http2 支持并发请求(单个连接多路复用),但当同时发送上千个请求时,消耗大量内存,浏览器会变的明显卡顿,所以一个解决方案是给并发添加最大数量限制。如果是 http1.1,浏览器会有默认的并发限制,并不需要我们处理这个问题,比如 Chrome 中并发数量是 6 个,所以这个问题的成立是建立在 http2 的基础上。
const concurrencyRequest = (urls, maxNum) => {
return new Promise((resolve) => {
if (urls.length === 0) {
resolve([]);
return;
}
const results = [];
let count = 0;
let index = 0;
async function request() {
if (index === urls.length) return;
const i = index; // 保存序号
const url = urls[i];
index++;
try {
const resp = await fetch(url);
results[i] = resp;
} catch (err) {
results[i] = err;
} finally {
count++;
if (count === urls.length) {
resolve(results);
}
request();
}
}
const times = Math.min(maxNum, urls.length);
for (let i = 0; i < times; i++) {
request();
}
})
}
// test
const urls = [];
for (let i = 1; i <= 20; i++) {
urls.push(`https://jsonplaceholder.typicode.com/todos/${i}`);
}
concurrencyRequest(urls, 3).then(res => {
console.log(res);
})
Promise.all
and Promise.resolve
by yourself.
``js
Promise._all = function (promises) {
return new Promise((resolve, reject) => {
let counter = 0;
const result = [];
for (let i = 0; i < promises.length; i++) {
// Use
Promise.resolve(promises[i])instead of
promises[i].then(),
// if it is non-promise value, wrap it
// if already promise,
Promise.resolve(promises[i])` returns the same Promise
Promise.resolve(promises[i]).then(res => { result[i] = res;
counter += 1;
if (counter === promises.length) {
resolve(result);
}
}, err => { reject(err);
});
}
});
};Promise._resolve = function (value) { if (value instanceof Promise) { return value; } else { return new Promise((resolve, reject) => { resolve(value); }); } };
3. Implement `debounce` and `throttle`.
```js
function debounce(fn, delay) {
let timer = null
return (...args) => {
if (timer) {
clearTimeout(timer)
timer = null
}
timer = setTimeout(() => {
fn(...args)
}, delay)
}
}
function throttle(fn, delay) {
let currentTime = Date.now()
return (...args) => {
if (Date.now() - currentTime > delay) {
fn(...args)
currentTime = Date.now()
}
}
}
bind
function by yourself.Function.prototype.myBind = function(context, ...args1) {
const fn = this;
return function(...args2) {
return fn.apply(context, [...args1, ...args2]);
};
};
function listToTree(list) { const map = {} const roots = []
list.forEach(item => { map[item.id] = { ...item, children: [] } })
list.forEach(item => { if (item.pid === 0) { roots.push(map[item.id]) } else { if (map[item.pid]) { map[item.pid].children.push(map[item.id]) } } })
return roots }
6. Use `setTimeout` to invoke a function multiple times in the fixed interval.
```js
function repeat(func, times, ms, immediate) {
let count = 0;
return function inner(...args) {
if (count === 0 && immediate) {
func(...args);
count++;
}
if (count >= times) {
return;
}
setTimeout(() => {
func(...args);
count++;
inner(...args);
}, ms);
}
}
// test
const repeatFunc = repeat(console.log, 4, 3000, true);
repeatFunc("hello");
Implement the functionality of lodash.get
.
```js
function get(obj, path, defaultValue = undefined) {
const keys = Array.isArray(path) ? path : path.split('.');
let result = obj;
for (const key of keys) { if (result == null || typeof result !== 'object') { return defaultValue; } result = result[key]; }
return result === undefined ? defaultValue : result; }
// test const obj = { a: { b: { c: 42 } } }; console.log(get(obj, 'a.b.c')); // 42 console.log(get(obj, ['a', 'b', 'c'])); // 42 console.log(get(obj, 'a.b.d', 'default')); // 'default' console.log(get(obj, 'x.y.z', 'not found')); // 'not found'
8. Implement the render function to convert the virtual dom JSON to real DOM.
```js
function render(vnode) {
const { tag, props, children } = vnode;
const el = document.createElement(tag);
if (props) {
for (const key in props) {
const value = props[key];
if (key.startsWith("on")) {
el.addEventListener(key.slice(2).toLowerCase(), value);
} else {
el.setAttribute(key, value);
}
}
}
if (children) {
if ([string, number].includes(typeof children)) {
el.textContent = children;
} else {
children.forEach((item) => {
el.appendChild(render(item));
});
}
}
return el;
}
renderToString
does.// written by Dan Abramov
// e.g. <div>hello<span>world</div>
// {
// $$typeof: Symbol("react.element"),
// type: "div",
// props: {
// children: [
// "hello",
// {
// $$typeof: Symbol("react.element"),
// type: "span",
// props: {
// children: "world"
// }
// }
// ]
// },
// }
async function renderJSXToHTML(jsx) {
if (typeof jsx === "string" || typeof jsx === "number") {
// This is a string. Escape it and put it into HTML directly.
return escapeHtml(jsx);
} else if (jsx == null || typeof jsx === "boolean") {
// This is an empty node. Don't emit anything in HTML for it.
return "";
} else if (Array.isArray(jsx)) {
const childHtmls = await Promise.all(
jsx.map((child) => renderJSXToHTML(child))
);
return childHtmls.join("");
} else if (typeof jsx === "object") {
// Check if this object is a React JSX element.
if (jsx.$$typeof === Symbol.for("react.element")) {
if (typeof jsx.type === "string") {
let html = "<" + jsx.type;
for (const propName in jsx.props) {
if (jsx.props.hasOwnProperty(propName) && propName !== "children") {
html += " ";
html += propName;
html += "=";
html += escapeHtml(jsx.props[propName]);
}
}
html += ">";
html += await renderJSXToHTML(jsx.props.children);
html += "</" + jsx.type + ">";
return html;
} else if (typeof jsx.type === "function") {
// Call the component with its props, and turn its returned JSX into HTML.
const Component = jsx.type;
const props = jsx.props;
const returnedJsx = await Component(props);
return renderJSXToHTML(returnedJsx);
}
}
};
}
function cycle(obj) {
const seen = new Set();
function helper(o) {
if (typeof o !== 'object' || o === null) return;
seen.add(o);
for (let key in o) {
if (typeof o[key] === 'object' && o[key] !== null) {
if (seen.has(o[key])) {
o[key] = 'cycle';
continue;
} else {
helper(o[key]);
}
}
}
}
helper(obj);
return obj;
}
// 嵌套很深的对象遍历
// dfs
function traverseDFS(obj) {
if (typeof obj !== 'object' || obj === null) return;
for (const key in obj) {
if (obj.hasOwnProperty(key)) {
console.log(key, obj[key]);
traverseDFS(obj[key]);
}
}
}
// bfs
function traverseBFS(obj) {
const queue = [obj];
while (queue.length > 0) {
const current = queue.shift();
for (const key in current) {
if (current.hasOwnProperty(key)) {
console.log(key, current[key]);
if (typeof current[key] === 'object' && current[key] !== null) {
queue.push(current[key]);
}
}
}
}
}