看代码输出

let a = { x: 1 },
  ref = a;
a.x = a = { h: 1 };
console.log(a, ref);

{h:1}, {x:{h:1}}

1. . 优先级更高，所以先执行a.x=a，ref = {x:a},
2. 接着执行a={h:1},由于引用类型，ref变成 {x:{h:1}}

看代码输出

setTimeout(() => {
  console.log(1);
}, 0);
console.log(2);
new Promise((resolve, reject) => {
  console.log(3);
  setTimeout(resolve, 1000);
}).then(() => {
  console.log(4);
});

2,3,1,4

大写驼峰转下划线

如 HelloWorld转成 hello_world

// 使用 replace
function underlineWord(str) {
  if (typeof str !== "string") {
    throw new Error("type Error!");
  }
  let newStr = str.replace(/[A-Z]/g, (pre, index) => {
    if (index === 0) return pre.toLowerCase();
    return "_" + pre.toLowerCase();
  });
  return newStr;
}
let str = "HelloWorld";
console.log(underlineWord(str));

字符串匹配-有效括号

// 使用栈
function isLegal(str) {
  if (typeof str !== "string") {
    throw new Error("type Error!");
  }
  let arr = [];
  for (let i = 0; i < str.length; i++) {
    if (str[i] === "{" || str[i] === "[" || str[i] === "(") {
      arr.push(str[i]);
    } else if (
      (str[i] === "}" && arr.at(-1) === "{") ||
      (str[i] === ")" && arr.at(-1) === "(") ||
      (str[i] === "]" && arr.at(-1) === "[")
    ) {
      arr.pop();
      continue;
    } else {
      return false;
    }
  }
  return arr.length === 0;
}
console.log(isLegal("{}[]()")); // true

事件代理

点击p标签时，输出对应p标签的内容和data-index

<div id="box">
  <p data-index="a">1</p>
  <p data-index="b">2</p>
  <p data-index="c">3</p>
  <div>4</div>
</div>

let dom = document.getElementById("box");
dom.addEventListener("click", (e) => {
    let tar = e.target;
    if (tar.nodeName.toLowerCase() === "p") {
      // 内容
      console.log(tar.innerHTML);
      console.log(tar.dataset.index);
    }
});