🔰trailing comma

• JS ⟩ grammar ⟩ token ⟩ literal ⟩ trailing comma

single trailing comma is allowed in array literal / object literal.

let arr = [ 1, 2, 3, ];         // 💡 single trailing comma allowed, 
//                 ^            //     won't create "undefined element".

let obj = { name: 'Joy', age: 17, }
//                               ^ <---- trailing comma

💾 replit：undefined elements in array literal

📘 手冊

• Array

• Expressions and operators ⟩

• Lexical Grammar ⟩ Array Literals

📗 參考