homobulla / odoe Goto Github PK
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one day one exercise
先看一段代码:
const homo = {};
homo.bulla= 123;
homo.bulla // 123
homo= {}; // TypeError: Assignment to constant variable.
const实际上保证的,并不是变量的值不得改动,而是变量指向的那个内存地址不得改动。对于简单类型的数据(数值、字符串、布尔值),值就保存在变量指向的那个内存地址,因此等同于常量。
但对于复合类型的数据(主要是对象和数组),变量指向的内存地址,保存的只是一个指针,const只能保证这个指针是固定的,至于它指向的数据结构是不是可变的,就完全不能控制。
const obj = {a:1};
Object.freeze(obj)
答案是Object.freeze
,对象一旦被冻结,则不能向这个对象添加新的属性,不能删除已有属性,不能修改该对象已有属性的可枚举性、可配置性、可写性,以及不能修改已有属性的值。即意味着也无法通过Object.defineProperty
来修改。
但这样的冻结还是浅冻结,假如对象某个属性值依然是对象,那么我们依旧可以修改这个属性值,解决办法是用递归来冻结所有属性值为对象的属性。
var deepFreeze= (obj) => {
Object.freeze(obj);
Object.keys(obj).forEach( (key, i) => {
if ( typeof obj[key] === 'object' ) {
deepFreeze( obj[key] );
}
});
}
DOM
的体积过大会影响页面性能,假如你想在用户关闭页面时统计(计算并反馈给服务器)当前页面中元素节点的数量总和、多少种HTML
标签种类、元素节点的最大嵌套深度以及最大子元素个数,请用 JS
配合原生 DOM API
实现该需求(不用考虑陈旧浏览器以及在现代浏览器中的兼容性,可以使用任意浏览器的最新特性;不用考虑 shadow DOM
)。
let maxChildrenCount = 0, // 元素节点的总和
maxDOMTreeDepth = 0, //元素最大嵌套问题
totalElementsCount = 1, //最大子元素
totalDOMTypeCount = new Set([...document.getElementsByTagName("*")].map(x=>x.tagName)).size; //DOM种类
function DFS(e, depth) {
// 没有子节点了, 意味着 DFS 已经抵达一个叶节点, 更新深度计数器
if (e.children.length === 0) {
maxDOMTreeDepth = depth > maxDOMTreeDepth ? depth : maxDOMTreeDepth;
return;
}
let stack = Array.from(e.children)
stack.forEach(n => {
// 更新总标签数
totalElementsCount += 1;
// 更新最大子元素数
if (stack.length > maxChildrenCount) maxChildrenCount = stack.length;
DFS(n, depth + 1);
});
}
DFS(document.querySelector("html"), 1);
console.log({
totalElementsCount,
maxDOMTreeDepth,
maxChildrenCount,
totalDOMTypeCount
});
如何获取页面中出现次数最多的元素及其出现次数?
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