How to get the last occurrence of the duplicated array in Javascript

Question

I have an array with duplicates:

nameList = [{name:"name1", filename:"filename1"}, {name:"name2", filename:"filename2"}, {name:"name3", filename:"filename2"}]

and I want to retain only the unique item but the last item only. I want:

nameList = [{name:"name1", filename:"filename1"}, {name:"name3", filename:"filename2"}]

I have tried:

var flags = {};
resultNameList = nameList.filter(function(entry) {
    if (flags[entry.filename]) {
        return false;
    }
    flags[entry.filename] = true;
    return true;
});

but it retains only the first occurrence of the duplicate. How can I get the last occurrence?

Please help. Thanks.

In the first object it key name is file. In rest of the object it is filename. Please confirm which one is correct — brk
– brk, Commented Jun 4, 2019 at 4:29

CertainPerformance · Accepted Answer · 2019-06-04 04:45:48Z

4

Reduce an object indexed by the filename property, overwriting the previous item at that key in the accumulator if it exists - this ensures that after all iterations, the object's values are composed only of the latest occurrence of a given filename. Then take the values of the accumulator:

const nameList = [{
  name: "name1",
  filename: "filename1"
}, {
  name: "name2",
  filename: "filename2"
}, {
  name: "name3",
  filename: "filename2"
}];

const output = Object.values(nameList.reduce((a, item ) => {
  a[item.filename] = item;
  return a;
}, {}));
console.log(output);

Note that key-value pairs in an object must be separated by :, not =.

The input given in the comment below looks to work as desired too:

const nameList = [{
  name: "name1",
  filename: "filename1"
}, {
  name: "name3",
  filename: "filename2"
}];

const output = Object.values(nameList.reduce((a, item) => {
  a[item.filename] = item;
  return a;
}, {}));
console.log(output);

edited Jun 4, 2019 at 4:45

answered Jun 4, 2019 at 4:24

CertainPerformance

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4 Comments

AlanBE Over a year ago

Doesn't work. I only returns: {name:"name3", filename="filename2"}. What about {name:"name1",filename:"filename1"}, which is also unique?

CertainPerformance Over a year ago

I changed the first item from file to filename per you edit in the question, but it still looks to work as expected - just press the "Run code snippet" button as in the answer. The output is [ { "name": "name1", "filename": "filename1" }, { "name": "name3", "filename": "filename2" } ]. That's correct, isn't it?

CertainPerformance Over a year ago

I edited that input into a second snippet in the answer, but it looks to perform as desired too: press "Run code snippet" and you get [ { "name": "name1", "filename": "filename1" }, { "name": "name3", "filename": "filename2" } ]

Sebastian Simon Over a year ago

Depending on the requirements, a better approach could be

Array.from(nameList.reduce((result, object) => { result.set(object.filename, object); return result; }, new Map()).values());

, using Map, so the order stays the same as in the original array and doesn’t change around due to Object key ordering semantics.

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