Underscore.js: create a map out of list of objects

问题:

I am using the excellent Underscore.js library. I have a specific task which I can do fine using JavaScript or jQuery but was wondering if there was some sort of abstraction avaialable in Underscore that I was missing out on.

Essentially I have an object like so -

var some_object_array = [{id: "a", val: 55}, {id: "b", val: 1}, {id: "c", val: 45}];

I want to convert this into -

var some_map = {"a": {id: "a", val: 55}, "b": {id: "b", val: 1}, "c": {id: "c", val: 45}};

I know that I can use _.groupBy(some_object_array, "id"). But this returns a map like so -

var some_grouped_map = {"a": [{id: "a", val: 55}], "b": [{id: "b", val: 1}], "c": [{id: "c", val: 45}]};

Note that this does what it is advertised to do. But I was hoping to get some_map without iterating over the objects myself.

Any help appreciated.

回答1:

FWIW since underscore.js you can now use _.object()

var some_map = _.object(_.map(some_object_array, function(item) {
   return [item.id, item]
}));

回答2:

for your case, you should use the indexBy function:

var some_object_array = [{id: "a", val: 55}, {id: "b", val: 1}, {id: "c", val: 45}];

var some_grouped_map = _.indexBy(some_object_array, 'id');

回答3:

There is also this method

_.reduce(data, function (o, item) { o[item.key] = item.value; return o }, {})

Which is one statement with two statements in the inner function.

回答4:

I don't think there's something closer than groupBy for your needs. Even if there was, it wouldn't do better than a simple:

var some_map = {};
_.each(some_object_array, function(val) {
  some_map[val.id] = val;
});

回答5:

I this case you don't need to iterate the array. Not map, not reduce, not transform. All you need is the old pluck

_.object(_.pluck(some_object_array, 'id'), some_object_array);

回答6:

for ES6, it's

var some_map = _.chain(some_object_array)
                .map(item => [item.id, item])
                .object()
                .value()

回答7:

I solved this way, using lodash:

const ar = [
    { key: '1', val: 'a' },
    { key: '2', val: 'b' },
    { key: '3', val: 'c' }
]

const obj = _.keyBy({ ...ar }, 'key')

console.log(obj)

<script src="https://cdn.jsdelivr.net/npm/lodash@4.17.5/lodash.min.js"></script>