在地图中获取上一个元素的功能方法 [英] Functional way to get previous element during map

问题描述

我有一个数组，我在map上.我需要将当前元素与前一个元素进行比较.我通过比较它们的id并根据此条件进行不同的操作来检测当前元素是否与先前的元素相同.有没有做索引数学的纯功能方法吗?

I have an array which I map over. I need to compare the current element with the previous. I am detecting if the current element is the same as the previous element by comparing their ids and doing something different based on this condition. Is there any purely functional way to do it without doing index math?

items.map((item, index) => {
    if(item.id === items[index - 1 > 0 ? index - 1 : 0].id) {
    // do something
} else {
   // do something else
 }
})

该代码有效，但我想避免对索引进行数学运算.有什么办法吗?

The code works but I would like to avoid doing math on the index. Is there any way to do it?

推荐答案

这不是一个特别快的实现，但是解构赋值使其特别优雅-

Not a particularly fast implementation, but destructuring assignment makes it particularly elegant -

const None =
  Symbol ()

const mapAdjacent = (f, [ a = None, b = None, ...more ] = []) =>
  a === None || b === None
    ? []
    : [ f (a, b), ...mapAdjacent (f, [ b, ...more ]) ]

const pair = (a, b) =>
  [ a, b ]

console.log(mapAdjacent(pair, [ 1, 2, 3 ]))
// [ [ 1, 2 ], [ 2, 3 ] ]

console.log(mapAdjacent(pair, "hello"))
// [ [ h, e ], [ e, l ], [ l, l ], [ l, o ] ]

console.log(mapAdjacent(pair, [ 1 ]))
// []

console.log(mapAdjacent(pair, []))
// []

或将其写为生成器-

const mapAdjacent = function* (f, iter = [])
{ while (iter.length > 1)
  { yield f (...iter.slice(0,2))
    iter = iter.slice(1)
  }
}

const pair = (a, b) =>
  [ a, b ]

console.log(Array.from(mapAdjacent(pair, [ 1, 2, 3 ])))
// [ [ 1, 2 ], [ 2, 3 ] ]

console.log(Array.from(mapAdjacent(pair, "hello")))
// [ [ h, e ], [ e, l ], [ l, l ], [ l, o ] ]

console.log(Array.from(mapAdjacent(pair, [ 1 ])))
// []

console.log(Array.from(mapAdjacent(pair, [])))
// []

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