所有 JavaScript 原始值包装对象都会转换为布尔值 true

JavaScript 高级程序设计读书笔记

let number = 0 
console.log(`number: ${!!number}`) // number: false

let wrapNumber = new Number(0)
console.log(`wrapNumber: ${!!wrapNumber}`) // wrapNumber: true

let arr = false
console.log(`arr: ${!!arr}`) // arr: false

let wrapArr = new Boolean(false)
console.log(`wrapArr: ${!!wrapArr}`) // wrapArr: true

let str = ''
console.log(`str: ${!!str}`) // str: false

let wrapStr = new String('')
console.log(`wrapStr: ${!!wrapStr}`) // wrapStr: true