lodash javascript array of duplicates that match multiple parameters

Question

I have an array, to simplify lets say persons with first, last, and age. I want to make a new array of all persons that have the same first name, same last name and same age. For example my starting array:

[
  {id: 1, first: 'fred', last: 'smith', age: 21},
  {id: 2, first: 'fred', last: 'smith', age: 21},
  {id: 3, first: 'tom', last: 'smith', age: 21},
  {id: 4, first: 'fred', last: 'smith', age: 32}
]

I would like to return the duplicates that match first/last/age:

[
  {id: 1, first: 'fred', last: 'smith', age: 21},
  {id: 2, first: 'fred', last: 'smith', age: 21}
]

I'm struggling with _.uniq to figure out how to do this, any help is appreciated.

Would .reduce() do what you want? Compare each item with the previous, and if they match, return the later? — evolutionxbox
– evolutionxbox, Commented May 4, 2016 at 15:42
What happens if there are 2 freds and 2 toms? What do you expect in return? — Felipe Skinner
– Felipe Skinner, Commented May 4, 2016 at 15:52
@ Felipe Skinner, if there are 2 freds with the same last name and age, they should be included, if there are 2 toms with same last name and age, they should also be included, basically all records that match on all three criteria. — edencorbin
– edencorbin, Commented May 4, 2016 at 16:00

ryeballar · Accepted Answer · 2016-05-04 18:12:56Z

You can make use of _.groupBy() to group values, make sure that it is grouped by values that you determine to be the criteria for a duplicate. You can then _.filter() each grouped values by the lengths of the arrays they accumulated and then _.flatten() it to obtain the final array.

var data = [
  {id: 1, first: 'fred', last: 'smith', age: 21},
  {id: 2, first: 'fred', last: 'smith', age: 21},
  {id: 3, first: 'tom', last: 'smith', age: 21},
  {id: 4, first: 'fred', last: 'smith', age: 32}
];

var result = _(data)
  .groupBy(i => _(i).pick('first', 'last', 'age').values().value())
  .filter(i => i.length > 1)
  .flatten()
  .value();

document.write('<pre>' + JSON.stringify(result, 0, 4) + '</pre>');

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.11.2/lodash.js"></script>

Thanks for introducing me you the multi-property form of pick.

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