访问也是数组 {React, Javascript} 的属性返回未定义

Question

This might looks stupid, but i'm lost. I want to Map through countries and regions array. I have a Countries Array that also includes Region's Array . I can access the Countries Array, but when i try to access the Regions Array i returned from the Countries Array , it returns undefined. Here is a minimal example.

Full code on CodeSandBox

 const Countries = [ { countryName: "Afghanistan", countryShortCode: "AF", regions: [ { name: "Badakhshan", shortCode: "BDS", }, { name: "Badghis", shortCode: "BDG", }, { name: "Helmand", shortCode: "HEL", }, { name: "Herat", shortCode: "HER", }, { name: "Jowzjan", shortCode: "JOW", }, { name: "Kabul", shortCode: "KAB", }, { name: "Kandahar", shortCode: "KAN", } ], }, { countryName: "Åland Islands", countryShortCode: "AX", regions: [ { name: "Brändö", shortCode: "BR", }, { name: "Eckerö", shortCode: "EC", }, { name: "Sund", shortCode: "SD", }, { name: "Vårdö", shortCode: "VR", }, ], } ] const mappedCountries = Countries.map(({ regions }) => regions); const mappedRegion = mappedCountries.map(({ name }) => name); console.log(mappedCountries); console.log(mappedRegion);

Answer 1

you are returing an array in the map method. so the result is an array of arrays.

you need to flatten your result. an easy alternative would be using reduce

const mappedCountries = Countries.reduce((memo, { regions }) => [...memo, ...regions], []);
const mappedRegion = mappedCountries.map(({ name }) => name);

Answer 2

您可以像这样更改第二张地图内的条件以访问区域名称 - mappedCountries.map(e => e.map(({name}) => name))