moy-fp/src/Relation/unionBy.js

A functional programming library.

import curry from '../Function/curry'

/**
 * (a -> String) -> [a] -> [a] -> [a]
 */
const unionBy = curry(
  (fn, list2, list1) => {
    const unionObj = {},
          result = []
    let id
    for(let item of list1){
      id = fn(item)
      if(!unionObj[id]){
        result.push(item)
        unionObj[id] = true
      }
    }
    for(let item of list2){
      id = fn(item)
      if(!unionObj[id]){
        result.push(item)
        unionObj[id] = true
      }
    }
    return result
  }
)

export default unionBy