es6中??与||区别小结

let foo = null;
let bar = foo ?? 'default'; // bar 的值为 'default'

let baz; 
let qux = baz ?? 'default'; // qux 的值为 'default'

let foo = 'hello';
let bar = foo || 'default'; // bar 的值为 'hello'

let baz; 
let qux = baz || 'default'; // qux 的值为 'default'

let person = {
    name: '拾柒',
    body: {
        color: 'yellow',
        height: 179,
        weight: 70,
        sex: null
    },
    action: function(name){
        console.log(name)
    }
}
let res = person?.body?.age; // => undefined 
// 等价于
let res = person&& person.body&& person.body.location // 作用就是判断person下的body下的age是否为null或者undefined，当其中一链为null或者undefined时就直接返回undefined

let res2 = person?.sex; // => undefined

person ?. action ?. ('peter'); // peter

function query(item) {
    return item ?? {}
}
// 等价于
function query(item) {
    if (item == null || item == undefined) {
        return {}
    } else {
        return item;
    }
}

function query(item) {
    return item || {}
}
// 等价于
function query(item) {
    if (item === 0 || item === "" || item === false || item === null || item === undefined) {
        return {}
    } else {
        return item;
    }
}

function config(options) {
    options.duration ??= 100;
    options.speed ??= 25;
    return options;
}

config({ duration: 125 }); // { duration: 125, speed: 25 }
config({}); // { duration: 100, speed: 25 }

let A = 'abc';
let a = 1;
let b = 0;
let c = false;
let d = null;
let e = undefined;
let f = '';
A &&= '拾柒'; // '拾柒'
a &&= '拾柒'; // '拾柒'
b &&= '拾柒'; // 0
c &&= '拾柒'; // false
d &&= '拾柒'; // null
e &&= '拾柒'; // undefined
f &&= '拾柒'; // ''

var a;
if(a != null && typeof(a) != undefined && a != ''){
    //a有内容才执行的代码  
}

if(!!a){
    //a有内容才执行的代码...  
}