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Suppose you have a bills array with this content:
const bills = [
{ date: '2018-01-20', amount: '220', category: 'Electricity' },
{ date: '2018-01-20', amount: '20', category: 'Gas' },
{ date: '2018-02-20', amount: '120', category: 'Electricity' }
]
and you want to extract the unique values of the category attribute of each item in the array.
Here’s what you can do:
const categories = [...new Set(bills.map(bill => bill.category))]
Explanation
Set is a new data structure that JavaScript got in ES6. It’s a collection of unique values. We put into that the list of property values we get from using map(), which how we used it will return this array:
['Electricity', 'Gas', 'Electricity']
Passing through Set, we’ll remove the duplicates.
... is the spread operator, which will expand the set values into an array.