JavascriptのMapを使わずに、ついObjectでやってしまっているので違いをまとめてみた
上リンク内のMapのコードを引用しつつ整理してみます
let contacts = new Map()
contacts.set('Jessie', {phone: "213-555-1234", address: "123 N 1st Ave"})
contacts.has('Jessie') // true
contacts.get('Hilary') // undefined
contacts.set('Hilary', {phone: "617-555-4321", address: "321 S 2nd St"})
contacts.get('Jessie') // {phone: "213-555-1234", address: "123 N 1st Ave"}
contacts.delete('Raymond') // false
contacts.delete('Jessie') // true
console.log(contacts.size) // 1これをObject使うと
let contacts = {}
contacts.Jessie = {phone: "213-555-1234", address: "123 N 1st Ave"}
contacts.hasOwnProperty('Jessie') // true
contacts.Hilary // undefined
contacts.Hilary = {phone: "617-555-4321", address: "321 S 2nd St"}
contacts.Jessie // {phone: "213-555-1234", address: "123 N 1st Ave"}
delete o.Raymond // true <- Mapと異なる結果
delete o.Jessie // true
Object.keys(o).length // 1となり、Mapのほうが統一感あってよいです
さらにプロパティでループしたいとき、Mapだと
for (let [key, value] of contacts) {
console.log(key, value)
}
// Hilary {phone: "617-555-4321", address: "321 S 2nd St"}もしくはforEachも使えるので
contacts.forEach(function(value, key) {
console.log(key, value)
})
// Hilary {phone: "617-555-4321", address: "321 S 2nd St"}Objectだと
for (let key of Object.keys(contacts)) {
console.log(key, contacts[key])
}
// Hilary {phone: "617-555-4321", address: "321 S 2nd St"}Mapのほうがすっきり書けます
MapはArrayと組み合わせても使えて
let kvArray = [["キー1", "値1"], ["キー2", "値2"]];
let myMap = new Map(kvArray)
console.log(myMap)
// Map(2) {"キー1" => "値1", "キー2" => "値2"}とArrayからMapを作れます
MapからArrayにするには
console.log(Array.from(myMap))
console.log([...myMap])
// [["キー1", "値1"], ["キー2", "値2"]]のどちらでもできます