🛡️strict mode

🚧 施工中

JS ⟩ concepts ⟩ environment ⟩ JS engine ⟩ mode ⟩ strict

🛡️ always use strict mode❗ (👉 see why)

a way to opt in to a restricted variant of JavaScript, thereby implicitly opting-out of "sloppy mode".

⭐️ 重點

in strict mode：

• duplicate parameter not allowed in strict mode❗️❗

• function in block (FiB) is local to the block.

JS is lenient with var, strict with let.

• var redeclaration applied even in strict mode❗️, even in strict mode❗

• let redeclaration not allowed even in sloppy mode❗️, even in sloppy mode❗

referencing a function in block (FiB) outside the block：

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

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

class / ES module always works in strict mode❗️

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