JavaScript開發(fā)人員需要知道的簡(jiǎn)寫技巧（高級(jí)篇）

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

const variable2 = variable1 || 'new';

let variable1;
let variable2 = variable1 || '';
console.log(variable2 === ''); // prints true
variable1 = 'foo';
variable2 = variable1 || '';
console.log(variable2); // prints foo

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

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

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
}

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

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;

// 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 === 'Dog' && 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

// object validation rules
const schema = {
 first: {
 required:true
 },
 last: {
 required:true
 }
}
 
// universal validation function
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