Creates a keyed JS object from an array, given a function to produce the keys for each value in the array.

This provides a convenient lookup for the array items if the key function produces unique results.

const phoneBook = [
  { name: 'Jon', num: '555-1234' },
  { name: 'Jenny', num: '867-5309' }
]

// { Jon: { name: 'Jon', num: '555-1234' },
//   Jenny: { name: 'Jenny', num: '867-5309' } }
const entriesByName = keyMap(
  phoneBook,
  entry => entry.name
)

// { name: 'Jenny', num: '857-6309' }
const jennyEntry = entriesByName['Jenny']