⛔'xxx' is not defined❗️

JS ⟩ error ⟩ ReferenceError ⟩ 'xxx' is not defined

⛔️ ReferenceError：'xxx' is not defined❗

✨ 範例

• eval has its own scope in strict mode

• accidental global variable in sloppy mode❗️

• function in block (FiB)❗

• dot notation (.) - when obj is undeclared.

⭐️ 重點

with a function in block (FiB)

• sloppy mode: ✅ hoisted / initialized to undefined (in outer scope)

• strict mode: ⛔ ReferenceError (invisible in outer scope)❗

💈範例

• replit：modify scope at runtime

function evalCantModifyScopeInStrictMode() {
    
    'use strict';    // ⭐️ strict mode
    
    // ---------------------------------------------------------------
    // ⭐️ in strict mode, `eval` doesn't introduce new variables into
    //    the surrounding scope, they're local to the script itself.
    eval("var b = 'bbb!';");
    //        ╰── `b` is "local" to the inner script❗
    // ---------------------------------------------------------------
    
    console.log(b);    // `b` is invisible.
    //          ^
    // ⛔ ReferenceError: `b` is not defined
}

evalCantModifyScopeInStrictMode();   // ⛔ ReferenceError (runtime error)

• more examples ...

  • eval has its own scope in strict mode

  • accidental global variable in sloppy mode❗️

👥 相關

• ⛔️ ReferenceError is a runtime error.