语法
_.indexBy(list, iteratee, [context])
说明
给定一个list，和 一个用来返回一个在列表中的每个元素键 的iterator 函数（或属性名）， 返回一个每一项索引的对象。和groupBy非常像，但是当你知道list的key是唯一的时候可以使用indexBy**。
什么时候用_.indexBy？
api都会给我们返回类似下面这样子的数据，现在我们要写一个方法。传入id,返回相对应的name或是其他。
var data = [{ ???id: 1, ???name: ‘Jon Doe‘, ???birthdate: ‘1/1/1991‘, ???height: ‘5 11‘}, { ???id: 2, ???name: ‘Jane Smith‘, ???birthdate: ‘1/1/1981‘, ???height: ‘5 6‘}, { ???id: 3, ???name: ‘Rockin Joe‘, ???birthdate: ‘4/4/1994‘, ???height: ‘6 1‘}, { ???id: 4, ???name: ‘Jane Blane‘, ???birthdate: ‘1/1/1971‘, ???height: ‘5 9‘}];function getUserInfoById(id){ ???//写一些代码，可能是这样子的 ???return _.find(data, function(element, index, list){ ???????return ?element.id == id; ???});}console.log(getUserInfoById(1)); //=> Object {id: 1, name: "Jon Doe", birthdate: "1/1/1991", height: "5 11"}console.log(getUserInfoById(3)); //=>Object {id: 3, name: "Rockin Joe", birthdate: "4/4/1994", height: "6 1"}//但是你很快发现，每次都遍历data里面的数据，如果频繁的取用户信息，应该把data的数据重组一下。//这时候_.indexBy就华丽的上场了。var dataClone = _.indexBy(data, ‘id‘);//这样子好多了。console.log(dataClone[1]); //=> Object {id: 1, name: "Jon Doe", birthdate: "1/1/1991", height: "5 11"}console.log(dataClone[3]); //=>Object {id: 3, name: "Rockin Joe", birthdate: "4/4/1994", height: "6 1"}代码示例
示例一：list可以是数组、对象、字符串、arguments等
var result1 = _.indexBy(‘1234‘);var result2 = _.indexBy([1, 2, 3, 4]);var result3 = _.indexBy({a:1, b:2, c: 3});var result4(function(){ ???result4 = _.indexBy(arguments);}(1, 2, 3));console.log(result1); //=> Object {1: "1", 2: "2", 3: "3", 4: "4"}console.log(result2); //=> Object {1: 1, 2: 2, 3: 3, 4: 4}console.log(result3); //=> Object {1: 1, 2: 2, 3: 3}console.log(result4); //=> Object {1: 1, 2: 2, 3: 3}示例二：iteratee的参数
var result1 = _.indexBy(‘1234‘, function(value, index, list){ ???console.log(value, index, list);});示例三：和_.groupBy对比
var stooges = [{name: ‘moe‘, age: 40}, {name: ‘iori‘, age: 40}, ?{name: ‘larry‘, age: 50}];var by1 = _.groupBy(stooges, ‘age‘);var by2 = _.indexBy(stooges, ‘age‘);console.log(by1);//=> Object {"40": [{name: ‘moe‘, age: 40}, {name: ‘iori‘, age: 40}], "50": {name: ‘larry‘, age: 50}}console.log(by2);//=> Object {"40": {name: ‘iori‘, age: 40}, "50": {name: ‘larry‘, age: 50}}示例四：context改成iteratee内的this
var stooges = [{name: ‘moe‘, age: 40}, {name: ‘iori‘, age: 40}, ?{name: ‘larry‘, age: 50}];var by1 = _.groupBy(stooges, function(value, index, list){ ???console.log(this);}, {text : ‘text‘});underscorejs之 _.indexBy(list, iteratee, [context])
原文地址：http://www.cnblogs.com/rechel/p/7615099.html