Ramda 过滤器 null 属性

Question

我有事件对象数组，看起来像：

{
    date: "2015-06-03T19:29:01.000Z",
    description: "Test",
    talks: [{
        author: "Nick",
        tags: ["tag1", "tag2", "tag3"]
    }]
}

我只想从这个对象中获取标签，所以我这样使用 Ramda:

let eventTags = pipe(prop('talks'), map(prop('tags')), flatten, uniq)
... 
eventTags(event); //and call function on event object

但有些情况下事件对象如下所示：

{
    date: "2015-06-03T19:29:01.000Z",
    description: "Test",
    talks: [{
        author: "Nick",
        tags: null
    }]
}

所以我在 eventTags 数组中得到了 [null]，但我想得到一个空数组。那么如何过滤空值？

Answer 1

我提倡一种解决方案，可以使用镜头访问 tags，并使用 Ramda 和 [=] 将 undefined 视为 Maybe Nothing 14=]

const x = [{
  date: "2015-06-03T19:29:01.000Z",
  description: "Test",
  talks: [{
    author: "Nick",
    tags: ["tag1", "tag2", "tag3"]
  }]
}, {
  date: "2015-06-03T19:29:01.000Z",
  description: "Test",
  talks: [{
    author: "Nick",
    tags: null
  }]
}]

const viewTalks = S.compose ( S.toMaybe ) ( 
  R.view ( R.lensProp( 'talks' ) ) 
)

const viewTags = S.compose ( S.toMaybe ) ( 
  R.view ( R.lensProp ( 'tags' ) ) 
)

const allTalkTags = S.map ( S.pipe ( [
  S.map ( viewTags ),
  S.justs,
  R.unnest
] ) )

const allTalksTags = S.pipe( [
  S.map ( S.pipe( [
    viewTalks,
    allTalkTags
  ] ) ),
  S.justs,
  R.unnest,
  R.uniq
] )

// outputs: ['tag1', 'tag2', 'tag3']
allTalksTags ( x )

Click to run a working sample

Answer 2

在 Matías Fidemraizer 的帮助下，我将 eventTags 功能更改为：

const viewTags = talk => !!talk.tags ? talk.tags : [];

export let eventTags = pipe(prop('talks'), map(viewTags), flatten, uniq)

Answer 3

您可以在此处利用 R.defaultTo([]) 创建一个函数，如果收到 null 或未定义的值，则 returns 一个空数组，否则通过未修改的值传递值。

const eventTags = pipe(
  prop('talks'),
  map(pipe(prop('tags'), defaultTo([]))),
  flatten,
  uniq
)

Ramda 过滤器 null 属性

Ramda filter null prop

javascript

ramda.js