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tonyjcamp/duplicateItems.js

Created February 12, 2019 17:37
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Return duplicate values in an Array
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duplicateItems.js
const names = ['Mike', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl', 'Nancy', 'Carl']
const uniq = names
.map((name) => {
return {count: 1, name: name}
})
.reduce((a, b) => {
a[b.name] = (a[b.name] || 0) + b.count
return a
}, {})
const duplicates = Object.keys(uniq).filter((a) => uniq[a] > 1)
console.log(duplicates) // [ 'Nancy', 'Carl' ]
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console.log(a, b, b.count, a[b.name])

// {Mike: 1} {count: 1, name: "Mike"} 1 1
// {Mike: 1, Matt: 1} {count: 1, name: "Matt"} 1 1
// {Mike: 1, Matt: 1, Nancy: 1} {count: 1, name: "Nancy"} 1 1
// {Mike: 1, Matt: 1, Nancy: 1, Adam: 1} {count: 1, name: "Adam"} 1 1
// {Mike: 1, Matt: 1, Nancy: 1, Adam: 1, Jenny: 1} {count: 1, name: "Jenny"} 1 1
// {Mike: 1, Matt: 1, Nancy: 2, Adam: 1, Jenny: 1} {count: 1, name: "Nancy"} 1 2
// {Mike: 1, Matt: 1, Nancy: 2, Adam: 1, Jenny: 1, …} {count: 1, name: "Carl"} 1 1
// {Mike: 1, Matt: 1, Nancy: 3, Adam: 1, Jenny: 1, …} {count: 1, name: "Nancy"} 1 3
// {Mike: 1, Matt: 1, Nancy: 3, Adam: 1, Jenny: 1, …} {count: 1, name: "Carl"} 1 2

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