Javascript 如何按值从不可变的 js 映射中获取特定对象？
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How do I get a specific object from an immutable js map by value?
提问by sspross
I created an immutable map (with Immutable-JS) from a list of objects:
我从对象列表中创建了一个不可变映射（使用Immutable-JS）：
var result = [{'id': 2}, {'id': 4}];
var map = Immutable.fromJS(result);
Now i want to get the object with id = 4.
现在我想用id = 4.
Is there an easier way than this:
有没有比这更简单的方法：
var object = map.filter(function(obj){
return obj.get('id') === 4
}).first();
回答by blgt
Essentially, no: you're performing a list lookup by value, not by index, so it will always be a linear traversal.
本质上，不：您正在按值而不是按索引执行列表查找，因此它将始终是线性遍历。
An improvement would be to use findinstead of filter:
一个改进是使用find而不是filter：
var result = map.find(function(obj){return obj.get('id') === 4;});
回答by Joshua
The first thing to note is that you're not actually creating a map, you're creating a list:
首先要注意的是，您实际上并不是在创建地图，而是在创建列表：
var result = [{'id': 2}, {'id': 4}];
var map = Immutable.fromJS(result);
Immutable.Map.isMap(map); // false
Immutable.List.isList(map); // true
In order to create a map you can use a reviverargument in your toJScall (docs), but it's certainly not the most intuitive api, alternatively you can do something like:
为了创建地图，您可以reviver在toJS调用中使用参数（docs），但这肯定不是最直观的 api，或者您可以执行以下操作：
// lets use letters rather than numbers as numbers get coerced to strings anyway
var result = [{'id': 'a'}, {'id': 'b'}];
var map = Immutable.Map(result.reduce(function(previous, current) {
previous[ current.id ] = current;
return previous;
}, {}));
Immutable.Map.isMap(map); // true
Now we have a proper Immutable.js map which has a get method
现在我们有一个合适的 Immutable.js 映射，它有一个 get 方法
var item = Map.get('a'); // {id: 'a'}
回答by eezing
It may be important to guarantee the order of the array. If that's the case:
保证数组的顺序可能很重要。如果是这种情况：
- Use an OrderedMap
- Do a set method on the OrderedMap at each iteration of your source array
- 使用 OrderedMap
- 在源数组的每次迭代中对 OrderedMap 执行 set 方法
The example below uses "withMutations" for better performance.
下面的示例使用“withMutations”以获得更好的性能。
var OrderedMap = Immutable.OrderedMap
// Get new OrderedMap
function getOm(arr) {
return OrderedMap().withMutations(map => {
arr.forEach(item => map.set(item.id, item))
})
}
// Source collection
var srcArray = [
{
id: 123,
value: 'foo'
},
{
id: 456,
value: 'bar'
}
]
var myOrderedMap = getOm(srcArray)
myOrderedMap.get(123)
// --> { id: 123, value: 'foo' }
myOrderedMap.toObject()
// --> { 123: {id: 123, value: 'foo'}, 456: {id: 456, value: 'bar'} }
myOrderedMap.toArray()
// --> [ {id: 123, value: 'foo'}, { id: 456, value: 'bar' } ]
回答by Anand Dayalan
When using fromJS for array, you'll get List not map. It will be better and easier if you create a map. The following code will convert the result into Immutable map.
将 fromJS 用于数组时，您将得到 List 而不是 map。如果您创建地图会更好更容易。以下代码将结果转换为不可变映射。
const map = result.reduce((map, json) =>
map.set(json.id, Immutable.fromJS(json))
, Map());
Now, you can
现在你可以
map.get('2'); //{'id': 2}
Note, if the result has nested structure and if that has array, it will be a List with the above code.
请注意，如果结果具有嵌套结构并且具有数组，则它将是具有上述代码的 List。
回答by roadev
With ES2015 syntax(and constants):
使用ES2015 语法（和常量）：
const result = map.find(o => o.get('id') === 4);
回答by Reed
Is there already a way thats easier? I don't know. but you can write your own function. Something like this should work:
有没有更简单的方法？我不知道。但是您可以编写自己的函数。这样的事情应该工作：
var myFunc = function(id){
var object = map.filter(function(obj){return obj.get('id') === id}).first();
return object;
}
Then you would just do:var myObj = myFunc(4);
然后你会这样做：var myObj = myFunc(4);
