Reverse a string
0 likes • Nov 19, 2022
JavaScript
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const nums = [1, 2, 3];const strs = Array.from('est');nums.push(6);nums.push(4, 9);strs.unshift('t');nums.length; // 6nums[nums.length - 1]; // 9strs[0]; // 't'strs[2]; // 's'nums.slice(1, 3); // [2, 3]nums.map(n => n * 2); // [2, 4, 6, 12, 8, 18]nums.filter(n => n % 2 === 0); // [2, 6, 4]nums.reduce((a, n) => a + n, 0); // 25strs.reverse(); // ['t', 's', 'e', 't']strs.join(''); // 'test'const nums = new Set([1, 2, 3]);nums.add(4);nums.add(1);nums.add(5);nums.add(4);nums.size; // 5nums.has(4); // truenums.delete(4);nums.has(4); // false[...nums]; // [1, 2, 3, 5]nums.clear();nums.size; // 0const items = new Map([[1, { name: 'John' }],[2, { name: 'Mary' }]]);items.set(4, { name: 'Alan' });items.set(2, { name: 'Jeff' });items.size; // 3items.has(4); // trueitems.get(2); // { name: 'Jeff' }items.delete(2);items.size; // 2[...items.keys()]; // [1, 4][...items.values()]; // [{ name: 'John' }, { name: 'Alan' }]items.clear();items.size; // 0
const MongoClient = require('mongodb').MongoClient;const assert = require('assert');// Connection URLconst url = 'mongodb://localhost:27017';// Database Nameconst dbName = 'myproject';// Use connect method to connect to the serverMongoClient.connect(url, function(err, client) {assert.equal(null, err);console.log("Connected successfully to server");const db = client.db(dbName);client.close();});
const toFixed = (n, fixed) => ~~(Math.pow(10, fixed) * n) / Math.pow(10, fixed);// ExamplestoFixed(25.198726354, 1); // 25.1toFixed(25.198726354, 2); // 25.19toFixed(25.198726354, 3); // 25.198toFixed(25.198726354, 4); // 25.1987toFixed(25.198726354, 5); // 25.19872toFixed(25.198726354, 6); // 25.198726
if (process.env.NODE_ENV === "production") {app.use(express.static("client/build"));app.get("*", (req, res) => {res.sendFile(path.resolve(__dirname, "client", "build", "index.html"));});}
const compactObject = val => {const data = Array.isArray(val) ? val.filter(Boolean) : val;return Object.keys(data).reduce((acc, key) => {const value = data[key];if (Boolean(value))acc[key] = typeof value === 'object' ? compactObject(value) : value;return acc;},Array.isArray(val) ? [] : {});};const obj = {a: null,b: false,c: true,d: 0,e: 1,f: '',g: 'a',h: [null, false, '', true, 1, 'a'],i: { j: 0, k: false, l: 'a' }};compactObject(obj);// { c: true, e: 1, g: 'a', h: [ true, 1, 'a' ], i: { l: 'a' } }
class TreeNode {constructor(key, value = key, parent = null) {this.key = key;this.value = value;this.parent = parent;this.children = [];}get isLeaf() {return this.children.length === 0;}get hasChildren() {return !this.isLeaf;}}class Tree {constructor(key, value = key) {this.root = new TreeNode(key, value);}*preOrderTraversal(node = this.root) {yield node;if (node.children.length) {for (let child of node.children) {yield* this.preOrderTraversal(child);}}}*postOrderTraversal(node = this.root) {if (node.children.length) {for (let child of node.children) {yield* this.postOrderTraversal(child);}}yield node;}insert(parentNodeKey, key, value = key) {for (let node of this.preOrderTraversal()) {if (node.key === parentNodeKey) {node.children.push(new TreeNode(key, value, node));return true;}}return false;}remove(key) {for (let node of this.preOrderTraversal()) {const filtered = node.children.filter(c => c.key !== key);if (filtered.length !== node.children.length) {node.children = filtered;return true;}}return false;}find(key) {for (let node of this.preOrderTraversal()) {if (node.key === key) return node;}return undefined;}}const tree = new Tree(1, 'AB');tree.insert(1, 11, 'AC');tree.insert(1, 12, 'BC');tree.insert(12, 121, 'BG');[...tree.preOrderTraversal()].map(x => x.value);// ['AB', 'AC', 'BC', 'BCG']tree.root.value; // 'AB'tree.root.hasChildren; // truetree.find(12).isLeaf; // falsetree.find(121).isLeaf; // truetree.find(121).parent.value; // 'BC'tree.remove(12);[...tree.postOrderTraversal()].map(x => x.value);// ['AC', 'AB']