Introduction to the abbreviation of js

const x = 20;
let answer;
if (x > 10) {
  answer = 'is greater';
} else {
  answer = 'is lesser';
}

const answer = x > 10 ? 'is greater' : 'is lesser';

const big = x > 10 ? " greater 10" : x

if (variable1 !== null || variable1 !== undefined || variable1 !== '') {
   let variable2 = variable1;
}

const variable2 = variable1 || 'new';

let x;
let y;
let z = 3;

let x, y, z=3;

if (likeJavaScript === true)

if (likeJavaScript)

let a;
if ( a !== true ) {
// do something...
}

let a;
if ( !a ) {
// do something...
}

for (let i = 0; i < allImgs.length; i++)

for (let index in allImgs)

function logArrayElements(element, index, array) {
 console.log("a[" + index + "] = " + element);
}
[2, 5, 9].forEach(logArrayElements);
// logs:
// a[0] = 2
// a[1] = 5
// a[2] = 9

let dbHost;
if (process.env.DB_HOST) {
 dbHost = process.env.DB_HOST;
} else {
 dbHost = 'localhost';
}

const dbHost = process.env.DB_HOST || 'localhost';

for (let i = 0; i < 10000000; i++) {}

for (let i = 0; i < 1e7; i++) {}

// 下面都是返回true
1e0 === 1;
1e1 === 10;
1e2 === 100;
1e3 === 1000;
1e4 === 10000;
1e5 === 100000;

const obj = { x:x, y:y };

const obj = { x, y };

function sayHello(name) {
 console.log('Hello', name);
}

setTimeout(function() {
 console.log('Loaded')
}, 2000);

list.forEach(function(item) {
 console.log(item);
});

sayHello = name => console.log('Hello', name);
setTimeout(() => console.log('Loaded'), 2000);
list.forEach(item => console.log(item));

function calcCircumference(diameter) {
 return Math.PI * diameter
}

var func = function func() {
 return { foo: 1 };
};

calcCircumference = diameter => (
 Math.PI * diameter;
)

var func = () => ({ foo: 1 });

function volume(l, w, h) {
 if (w === undefined)
  w = 3;
 if (h === undefined)
  h = 4;
 return l * w * h;
}

volume = (l, w = 3, h = 4 ) => (l * w * h);
volume(2) //output: 24

const welcome = 'You have logged in as ' + first + ' ' + last + '.'

const db = 'http://' + host + ':' + port + '/' + database;

const welcome = `You have logged in as ${first} ${last}`;

const db = `http://${host}:${port}/${database}`;

const observable = require('mobx/observable');
const action = require('mobx/action');
const runInAction = require('mobx/runInAction');

const store = this.props.store;
const form = this.props.form;
const loading = this.props.loading;
const errors = this.props.errors;
const entity = this.props.entity;

import { observable, action, runInAction } from 'mobx';

const { store, form, loading, errors, entity } = this.props;

const { store, form, loading, errors, entity:contact } = this.props;
//最后一个变量名为contact

const lorem = 'Lorem ipsum dolor sit amet, consectetur\n\t'
  + 'adipisicing elit, sed do eiusmod tempor incididunt\n\t'
  + 'ut labore et dolore magna aliqua. Ut enim ad minim\n\t'
  + 'veniam, quis nostrud exercitation ullamco laboris\n\t'
  + 'nisi ut aliquip ex ea commodo consequat. Duis aute\n\t'
  + 'irure dolor in reprehenderit in voluptate velit esse.\n\t'

const lorem = `Lorem ipsum dolor sit amet, consectetur
  adipisicing elit, sed do eiusmod tempor incididunt
  ut labore et dolore magna aliqua. Ut enim ad minim
  veniam, quis nostrud exercitation ullamco laboris
  nisi ut aliquip ex ea commodo consequat. Duis aute
  irure dolor in reprehenderit in voluptate velit esse.`

// joining arrays
const odd = [1, 3, 5];
const nums = [2 ,4 , 6].concat(odd);

// cloning arrays
const arr = [1, 2, 3, 4];
const arr2 = arr.slice()

// joining arrays
const odd = [1, 3, 5 ];
const nums = [2 ,4 , 6, ...odd];
console.log(nums); // [ 2, 4, 6, 1, 3, 5 ]

// cloning arrays
const arr = [1, 2, 3, 4];
const arr2 = [...arr];

const odd = [1, 3, 5 ];
const nums = [2, ...odd, 4 , 6];

const { a, b, ...z } = { a: 1, b: 2, c: 3, d: 4 };
console.log(a) // 1
console.log(b) // 2
console.log(z) // { c: 3, d: 4 }

function foo(bar) {
 if(bar === undefined) {
  throw new Error('Missing parameter!');
 }
 return bar;
}

mandatory = () => {
 throw new Error('Missing parameter!');
}

foo = (bar = mandatory()) => {
 return bar;
}

const pets = [
 { type: 'Dog', name: 'Max'},
 { type: 'Cat', name: 'Karl'},
 { type: 'Dog', name: 'Tommy'},
]

function findDog(name) {
 for(let i = 0; i<pets.length; ++i) {
  if(pets[i].type === &#39;Dog&#39; && pets[i].name === name) {
   return pets[i];
  }
 }
}

pet = pets.find(pet => pet.type ==='Dog' && pet.name === 'Tommy');
console.log(pet); // { type: 'Dog', name: 'Tommy' }

function validate(values) {
 if(!values.first)
  return false;
 if(!values.last)
  return false;
 return true;
}

console.log(validate({first:'Bruce',last:'Wayne'})); // true

// 对象验证规则
const schema = {
 first: {
  required:true
 },
 last: {
  required:true
 }
}

// 通用验证函数
const validate = (schema, values) => {
 for(field in schema) {
  if(schema[field].required) {
   if(!values[field]) {
    return false;
   }
  }
 }
 return true;
}


console.log(validate(schema, {first:'Bruce'})); // false
console.log(validate(schema, {first:'Bruce',last:'Wayne'})); // true

Math.floor(4.9) === 4 //true

~~4.9 === 4 //true