  

当然,这就是为什么我们等了15年才将其包含在语言中的原因。 - Alsciende
2
@stereofrog,现在许多库和脚本都使用异步函数调用(直到WebWorkers得到普遍支持,但即使如此),目前,您认为如何使用简单的.each()函数或“for in”语句以非阻塞方式异步迭代数据?迭代器是最好的解决方案;它们可以通过异步函数传递以便轻松恢复迭代,而不管底层实现如何。(这毕竟是迭代器模式)。JavaScript不仅是一种函数式语言...但即使函数式语言有迭代器... - Yanick Rochon
在第二次编辑中,当您提到ES5的each方法时,您是指.forEach()方法吗? - dcorking
7个回答

5

好的,枚举模式不是一个真正的迭代器。

以下内容对您有用吗?至少它符合您给出的语义。通常在做决策时需要权衡利弊,这次我没有仔细考虑:)
也许您想能够输入一个或两个数字并以这种方式迭代范围。但这可能是一个开始(支持迭代哈希、数组和字符串)。

这是一个完整的演示页面,可以自行运行并输出一些调试信息,但(可能)有趣的东西在于

window.npup = (function() {
    [...]
})();

点。

也许只有我完全不明白,但在实际情况下你会用类似Java的迭代器做什么呢?

最好 /npup

<html>
<head>
<title>untitled</title>
</head>

<body>
    <ul id="output"></ul>


<script type="text/javascript">
window.log = (function (outputAreaId) {
    var myConsole = document.getElementById(outputAreaId);
    function createElem(color) {
        var elem = document.createElement('li');
        elem.style.color = color;
        return elem;
    }
    function appendElem(elem) {
        myConsole.appendChild(elem);
    }
    function debug(msg) {
        var elem = createElem('#888');
        elem.innerHTML = msg;
        appendElem(elem);
    }
    function error(msg) {
        var elem = createElem('#f88');
        elem.innerHTML = msg;
        appendElem(elem);
    }
    return {
        debug: debug
        , error: error
    };
})('output');


window.npup = (function () {
    // Array check as proposed by Mr. Crockford
    function isArray(candidate) {
        return candidate &&
            typeof candidate==='object' &&
            typeof candidate.length === 'number' &&
            typeof candidate.splice === 'function' &&
            !(candidate.propertyIsEnumerable('length'));
    }
    function dontIterate(collection) {
        // put some checks chere for stuff that isn't iterable (yet)
        return (!collection || typeof collection==='number' || typeof collection==='boolean');
    }
    function Iterator(collection) {
        if (typeof collection==='string') {collection = collection.split('');}
        if (dontIterate(collection)) {throw new Error('Oh you nasty man, I won\'t iterate over that ('+collection+')!');}
        var arr = isArray(collection);
        var idx = 0, top=0;
        var keys = [], prop;
        if (arr) {top = collection.length;}
        else {for (prop in collection) {keys.push(prop);}}
        this.next = function () {
            if (!this.hasNext()) {throw new Error('Oh you nasty man. I have no more elements.');}
            var elem = arr ? collection[idx] : {key:keys[idx], value:collection[keys[idx]]};
            ++idx;
            return elem;
        };
        this.hasNext = function () {return arr ? idx<=top : idx<=keys.length;};
    }
    return {Iterator: Iterator};
})();

var element;

log.debug('--- Hash demo');
var o = {foo:1, bar:2, baz:3, bork:4, hepp: {a:1,b:2,c:3}, bluff:666, bluff2:777};
var iterator = new npup.Iterator(o);
for (element = iterator.next(); iterator.hasNext(); element = iterator.next()) {
    log.debug('got elem from hash: '+element.key+' => '+element.value);
    if (typeof element.value==='object') {
        var i2 = new npup.Iterator(element.value);
        for (var e2=i2.next(); i2.hasNext(); e2=i2.next()) {
            log.debug('&nbsp;&nbsp;&nbsp;&nbsp;# from inner hash: '+e2.key+' => '+e2.value);
        }
    }
}
log.debug('--- Array demo');
var a = [1,2,3,42,666,777];
iterator = new npup.Iterator(a);
for (element = iterator.next(); iterator.hasNext(); element = iterator.next()) {
    log.debug('got elem from array: '+ element);
}
log.debug('--- String demo');
var s = 'First the pants, THEN the shoes!';
iterator = new npup.Iterator(s);
for (element = iterator.next(); iterator.hasNext(); element = iterator.next()) {
    log.debug('got elem from string: '+ element);
}
log.debug('--- Emptiness demo');
try {
    log.debug('Try to get next..');
    var boogie = iterator.next();
}
catch(e) {
    log.error('OW: '+e);
}

log.debug('--- Non iterables demo');
try{iterator = new npup.Iterator(true);} catch(e) {log.error('iterate over boolean: '+e);}
try{iterator = new npup.Iterator(6);} catch(e) {log.error('iterate over number: '+e);}
try{iterator = new npup.Iterator(null);} catch(e) {log.error('iterate over null: '+e);}
try{iterator = new npup.Iterator();} catch(e) {log.error('iterate over undefined: '+e);}

</script>
</body>
</html>

做得好。我喜欢你如何迭代字符串,为什么不呢?由于这个问题需要解释,我将编辑我的问题。 - Alsciende
1
在JavaScript中,如果需要以异步方式迭代所有数组元素,则迭代器非常有用(循环遍历前n个元素,然后在第n + 1个元素之后暂停一段时间后恢复,等等)。 - Yanick Rochon

5

感谢提供 JS 1.7 的链接。我差点忘了这个。我很想看到在 ECMA4 中的实现。 - Alsciende

3
自从这个问题被提出以来,JavaScript已经添加了实际的迭代器。 一些内置类型,例如ArrayMapString现在具有默认的迭代行为,但您可以通过包含返回两个对象之一的next()函数来将其添加到任何对象中。
{done:true}     /*or*/
{done:false, value:SOMEVALUE}

访问对象迭代器的一种方式是使用:

for ( var of object ) { }

循环。这里是一个(相当愚蠢的)例子,我们定义了一个迭代器,然后在这样的循环中使用它来生成一个字符串1, 2, 3

"use strict";

function count ( i ) {
  let n = 0;
  let I = {};
  I[Symbol.iterator] = function() {
     return { next: function() { return (n > i) ? {done:true}
                                                : {done:false, value:n++} } } };
  let s = "";
  let c = "";
  for ( let i of I ) {       /* use the iterator we defined above */
      s += c + i;
      c = ", "
  }
  return s;
}


let s = count(3);
console.log(s);

3
这是我为ECMAScript 262第5版(又名JavaScript)做出的尝试(jsfiddle)。 (例如使用Object.keys和Array.isArray)。
//Usage
b=Iterator(a);
while(b()){
  console.log(b.value);
}

代码:

function Iterator(input,keys) {
  // Input:
  //  input : object|array
  //  keys   : array|undefined|boolean
  function my() {
    ++my.index;
    if (my.index >= my.keys.length) {
      my.index = my.keys.length -1;
      my.key = my.value = undefined;
      return false;
    }
    my.key = my.useIndex ? my.index : my.keys[my.index];
    my.value = my.input[my.key];
    return my.index < my.keys.length;
  }
  if (input === null || typeof input !== 'object') {
    throw new TypeError("'input' should be object|array");
  }
  if (
    !Array.isArray(keys)
    && (typeof keys !== 'undefined')
    && (typeof keys !== 'boolean')
    ) {
    throw new TypeError("'keys' should be array|boolean|undefined");
  }
  // Save a reference to the input object.
  my.input = input;
  if (Array.isArray(input)) {
    //If the input is an array, set 'useIndex' to true if 
    //the internal index should be used as a key.
    my.useIndex = !keys;
    //Either create and use a list of own properties,
    // or use the supplied keys
    // or at last resort use the input (since useIndex is true in that
    // case it is only used for the length)
    my.keys = keys===true ? Object.keys(input) : keys || input;
  } else {
    my.useIndex = false;
    my.keys = Array.isArray(keys) ? keys : Object.keys(input);
  }
  // Set index to before the first element.
  my.index = -1;
  return my;
}

例子:

function Person(firstname, lastname, domain) {
  this.firstname = firstname;
  this.lastname = lastname;
  this.domain = domain;
}
Person.prototype.type = 'Brillant';

var list = [
  new Person('Paula','Bean','some.domain.name'),
  new Person('John','Doe','another.domain.name'),
  new Person('Johanna','Doe','yet.another.domain.name'),
];

var a,b; 
var data_array = ['A','B','C','D','E','F'];
data_array[10]="Sparse";


console.log('Iterate over own keys in an object, unknown order');
a = Iterator(list[0]);
while(a()) console.log("  ",a.key, a.value);

console.log('Iterate over keys from anywhere, in specified order');
a = Iterator(list[0], ['lastname','firstname','type']);
while(a()) console.log("  ",a.key, a.value);

console.log('Iterate over all values in an array');
a = Iterator(list);
while(a()) console.log(a.key, a.value.firstname, a.value.lastname);


//Some abusing, that works for arrays (if the iterator.keys is modified
//it can also be used for objects)
console.log('Add more entries to the array, reusing the iterator...');
list.push(new Person('Another','Name','m.nu'));
while(a()) console.log(a.key, a.value.firstname, a.value.lastname);

console.log('Reset index and print everything again...');
a.index=-1; //Reset the index.
while(a()) console.log(a.key, a.value.firstname, a.value.lastname);

//With arrays, if setting 'keys' to true it will only print the
//elements that has values (If the array has more own enumerable values
//they too will be included)
console.log('Print sparce arrays...');
a = Iterator(data_array,true);
while(a()) console.log(a.key, a.value);

1

我在几个项目中使用过LINQ to Javascript。

http://jslinq.codeplex.com/Wikipage

var myList = [
            {FirstName:"Chris",LastName:"Pearson"},
            {FirstName:"Kate",LastName:"Johnson"},
            {FirstName:"Josh",LastName:"Sutherland"},
            {FirstName:"John",LastName:"Ronald"},
            {FirstName:"Steve",LastName:"Pinkerton"}
            ];

var exampleArray = JSLINQ(myList)
                   .Where(function(item){ return item.FirstName == "Chris"; })
                   .OrderBy(function(item) { return item.FirstName; })
                   .Select(function(item){ return item.FirstName; });

1
LINQ 看起来不错,但它与迭代器有什么关联呢?它似乎是为查询数据集而设计的。 - Alsciende
LINQ是一种函数式编程库,采用DSL风格编写,因此看起来像SQL。 - Dan

1

由于还没有提到,数组内置了高阶函数。

Map 的工作方式类似于迭代器,只能进行单次遍历。

[1,2,3,4,5].map( function(input){ console.log(input); } );

这段代码将列表中的每个元素传递到一个函数中,本例中是一个简单的打印机。

1
2
3
4
5

1

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