❗deplicate parameters

🚧 施工中

JS ⟩ objects ⟩ built-in ⟩ Function ⟩ parameter ⟩ duplicate parameters

in non-strict mode, JavaScript allows a function to have several parameters with the same name, where later parameters shadow earlier parameters.

🧨 雷區

⭐️ duplicate parameters not allowed in strict mode.

function g(x, y, x, y) {
    //           ^
    // ⛔ SyntaxError: 
    //    Duplicate parameter name not allowed in this context
    //    (note: this is a compile-time error)
    
    'use strict';        // ⭐️ strict mode
    
    return [x, y];
}

👉 duplicate parameter not allowed in strict mode❗️

📗 參考

• YDKJS: Scope & Closures (v.2) ⟩ Ch. 1 ⟩ Required: Two Phases

• Function With Duplicate Parameters. Your Turn, JS!

• Duplicate Function Parameters in JavaScript

💈範例

• replit：duplicate parameters

// function with duplicate parameters
function f(x, y, x, y) {
    return [x, y];
}

//    x = 3
//      ⭡ ↱ y = undefined
f(1, 2, 3  )                    // [ 3, undefined ]

⭐️ 重點

duplicate parameters

mode         FD      FE      AF
----------------------------------
strict       ⛔️      ⛔️      ⛔️         // ⛔️ SyntaxError
non-strict   ✅      ✅      ⛔️         // ✅ valid
----------------------------------
• FD: function declaration, 
• FE: function expression, 
• AF: arrow function

duplicate parameters Early Error rule (🚧) is always applied to arrow function definition even if 'strict mode' (🚧) is not defined explicitly.

👥 相關

• duplicate parameters can cause ⛔️ early errors❗️

🗣 討論

• Javascript function declaration with same arguments

❓

why does JavaScript even allow you to do such an obviously stupid thing in the first place?

🚧

• strict mode

• Early Error rule