Question

只是想知道是否有人可以向我解释这一点：我已经设法通过在地图中key手动声明“function”来实现我之后的结果。

我很难理解如何在地图函数中传递密钥，因此我可以轻松调用valueMapper('key')以获得相同的结果（下面的示例）。

我发现了一些解决方案，但它们似乎都没有意义。请为方法添加一些背景知识，我对该过程比解决方案更感兴趣。

感谢您的帮助！

var users = [
  {firstName: 'Pete', lastName: 'Barrat', favoriteFood: 'Pizza', age: 30},
  {firstName: 'Lisa', lastName: 'Jenkins', favoriteFood: 'Curry', age: 34},
  {firstName: 'Bob', lastName: 'Yates', favoriteFood: 'Fish', age: 54},
  {firstName: 'Claire', lastName: 'Smith', favoriteFood: 'Steak', age: 21},
  {firstName: 'Adam', lastName: 'Johnson', favoriteFood: 'Pasta', age: 27}
 ]

// write a function call valueMapper
// valueMapper takes one argument which is the name of a key 
// it returns an array of all the values in the users array that correspond to that key
// examples: 

// valueMapper('favoriteFood')
// -> ['Pizza', 'Curry', 'Fish', 'Steak', 'Pizza']

// valueMapper('firstName')
// -> ['Pete', 'Lisa', 'Bob', 'Claire', 'Adam']


/* #1st solution, I managed to get the answers I'm looking for, but I haven't defined
the valueMapper function and I haven't passed it a key yet */

var lastName = users.map(function(users) {
    return users['lastName'];
 })

 console.log(lastName);



/* #2nd solution is an attempt to create an anonymous function so that
I can pass through a key to the function in order to read the same results*/


function valueMapper() {Object.keys(users).map(function(key) {
   return users[key];
});
}

console.log(valueMapper('firstName'));

Answer 1

当你map一个带有函数fn的数组时，你正在创建一个新数组，其中包含对该数组中每个项目调用fn的结果。

[a, b, c].map(fn)  // => [fn(a), fn(b), fn(c)]

// e.g.
[1.5, 2.2, 0.3].map(Math.floor)  // => [1, 2, 0]

您正在寻找的功能存在于流行的JavaScript库中，有时也称为pluck。它使用映射到＆＃34; pluck＆＃34;集合中每个对象的字段：

var pluck = function (collection, field) {
    return collection.map(function (item) {
        return item[field];
    });
};

// e.g.
var albums = [
    { title: 'Colony', release: '1999' },
    { title: 'Clayman', release: '2000' },
    { title: 'Reroute to Remain', release: '2002' }
];

pluck(albums, 'title');  // => ['Colony', 'Clayman', 'Reroute to Remain']

在您的具体情况下，它看起来像这样：

var valueMapper = function (key) {
    return users.map(function (user) {
        return user[key];
    });
};

Answer 2

如果您命名一个函数（valueMapper），那么它不是一个匿名函数。
当您将属性值作为参数传递给函数时，不需要使用Object.Keys迭代属性。但是，作为gaurd，您可以检查传入的属性名称是否与对象上的现有属性匹配。

您可能正在寻找如下所示的解决方案。

var users = [
  {firstName: 'Pete', lastName: 'Barrat', favoriteFood: 'Pizza', age: 30},
  {firstName: 'Lisa', lastName: 'Jenkins', favoriteFood: 'Curry', age: 34},
  {firstName: 'Bob', lastName: 'Yates', favoriteFood: 'Fish', age: 54},
  {firstName: 'Claire', lastName: 'Smith', favoriteFood: 'Steak', age: 21},
  {firstName: 'Adam', lastName: 'Johnson', favoriteFood: 'Pasta', age: 27}
 ]

// write a function call valueMapper
// valueMapper takes one argument which is the name of a key 
// it returns an array of all the values in the users array that correspond to that key
// examples: 

// valueMapper('favoriteFood')
// -> ['Pizza', 'Curry', 'Fish', 'Steak', 'Pizza']

// valueMapper('firstName')
// -> ['Pete', 'Lisa', 'Bob', 'Claire', 'Adam']


/* #1st solution, I managed to get the answers I'm looking for, but I haven't defined
the valueMapper function and I haven't passed it a key yet */

var lastName = users.map(function(users) {
    return users['lastName'];
 })

 console.log(lastName);



/* #2nd solution is an attempt to create an anonymous function so that
I can pass through a key to the function in order to read the same results*/


function valueMapper(passedInKey) {
  
 return users.map(function(user) {
   if(user.hasOwnProperty(passedInKey)){     
    return user[passedInKey];
   }
});
}

console.log(valueMapper('firstName'));

Answer 3

一种解决方案是构建一个看似如下的hash表：

{ key1: Array('all values of key1'), key2: Array('all values of key2'), ...}

然后使用此哈希表快速获得结果。这将是一个很好的解决方案，因为您需要构建一次哈希，然后可以使用O(1)的复杂度检索结果。

见下面的例子：

var users = [
  {firstName: 'Pete', lastName: 'Barrat', favoriteFood: 'Pizza', age: 30},
  {firstName: 'Lisa', lastName: 'Jenkins', favoriteFood: 'Curry', age: 34},
  {firstName: 'Bob', lastName: 'Yates', favoriteFood: 'Fish', age: 54},
  {firstName: 'Claire', lastName: 'Smith', favoriteFood: 'Steak', age: 21},
  {firstName: 'Adam', lastName: 'Johnson', favoriteFood: 'Pasta', age: 27}
 ]
var hash = {};

users.forEach((user) => {
  Object.keys(user).forEach((key) => {
    hash[key] = hash[key] || new Array();
    hash[key].push(user[key]);
  })
});

function getKey(key) {
  return hash[key] || [];
}

console.log(getKey('age'));
console.log(getKey('favoriteFood'));
console.log(getKey('hey'));

如何将键值传递给.map函数

3 个答案: