✅let can't shadow parameter even in sloppy mode❗️

JS ⟩ variable ⟩ shadowing ⟩ let can't shadow parameter even in sloppy mode.

✅let can't shadow parameter(s) even in sloppy mode❗

👥 相關

• function declaration instantiation

• lexical environment

• parameter / argument

• strict mode

• ✅stop using var❗️

• ❗var can shadow parameter even in strict mode❗️

💈範例

• replit：let can't shadow parameters

'use strict';    // ⭐ toggle sloppy/strict mode

const { log } = console;

// ⭐ only "simple" parameters
//    "parameters" / "body declarations" in the same environment record.
function f(x) {
    let x = 1;        // ⛔ SyntaxError
    log(x);
}

// ⭐ "non-simple" parameters exist,
//    "parameters" / "body declarations" in different environment records.
function g(x = 0) {
    let x = 100;      // ⛔ SyntaxError
    log(x);
}

function h(x = 0) {
    var x = 300;      // ❗ "var" do shadow parameter (even in strict mode)
    log(x);
}

function k(x) {
    var x = 400;      // ❗ "var" do shadow parameter
    log(x);
}

//   mode:   sloppy            strict
// -----------------------------------------------
f(10);    // ⛔ SyntaxError    ⛔ SyntaxError
g(20);    // ⛔ SyntaxError    ⛔ SyntaxError
h(30);    // ✅ 300            ✅ 300
k(40);    // ✅ 400            ✅ 400

// ⛔ SyntaxError: Identifier 'x' has already been declared

📘 手冊

• JS spec ⟩

  • FunctionDeclarationInstantiation

  • Function Environment Record

🗣 討論

• shadowing parameters by local variables? (my question)

• Where are arguments positioned in the lexical environment? ⭐️

📗 參考

• YDKJS: Scope & Closures (v.2) ⟩ Ch. 8 ⟩ Parameter Scope