js中 ?.可选链接操作符，??控制合并操作符和||

var test = 1, obj = {}

• test = obj || '嘿嘿'

等价于

if(
    obj === '' ||
    obj === null ||
    obj === undefined ||
    obj === 0 ||
    obj === false ||
) {
    test = '嘿嘿'
} else {
    test = obj
}

• test = obj ?? {}

等价于

if (
    obj === null ||
    obj === undefined ||
) {
    test = '嘿嘿'
} else {
    test = obj
}

?.可选链接操作符 表达式中的引用如不存在会返回undefined，不会报错

let obj ={}
console.log(obj.a.b)
//  TypeError: Cannot read property 'b' of undefined

console.log(obj?.a?.b)
// undefined