如何为数组中每个对象的username字段添加感叹号

你得到['[object Object]!']是因为forEach回调里的参数是数组中的整个对象，不是username属性。当你把对象和字符串'!'拼接时，JavaScript会把对象转换成默认的字符串表示[object Object]，所以最终得到了错误的结果。

方式一：修正forEach的写法 #

把回调参数改为代表数组元素的对象（比如命名为user），然后访问它的username属性再拼接'!'：

const array = [
  { username: "john", team: "red", score: 5, items: ["ball", "book", "pen"] },
  { username: "becky", team: "blue", score: 10, items: ["tape", "backpack", "pen"] },
  { username: "susy", team: "red", score: 55, items: ["ball", "eraser", "pen"] },
  { username: "tyson", team: "green", score: 1, items: ["book", "pen"] },
];

const players = [];
array.forEach((user) => {
  players.push(user.username + '!');
});
console.log(players); // 输出: ["john!", "becky!", "susy!", "tyson!"]

方式二：用map更高效（推荐） #

如果是要从原数组生成新数组，map方法比forEach更合适，它会直接返回处理后的新数组，不需要额外声明空数组：

const array = [
  { username: "john", team: "red", score: 5, items: ["ball", "book", "pen"] },
  { username: "becky", team: "blue", score: 10, items: ["tape", "backpack", "pen"] },
  { username: "susy", team: "red", score: 55, items: ["ball", "eraser", "pen"] },
  { username: "tyson", team: "green", score: 1, items: ["book", "pen"] },
];

const players = array.map(user => user.username + '!');
console.log(players); // 输出: ["john!", "becky!", "susy!", "tyson!"]

内容的提问来源于stack exchange，提问作者Mike Case