var varName = 'variant';
//瀏覽器環境
console.log(window.varName); //? => variant
console.log(self.varName);? //? => variant
// 通用環境下
console.log(this.varName);? //? => variant
//Node 環境下
console.log(global.varName);? // 報錯
let letName = 'letName';
console.log(window.letName);? //undefined
console.log(this.letName)? ? //undefined
