Javascript实现Linq查询方式

    Linq是.net平台一个重要的技术，全称Language Integrated Query。通过构建快速查询语句，可快速从数据库或集合中筛选数据集、以查询数据库相同的方式操作内存数据。

    在ECMAScript 5^th以后的版本中，Javascript实现了有限的有限的Linq查询方式，包括forEach, every, some, filter, map, reduce and reduceRight.

     首先需要说明，以上这些方法并不是跨浏览器的，对版本有相应的限制。我们知道Linq的对象需要实现Enumerable接口,本篇文章主要介绍使用JS模拟实现C# 中的Linq查询,包括 聚集查询、迭代查询、条件查询、构建Selector查询器等。

Javascript本质上并不支持类的继承，通过属性的继承可实现类的面向对象的功能，所以这也被认为是面向对象的一种方式，这就意味着可以使用它的属性构建更多面向对象的接口。例如Array,它是继承自Array.prototype 。如果更改了Array.prototype,那么基于这个属性继承的数组必然会变化。有了这些依据后开始构建我们的Linq功能。

举个例子,JS API不支持union方法，但支持concat方法合并数据。

Array.prototype.union

Array.prototype.union = Array.prototype.concat; 

先来看看一个.NET下的简单查询方式

var someArray = new int[] { 1, 2, 3, 4 };
var otherArray = someArray.Select(t => t * 2);   

在C#下使用查询数据时使用的是Select,使用一个 Delegate 构建查询。在这个例子中，我们使用了  t => t * 2 是一个 Lambda表达式。 将这个功能嫁接到JS下，定义一个function(){}

JS下的Select查询可以是这样

var someArray = [1, 2, 3, 4];
var otherArray = someArray.select(function (t) { return t * 2 }); 

以下我们将一一实现JS的 比较(EqualityComparer)、排序(SortComparer)、条件(Predicate)、查询器(Selector)

• Javascript Linq 查询器

 Select

遍历元素下的每一个元素,调用JS.Call方法返回数据。

Array.prototype.select = Array.prototype.map || function (selector, context) {
    context = context || window;
    var arr = [];
    var l = this.length;
    for (var i = 0; i < l; i++)
        arr.push(selector.call(context, this[i], i, this));
    return arr;
};


var arr = [1, 2, 3, 4, 5];
var doubled = arr.select(function(t){ return t * 2 }); 

SelectMany

SelectMany

Array.prototype.selectMany = function (selector, resSelector) {
    resSelector = resSelector || function (i, res) { return res; };
    return this.aggregate(function (a, b, i) {
        return a.concat(selector(b, i).select(function (res) { return resSelector(b, res) }));
    }, []);
}; 

var arr = [{Name:"A", Values:[1, 2, 3, 4]}, {Name:"B", Values:[5, 6, 7, 8]}];  
var res1 = arr.selectMany(function(t){ return t.Values });  
var res2 = arr.selectMany(function(t){ return t.Values }, function(t, u){ return {Name:t.Name, Val:u}}); 

Take

Array.prototype.take = function (c) {
    return this.slice(0, c);
};  

var arr = [1, 2, 3, 4, 5]; 
var res = arr.take(2);

Skip

跳过指定数后返回集合数据,使用slice。

Array.prototype.skip = function (c) {
    return this.slice(c);
}; 

var arr = [1, 2, 3, 4, 5]; 
var res = arr.skip(2);  

First

返回序列的第一个元素,如果没有元素,可以指定一个默认元素。

Array.prototype.first = function (predicate, def) {
    var l = this.length;
    if (!predicate) return l ? this[0] : def == null ? null : def;
    for (var i = 0; i < l; i++)
        if (predicate(this[i], i, this))
            return this[i];
    return def == null ? null : def;
}; 

var arr = [1, 2, 3, 4, 5];
var t1 = arr.first();  
var t2 = arr.first(function(t){ return t > 2 }); 
var t3 = arr.first(function(t){ return t > 10 }, 10); //默认值是10

Union

合并两个集合中的数据,使用concat,不合并重复数据。

Array.prototype.union = function (arr) {
    return this.concat(arr).distinct();
};  

var arr1 = [1, 2, 3, 4, 5]; 
var arr2 = [5, 6, 7, 8, 9];
var res = arr1.union(arr2);  

Distinct

找出不重复的数据。当有重复元素是只push一个元素进集合。

Array.prototype.distinct = function (comparer) {
    var arr = [];
    var l = this.length;
    for (var i = 0; i < l; i++) {
        if (!arr.contains(this[i], comparer))
            arr.push(this[i]);
    }
    return arr;
};    


var arr1 = [1, 2, 2, 3, 3, 4, 5, 5];   
var res1 = arr.distinct();  // [1, 2, 3, 4, 5]

var arr2 = [{Name:"A", Val:1}, {Name:"B", Val:1}];
var res2 = arr2.distinct(function(a, b){ return a.Val == b.Val }); //返回[{Name:"A", Val:1}] 

IndexOf

查找指定的值第一次出现的位置。

Array.prototype.indexOf = Array.prototype.indexOf || function (o, index) {
    var l = this.length;
    for (var i = Math.max(Math.min(index, l), 0) || 0; i < l; i++)
        if (this[i] === o) return i;
    return -1;
};  

var arr = [1, 2, 3, 4, 5];
var index = arr.indexOf(2);  // 1 

Remove

从集合中移除指定元素。

Array.prototype.remove = function (item) {
    var i = this.indexOf(item);
    if (i != -1)
        this.splice(i, 1);
}; 

var arr = [1, 2, 3, 4, 5];
arr.remove(2);   // [1, 3, 4, 5]

OrderBy

Array.prototype.orderBy = function (selector, comparer) {
    comparer = comparer || DefaultSortComparer;
    var arr = this.slice(0);
    var fn = function (a, b) {
        return comparer(selector(a), selector(b));
    };

    arr.thenBy = function (selector, comparer) {
        comparer = comparer || DefaultSortComparer;
        return arr.orderBy(DefaultSelector, function (a, b) {
            var res = fn(a, b);
            return res === 0 ? comparer(selector(a), selector(b)) : res;
        });
    };

    arr.thenByDescending = function (selector, comparer) {
        comparer = comparer || DefaultSortComparer;
        return arr.orderBy(DefaultSelector, function (a, b) {
            var res = fn(a, b);
            return res === 0 ? -comparer(selector(a), selector(b)) : res;
        });
    };

    return arr.sort(fn);
}; 





var arr = [{Name:"A", Val:1}, {Name:"a", Val:2}, {Name:"B", Val:1}, {Name:"C", Val:2}];

var res1 = arr.orderBy(function(t){ return t.Name });   

var res2 = arr.orderBy(function(t){ return t.Name }, function(a, b){
    if(a.toUpperCase() > b.toUpperCase()) return 1;
    if(a.toUpperCase() < b.toUpperCase()) return -1;
    return 0;
});    

OrderByDescending

Array.prototype.orderByDescending = function (selector, comparer) {
    comparer = comparer || DefaultSortComparer;
    return this.orderBy(selector, function (a, b) { return -comparer(a, b) });
};

var arr = [{Name:"A", Val:1}, {Name:"a", Val:2}, {Name:"B", Val:1}, {Name:"C", Val:2}];
var res = arr.orderByDescending(function(t){ return t.Name });   

GroupBy

Array.prototype.groupBy = function (selector, comparer) {
    var grp = [];
    var l = this.length;
    comparer = comparer || DefaultEqualityComparer;
    selector = selector || DefaultSelector;

    for (var i = 0; i < l; i++) {
        var k = selector(this[i]);
        var g = grp.first(function (u) { return comparer(u.key, k); });

        if (!g) {
            g = [];
            g.key = k;
            grp.push(g);
        }

        g.push(this[i]);
    }
    return grp;
};  


var arr = [{Name:"A", Val:1}, {Name:"B", Val:1}, {Name:"C", Val:2}, {Name:"D", Val:2}]; 
var res = arr.groupBy(function(t){ return t.Val }); 
// [[{Name:"A", Val:1}, {Name:"B", Val:1}], [{Name:"C", Val:2}, {Name:"D", Val:2}]] 

res.forEach(function(t){ 
    console.log("Key: " + t.key, "Length: " + t.length); 
});   

• Javascript Linq 聚合

Min

Array.prototype.min = function (s) {
    s = s || DefaultSelector;
    var l = this.length;
    var min = s(this[0]);
    while (l-- > 0)
        if (s(this[l]) < min) min = s(this[l]);
    return min;
};  


var arr1 = [1, 2, 3, 4, 5, 6, 7, 8];
var min1 = arr.min();  // 1 

var arr2 = [{Name:"A", Val:1}, {Name:"B", Val:2}];
var min2 = arr2.min(function(t){ return t.Val });   // 1 

Max

Array.prototype.max = function (s) {
    s = s || DefaultSelector;
    var l = this.length;
    var max = s(this[0]);
    while (l-- > 0)
        if (s(this[l]) > max) max = s(this[l]);
    return max;
};  

var arr1 = [1, 2, 3, 4, 5, 6, 7, 8];
var max1 = arr.max();  // 8 

var arr2 = [{Name:"A", Val:1}, {Name:"B", Val:2}];
var max2 = arr2.max(function(t){ return t.Val });   // 2  

Sum

Array.prototype.sum = function (s) {
    s = s || DefaultSelector;
    var l = this.length;
    var sum = 0;
    while (l-- > 0) sum += s(this[l]);
    return sum;
}; 

var arr1 = [1, 2, 3, 4, 5, 6, 7, 8];
var sum1 = arr.sum();  // 36 

var arr2 = [{Name:"A", Val:1}, {Name:"B", Val:2}];
var sum2 = arr2.sum(function(t){ return t.Val });   // 3 

• Javascript Linq 条件查询

Where

Array.prototype.where = Array.prototype.filter || function (predicate, context) {
    context = context || window;
    var arr = [];
    var l = this.length;
    for (var i = 0; i < l; i++)
        if (predicate.call(context, this[i], i, this) === true) arr.push(this[i]);
    return arr;
}; 

var arr = [1, 2, 3, 4, 5];
var res = arr.where(function(t){ return t > 2 }) ;  // [3, 4, 5] 

Any

Array.prototype.any = function (predicate, context) {
    context = context || window;
    var f = this.some || function (p, c) {
        var l = this.length;
        if (!p) return l > 0;
        while (l-- > 0)
            if (p.call(c, this[l], l, this) === true) return true;
        return false;
    };
    return f.apply(this, [predicate, context]);
};  


var arr = [1, 2, 3, 4, 5];
var res1 = arr.any();  // true
var res2 = arr.any(function(t){ return t > 5 });  // false 

All

Array.prototype.all = function (predicate, context) {
    context = context || window;
    predicate = predicate || DefaultPredicate;
    var f = this.every || function (p, c) {
        return this.length == this.where(p, c).length;
    };
    return f.apply(this, [predicate, context]);
};  

var arr = [1, 2, 3, 4, 5];
var res = arr.all(function(t){ return t < 6 });  // true 

Contains

Array.prototype.contains = function (o, comparer) {
    comparer = comparer || DefaultEqualityComparer;
    var l = this.length;
    while (l-- > 0)
        if (comparer(this[l], o) === true) return true;
    return false;
}; 

var arr1 = [1, 2, 3, 4, 5]; 
var res1 = arr.contains(2);  // true 

var arr2 = [{Name:"A", Val:1}, {Name:"B", Val:1}]; 
var res2 = arr2.contains({Name:"C", Val:1}, function(a, b){ return a.Val == b.Val }) ;  // true 

• Javasciprt Linq 迭代

ForEach

Array.prototype.forEach = Array.prototype.forEach || function (callback, context) {
    context = context || window;
    var l = this.length;
    for (var i = 0; i < l; i++)
        callback.call(context, this[i], i, this);
};  

var arr = [1, 2, 3, 4, 5];
arr.forEach(function(t){ if(t % 2 ==0) console.log(t); });   

DefaultIfEmpty

Array.prototype.defaultIfEmpty = function (val) {
    return this.length == 0 ? [val == null ? null : val] : this;
};  

var arr = [1, 2, 3, 4, 5];
var res = arr.where(function(t){ return t > 5 }).defaultIfEmpty(5);  // [5]