声明性循环与命令式循环

Question

我试图将我的编程风格从命令式切换到声明式，但有一些概念困扰着我，比如当涉及到循环时的性能。例如，我有一个原始数据，在操作它之后，我希望得到3个预期结果:itemsHash、namesHash、rangeItemsHash

// original data

const DATA = [
  {id: 1, name: 'Alan', date: '2021-01-01', age: 0},
  {id: 2, name: 'Ben', date: '1980-02-02', age: 41},
  {id: 3, name: 'Clara', date: '1959-03-03', age: 61},
]

...

// expected outcome

// itemsHash => {
//   1: {id: 1, name: 'Alan', date: '2021-01-01', age: 0},
//   2: {id: 2, name: 'Ben', date: '1980-02-02', age: 41},
//   3: {id: 3, name: 'Clara', date: '1959-03-03', age: 61},
// }

// namesHash => {1: 'Alan', 2: 'Ben', 3: 'Clara'}

// rangeItemsHash => {
//   minor: [{id: 1, name: 'Alan', date: '2021-01-01', age: 0}],
//   junior: [{id: 2, name: 'Ben', date: '1980-02-02', age: 41}],
//   senior: [{id: 3, name: 'Clara', date: '1959-03-03', age: 61}],
// }

js prettyprint-override">// imperative way

const itemsHash = {}
const namesHash = {}
const rangeItemsHash = {}

DATA.forEach(person => {
  itemsHash[person.id] = person;
  namesHash[person.id] = person.name;
  if (person.age > 60){
    if (typeof rangeItemsHash['senior'] === 'undefined'){
      rangeItemsHash['senior'] = []
    }
    rangeItemsHash['senior'].push(person)
  }
  else if (person.age > 21){
    if (typeof rangeItemsHash['junior'] === 'undefined'){
      rangeItemsHash['junior'] = []
    }
    rangeItemsHash['junior'].push(person)
  }
  else {
    if (typeof rangeItemsHash['minor'] === 'undefined'){
      rangeItemsHash['minor'] = []
    }
    rangeItemsHash['minor'].push(person)
  }
})

// declarative way

const itemsHash = R.indexBy(R.prop('id'))(DATA);
const namesHash = R.compose(R.map(R.prop('name')),R.indexBy(R.prop('id')))(DATA);

const gt21 = R.gt(R.__, 21);
const lt60 = R.lte(R.__, 60);
const isMinor = R.lt(R.__, 21);
const isJunior = R.both(gt21, lt60);
const isSenior = R.gt(R.__, 60);


const groups = {minor: isMinor, junior: isJunior, senior: isSenior };

const rangeItemsHash = R.map((method => R.filter(R.compose(method, R.prop('age')))(DATA)))(groups)

为了达到预期的结果，命令式只循环一次，而声明式至少循环3次(itemshash、nameshash、rangeitemshash)。哪一个比较好？在性能上有什么权衡吗？

Answer 1

与.map(f).map(g)==.map(compose(g,f))类似，您可以编写简化程序，以确保一次传递就能给出所有结果。

编写声明性代码实际上与循环一次或多次的决定没有任何关系。

// Reducer logic for all 3 values you're interested in
// id: person
const idIndexReducer = (idIndex, p) => 
  ({ ...idIndex, [p.id]: p });

// id: name
const idNameIndexReducer = (idNameIndex, p) => 
  ({ ...idNameIndex, [p.id]: p.name });
  
// Age
const ageLabel = ({ age }) => age > 60 ? "senior" : age > 40 ? "medior" : "junior";
const ageGroupReducer = (ageGroups, p) => {
  const ageKey = ageLabel(p);
  
  return {
    ...ageGroups,
    [ageKey]: (ageGroups[ageKey] || []).concat(p)
  }
}

// Combine the reducers
const seed = { idIndex: {}, idNameIndex: {}, ageGroups: {} };
const reducer = ({ idIndex, idNameIndex, ageGroups }, p) => ({
  idIndex: idIndexReducer(idIndex, p),
  idNameIndex: idNameIndexReducer(idNameIndex, p),
  ageGroups: ageGroupReducer(ageGroups, p)
})

const DATA = [
  {id: 1, name: 'Alan', date: '2021-01-01', age: 0},
  {id: 2, name: 'Ben', date: '1980-02-02', age: 41},
  {id: 3, name: 'Clara', date: '1959-03-03', age: 61},
]

// Loop once
console.log(
  JSON.stringify(DATA.reduce(reducer, seed), null, 2)
);