underscorejs-groupBy学习

2.18 groupBy

2.18.1 语法

_.groupBy(list, iteratee, [context])

2.18.2 说明

把list分为多个集合，iterator为分组的依据,返回值为Object

• list可以是数组、对象、字符串或arguments等
• iteratee为分组的依据.
• iterator的参数(value, key, list)
• iterator如果是function需要返回值
• context可以改变iterator内部的this

2.18.3 代码示例

示例一：list可以是数组、对象、字符串或arguments等

var parity;
var iteratee = function(value, key, list){return value % 2; // value % 2的结果是0或是1，所以key就是0或是1//return value % 2 === 0; //这样子就变成了true或false
};//list为数组
parity = _.groupBy([1, 2, 3], iteratee);
console.log(parity); //=> {0:[2], 1:[1, 3]}//list为对象
parity = _.groupBy({a:1, b:2, c:3}, iteratee);
console.log(parity); //=> {0:[2], 1:[1, 3]}//list为字符串
parity = _.groupBy('123', iteratee);
console.log(parity); //=> {0:[2], 1:[1, 3]}//list为arguments
(function(){parity = _.groupBy(arguments, iteratee);console.log(parity); //=> {0:[2], 1:[1, 3]}
}(1, 2, 3));

示例二：iteratee可以全局的方法

var parity;//iteratee可以是全局的方法
parity = _.groupBy([1, 1.4, 1.6, 1.9], Math.floor);
console.log(parity); //=> {1 : [1, 1.4, 1.6, 1.9]}parity = _.groupBy([1, 1.4, 1.6, 1.9], Math.ceil);
console.log(parity); //=> {1 : [1], 2: [1.4, 1.6, 1.9]}

示例三：iteratee可以是list内元素的属性

var parity = _.groupBy(['a', 'b', 'cc'], 'length');
console.log(parity); //=> {1:['a', 'b'], 2:['c']}

示例四：iteratee可以是list内，元素的key

这种情况其实是用的最多的。

var array = [{"type": "stream","price": "3.99","id": "13nee"
}, {"type": "stream","price": "2.99","id": "8ejwj"
}, {"type": "buy","price": "3.99"."id": "9akwk"
}];var parity = _.groupBy(array, 'type');
console.log(parity); 
//=>
// {
//     stream: [{
//         "type": "stream",
//         "price": "3.99",
//         "id": "13nee"
//     }, {
//         "type": "stream",
//         "price": "2.99",
//         "id": "8ejwj"
//     }],
//     buy: [{
//         "type": "buy",
//         "price": "3.99".
//         "id": "9akwk"
//     }]
// }

示例五：iteratee的参数

_.groupBy('abc', function(v, i, l){console.log(v, i, l);//=> a 0 abc//=> b 1 abc//=> c 2 abcreturn v;
});

示例六：context可以改变iterator内部的this(坑)

_.groupBy('1', function(v, i, l){console.log(this);//=> Object {txt: "moe"}
}, {txt : 'moe'});_.groupBy('1', function(v, i, l){console.log(this ===1 ); //true or false?
}, 1);

2.18.4 list的特殊情况

console.log(_.groupBy(null)); //=> Object {}
console.log(_.groupBy(undefined)); //=> Object {}
console.log(_.groupBy(NaN)); //=> Object {}
console.log(_.groupBy(true)); //=> Object {}
console.log(_.groupBy(false)); //=> Object {}

2.18.5 将下列数组，按是否数字分类

var arr = [1, '1', '2', 2, '3', '3'];var parity = (function(arr){//写下你的代码
}(arr));console.log(parity);
//=> {false:['1', '2', '3', '3'], true: [1, 2]}