如何通过值从不可变的js映射中获取特定对象？

Question

I created an immutable map (with Immutable-JS ) from a list of objects:

var result = [{'id': 2}, {'id': 4}];
var map = Immutable.fromJS(result);

Now i want to get the object with id = 4 .

Is there an easier way than this:

var object = map.filter(function(obj){
 return obj.get('id') === 4
}).first();

Answer 1

Essentially, no: you're performing a list lookup by value, not by index, so it will always be a linear traversal.

An improvement would be to use find instead of filter :

var result = map.find(function(obj){return obj.get('id') === 4;});

Answer 2

The first thing to note is that you're not actually creating a map, you're creating a list:

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 reviver argument in your toJS call ( docs ), but it's certainly not the most intuitive api, alternatively you can do something like:

// 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

var item = Map.get('a'); // {id: 'a'}

Answer 3

It may be important to guarantee the order of the array. If that's the case:

1. Use an OrderedMap
2. Do a set method on the OrderedMap at each iteration of your source array

The example below uses "withMutations" for better performance.

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' } ]

Answer 4

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.

  const map = result.reduce((map, json) =>
    map.set(json.id, Immutable.fromJS(json))
  , Map());

Now, you can

   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.

Answer 5

使用ES2015语法 （和常量）：

const result = map.find(o => o.get('id') === 4);

Answer 6

Is there already a way thats easier? I don't know. but you can write your own function. Something like this should work:

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);