2.6 filter

2.6.1 语法：

_.filter(list, predicate, [context])

2.6.2 说明：

对list集合的每个成员依次进行匹配(根据predicate迭代函数检测)，返回匹配成功的集合

• list可以为数组，对象，字符串和arguments
• predicate会传第三个参数value, key, list(参数名可自定义)
• predicate函数需要返回值
• context可以改变predicate函数内部的this

2.6.3 代码示例：

示例一：filter对数组，对象，字符串，arguments进行操作并返回匹配成功的数据

var result;// 操作数组
result = _.filter([1, 2, 3], function (value) {
    return value % 2 === 1;
});
console.log(result) //=> [1, 3]// 操作对象
result = _.filter({ one: '一', two: '二', three: '三', num: 123, bool: false }, function (value) {
    return typeof value === "string";
});
console.log(result) //=> ["一", "二", "三"]// 操作复杂的对象
var obj = {
    levelA: {
        level0: 'level0',
        level1: 'level1'
    },
    levelB: '一',
    levelC: 1,
    levelD: {
        level3: 'level3'
    }
}
result = _.filter(obj, function (value) {
    return typeof value === "object";
});
console.log(result) //=> [{level0: "level0", level1: "level1", {level3: 'level3'}}]// 操作字符串(此处将字符拆分为数组)
result = _.filter('123', function (value) {
    return value === '2' || value === '3';
});
console.log(result) //=> ["2", "3"]//操作arguments
function abc() {
    result = _.filter(arguments, function (value) {
        return value % 2 === 1;
    });
    console.log(result); //=> [1, 3]
}
abc(1, 2, 3);

示例二：predicate函数传递的参数(函数内部需要return返回值，否则返回[])

var result;//数组的情况
result = _.filter([1, 2, 3], function (value, key, list) {
    console.log(value, key, list);
    //=> 1 0 [1, 2, 3]
    //=> 2 1 [1, 2, 3]
    //=> 3 2 [1, 2, 3]
});
console.log(result) //=> []//对象的情况
result = _.filter({one: '一', two: '二', three: '三'}, function(value, key, list){
    console.log(value, key, list);
    //=> 一 one Object {one: "一", two: "二", three: "三"}
    //=> 二 two Object {one: "一", two: "二", three: "三"}
    //=> 三 three Object {one: "一", two: "二", three: "三"}
});
console.log(result) //=> []

示例三：context可以改变predicate内部的this

var result;// 数组的情况
result = _.filter([1, 2, 3], function (value, key, list) {
    console.log(this); //=> [1, 2, 3] this是数组
}, [1, 2, 3]);// 对象的情况
result = _.filter([1, 2, 3], function (value, key, list) {
    console.log(this); //=> Object {no: 10} this是对象
}, { "no": 10 });// 字符串的情况
result = _.filter([1, 2, 3], function (value, key, list) {
    console.log(this); //=> String {0: "1", 1: "2", 2: "3", length: 3, [[PrimitiveValue]]: "123"}
                       //   this是将字符串拆分后的对象
}, "123");

2.6.4 _.select的功能和_.filter是一样的

var result = _.filter([1, 2, 3], function (value, key, list) {
    return value % 2 === 1;
});
console.log(result) //=> [1, 3]

2.6.5 特殊情况

示例一：list的特殊情况

//例如:null,undefined,0,true,this等；
var result = _.filter(null, function (value, key, list) {
    return true;
});
console.log(result) //=> []

示例二：predicate函数的this为window全局对象的情况

// 例如:null,undefined,window,this等
var result = _.filter([1, 2, 3], function (value, key, list) {
    console.log(this); //=> this是window全局对象
}, null);

2.6.6 list参数可为真假值？

var  result = _.filter([1, true, '1', 0, undefined, null]);
console.log(result) //=> [1, true, "1"]

2.6.7 predicate还有其他写法？

示例一：predicate参数为空的时候

var  result = _.filter({x: 1, y: 2});
console.log(result) //=> [1, 2]

示例二：predicate参数为一个字符的时候

var  result = _.filter([{x: 1}, {y: 2}], 'x');
console.log(result) //=> [{x: 1}]

示例三：predicate参数为对象的时候

var obj = [
    {x: 1, y: 2},
    {x: 1},
    {y: 2, z: 3}
]
var  result = _.filter(obj, {x: 1});
console.log(result) //=> [{x: 1, y: 2},{x: 1}]