reduce() can build any result:
let words = ["hello", "world"]
let sentence = words.reduce((acc, w) => acc + " " + w)
console.log(sentence) // "hello world"
// Count occurrences
let letters = ["a", "b", "a", "c", "a"]
let counts = letters.reduce((acc, letter) => {
acc[letter] = (acc[letter] || 0) + 1
return acc
}, {})
console.log(counts) // { a: 3, b: 1, c: 1 }