懒惰map，JavaScript中的filter，reduce等

Question

Array#map , Array#filter create a new array and hence effectively iterating over the array (or creating new array).

Whereas in rust, python, java, c#, etc. such expression chain will iterate only once.

In most cases this is irrelevant and we do not have to care about that. However in some cases the performance hit could be a deal breaker to leverage the function api of the Array class.

How do you mitigate this? So you have any preference on a library enabling lazy evaluation for functional expression?

Answer 1

If you don't want it to iterate more than once. You can use a loop

const numbers = [1,2,3,4,5,6]
let result = 0;
for(const number of numbers) {
  const square = number * number
  if(square % 2) {
    result += square
  }
}
console.log(result)

Or reduce

const numbers = [1,2,3,4,5,6]
const result = numbers.reduce((acc, number) => {
  const square = number * number
  if(square % 2) {
    return acc + square
  }
  return acc
}, 0)
console.log(result)

Array methods aren't functional so the whole premise is flawed. The fact that they exist on the array object means they aren't open composition the way pure functions are. You can get closer though

const square = (n) => n * n
const oddNumberOrZero = (n) => n % 2 ? n : 0
const add = (a, b) => a + b
const addOddSquare = (a, b) => add(a, oddNumberOrZero(square(b)))
const reduce = (arr, fn, acc) => arr.reduce(fn,acc)
const numbers = [1,2,3,4,5,6]

const result = reduce(numbers, addOddSquare, 0)

console.log(result)

You also seem to be conflating fluent interfaces with functional programming.

Answer 2

I believe what you are looking for is processing the array as a stream. You can do that with highland :

import _ from "highland";
_([1, 2, 3, 4])
    .filter(v => v % 2 === 0)
    .map(v => v * 2)
    .toArray((result: number[]) => {
        // result is the new array
    });

Relevant part from the docs :

Arrays - Streams created from Arrays will emit each value of the Array (...)

Answer 3

Please check the following code lines. In the code, in which v * v is divided 2 means that v is divided.

 const numbers = [1,2,3,4,5,6]; const res = numbers.reduce((sum, v)=> sum + (v % 2? v * v: 0), 0); console.log(res)