如何使用“过滤器”和“映射”判断列表是否为空

Question

      list
        .filter((val) => {
          if (
            val.name.toLocaleLowerCase().includes(input))
          ) {
            return val
          }
        })
        .map((val, idx) => (
          <Li
            key={idx}
          />
        ))

Using this structure, I wanted to do something if returns empty.

Ex: When searching for "abc" and there is no "abc" in the list, a message appears saying that "abc" was not found.

Answer 1

Several things to change:

1. Your filter expression is only working by accident

list
    .filter((val) => {
      if (
        val.name.toLocaleLowerCase().includes(input))
      ) {
        return val
      }
    })

can be just

list
    .filter(val => val.name.toLocaleLowerCase().includes(input)

Remember that the function inside filter just returns a true or false . By chance your expression, which returns the actual list element, is interpreted as true , and when your expression does not match, you don't return anything, which has the same effect as returning false . Expressing it as above makes it clearer what you are doing (and is very marginally faster).

2. To get your error message, save the output of the filter so you can test its length.

const matches = list.filter(val => val.name.toLocaleLowerCase().includes(input)

const output =  (matches.length>0) ? 
    matches.map((val, idx) => (
      <Li
        key={idx}
      />
    )) : (
      <div>Not found</div>
    )

Answer 2

You can tell if the list is empty using "filter" and "map" by using following JavaScript Code:

list
.filter((value) => (value!==undefined) && (value!==null) && value.name.toLocaleLowerCase().includes(input)))
.map((value, id) => (
          <Li
            key={id}
          />
))