Javascript 中用于键值对的 forEach() 方法

Question

如何仅打印显示屏中的值？

let room1 = new Map();
room1.set("candles", 8)
room1.set("fridge", 1)
room1.set("washing machine", 3)
room1.set("chairs", 4)
room1.set("sofa", 2)
room1.set("jacuzzi", 6)

let text4 = "";
room1.forEach(function(key) {
  text4 += key + "<br>"
});

document.getElementById("demo8").innerHTML = text4;

<p id="demo8"></p>

Answer 1

现在是 2022 年。忘记 forEach 并使用 针对...的。它在迭代 Map 时获取键值对：

let room1 = new Map();
room1.set("candles", 8)
room1.set("fridge", 1)
room1.set("washing machine", 3)
room1.set("chairs", 4)
room1.set("sofa", 2)
room1.set("jacuzzi", 6)

let text4 = "";
for(let [key,value] of room1) {
  text4 += `${key} - ${value}<br>`; // this could be key+" - "+value+"<br>" of course
};

document.getElementById("demo8").innerHTML = text4;

<p id="demo8"></p>

Answer 2

有一些解决方案，请遵循四种可能性...

1) 对于 更好的性能 使用 for...of 代替 forEach:

for(let [key] of room1) {
    text4 += key + "<br>";
};

2) 短代码（扩展语法）到获取 Map 的 keys 并转换为 Array：

[...room1.keys()].forEach(function(key) {
    text4 += key + "<br>"
});

3) 与第一个解决方案相同，但使用 Array.from(...) 显式：

Array.from(room1.keys()).forEach(function(key) {
    text4 += key + "<br>"
});

4) 将第一个参数 value 添加到 forEach 函数：

room1.forEach(function(value, key) {
    text4 += key + "<br>"
});

示例：

let room1 = new Map();
room1.set("candles", 8)
room1.set("fridge", 1)
room1.set("washing machine", 3)
room1.set("chairs", 4)
room1.set("sofa", 2)
room1.set("jacuzzi", 6)

let display = document.getElementById("demo8");
for (let [key] of room1) {
  display.innerHTML += key + "<br>";
};

<p id="demo8"></p>

参考：

• 如何在地图上使用 .map() Javascript 中的键

• https://www.geeksforgeeks.org/javascript-map-foreach-method/

Answer 3

您只能直接获得钥匙。

let rooms = new Map();
rooms.set("candles", 8)
rooms.set("fridge", 1)
rooms.set("washing machine", 3)
rooms.set("chairs", 4)
rooms.set("sofa", 2)
rooms.set("jacuzzi", 6)

console.log([...rooms.keys()]);