JS Boolean，Array，Object的基础知识

Boolean

console.log(true); //true
console.log(false); //false

console.log(0); //0
console.log(1); //1
console.log(2); //2

console.log(!0); //true
console.log(!1); //false
console.log(!2); //false

console.log(!!0); //false
console.log(!!1); //true
console.log(!!2); //true

if (array.length) {
    console.log(true)  // 1
}else
    console.log(false)  //0
}

空Array，空Object

//空对象
let a = {}; // new Object({}) , new Object(null)
console.log(a); //{}
console.log(a.name); //undefined
console.log(typeof a.name === 'undefined'); //true
console.log(a.length); //undefined ，无length
console.log(JSON.stringify(a) === '{}'); //true  [√]
console.log(!Object.keys(a).length); //true      [√]
console.log(Object.keys(a)); //[]

a.name?console.log('true'):console.log('false'); //false
typeof a.name === 'undefined'?console.log('true'):console.log('false'); //true

//空数组
let a = []; // new Array()
console.log(a); //[]
console.log(a.length === 0); //true              [√]
console.log(JSON.stringify(a) === '[]'); //true  [√]
console.log(!Object.keys(a).length); //true      [√]
console.log(Object.keys(a)); //[]

Array，Object

//对象
let a = {id:'001',name:'Moonsic'};
console.log(a);
console.log(b); //报错： b is not defined
console.log(typeof b === 'undefined'); //true
console.log(a.id); //001
console.log(a.age); //undefined
console.log(typeof a.age === 'undefined'); //true
console.log(Object.keys(a)); //["id", "name"]

//数组
let a = ['001','002','003','004','005'];
console.log(a);
console.log(typeof b === 'undefined'); //true
console.log(a[0]); //001
console.log(a[5]); //undefined
console.log(typeof a[5] === 'undefined'); //true
console.log(Object.keys(a)); //["0", "1", "2", "3", "4"]

null

let a = null;
a.name,
a.length,
typeof a.name === 'undefined'
全部报错：Cannot read property 'name' of null