从数组中获取具有唯一ID的第一个对象，并将其放入新列表中

Question

I am making a dropdown from json file containing array of objects, the problem is some of the objects have same IDs, and I want to extract only the first objects with unique ID (for example 1, then take the second object with unique ID 2 etc..) and put them in a list. That is, I need a list with unique IDs only.

What I have tried so far:

var distinctId = [];

for (var i = 0; i < data.length; i++) {
            var id = data[i]["id"];
            if (distinctId[id] == undefined) {
                distinctId[id] = [];
            }
            distinctId[id].push(data[i]);
            console.log(data[i]);
        }

The json file looks something like this:

[
{
id: 1,
field1: ...,
field2: ...
},
{
id: 1,
field1: ...,
field2: ...
},
{
id: 2,
field1: ...,
field2: ...
},
{
id: 2,
field1: ...,
field2: ...
},
{
id: 3,
field1: ...,
field2: ...
},
{
id: 3,
field1: ...,
field2: ...
},
]

Answer 1

If all you wish to do is get the first two unique id s of your objects you can do this by mapping your array to an array of id's using .map and destructing assignment .

Once you have done this you can use a set to remove all the duplicate's from the array. Lastly, you can use .splice to keep the first 2 unique id s from your array:

 const arr = [{id:1,field1:'foo',field2:'bar'},{id:1,field1:'foo',field2:'bar'},{id:2,field1:'foo',field2:'bar'},{id:2,field1:'foo',field2:'bar'},{id:3,field1:'foo',field2:'bar'},{id:3,field1:'foo',field2:'bar'}], res = [...new Set(arr.map(({id}) => id))].splice(0, 2); console.log(res); 

If you wish to have an array of objects which are unique you can use .reduce to create a new array. Essentially the new array is created by adding the first object from the array into it, then checking if the next object has the same id as that object. To do this we use .every . If it does have the same id we go to the next object, if the id is different then we can add this to our array. Then when we look at our next object we check if it matches any of the now 2 object id's in our array, if it doesn't we can add it and so on.

 const arr = [{id:1,field1:'foo1',field2:'bar1'},{id:1,field1:'foo2',field2:'bar2'},{id:2,field1:'foo3',field2:'bar3'},{id:2,field1:'foo4',field2:'bar4'},{id:3,field1:'foo5',field2:'bar5'},{id:3,field1:'foo6',field2:'bar6'}], res = arr.splice(1).reduce((acc, elem) => acc.every(({id}) => id != elem.id) ? [...acc, elem] : acc, [arr[0]]); console.log(res); 

Answer 2

You can reduce your array, and check if there is an element that matches your criteria:

your_json_array.reduce((destArray, obj) => {
    if (destArray.findIndex(i => i.id === obj.id) < 0) {
      return destArray.concat(obj);
    } else {
      return destArray;
    }
  }, []);

That will give you another array with the desired data.

Answer 3

You can achieve this by using something along the lines of this, however it may be a better idea to do what I've done in my second example. Within the second example, you can see that it's storing a list of all duplicated ID's, the first example will only work for an even number of duplicated ID's... IE if there were three objects with the id property set to a value of 1 , you'd still get the value 1 within the array variable.

I mean there are other ways in which you could achieve this, you could use a filter function in there somewhere, etc. If you require more documentation on how to use the reduce function, I'd suggest the MDN Docs .

Finally with example 3, that's simply removing all duplicates, making use of more modern syntax and features such as destructuring .

Example 1

 var objects = [{id: 1,a:'x'},{id:1,a:'x'},{id:2,a:'x'},{id:3,a:'x'}]; var array = objects.reduce((a, i) => { a.indexOf(i.id) == -1 ? a.push(i.id) : a.splice(a.indexOf(i.id), 1); return a; }, []); console.log(array); 

Example 2

 var objects = [{id: 1,a:'x'},{id:1,a:'x'},{id:2,a:'x'},{id:3,a:'x'}]; var array = objects.reduce((a, i) => { objects.filter(e => e.id == i.id).length == 1 ? a.push(i.id) : null; return a; }, []); console.log(array); 

Example 3

 const objects = [{id: 1,a:'x'},{id:1,a:'x'},{id:2,a:'x'},{id:3,a:'x'}]; const removeDuplicates = ([...o]) => o.reduce((a, i) => { a.indexOf(i.id) == -1 ? a.push(i) : null; return a }, []); console.log(removeDuplicates(objects)); 

Answer 4

1) To remove duplicates from the original array you can use filter() in conjunction with the optional argument that is passed as this to the filtering method.

2) To get an array with unique ids you can use map() on the previous result.

This is shown on next example:

 const input = [ {id: 1, field1: "f1", field2: "f2"}, {id: 1, field1: "f1", field2: "f2"}, {id: 2, field1: "f1", field2: "f2"}, {id: 2, field1: "f1", field2: "f2"}, {id: 3, field1: "f1", field2: "f2"}, {id: 3, field1: "f1", field2: "f2"}, ]; // Remove duplicates elements. let res = input.filter( function({id}) {return !this.has(id) && this.add(id)}, new Set() ); console.log("No duplicates:", res); // Get a map with only the IDs. let ids = res.map(({id}) => id); console.log("Ids:", ids);