🔰scope

🚧 under construction

JS ⟩ scope

( visibility of variable)

the current context of execution in which values and expressions are "visible" or can be referenced.

🔴 主題

• concepts about scopes

  • context vs. scope

  • lexical environment vs. scope

  • scope chain

  • lexical scope / dynamic scope / static vs. dynamic scoping

  • lexical environment - stores local variables/functions/this.

• kindes of scopes

  • top-level scope - the scope of a JS file.

  • global scope： default scope for all code running in script mode.

  • module scope： scope for code running in module mode.

  • function scope： scope created with a function.

  • block scope：scope created with a pair of curly braces. (where let / const can belong to)

• special scopes

  • function name scope - where name of function expression lives in.

  • parameter scope

  • "for-init" scope - where the for loop initialization variables live in.

• hoisting

  • function hoisting

  • var hoisting

  • function expression not hoisted

• other topics

  • modify current scope at runtime❗️

  • eval has its own scope in strict mode

⭐️ 重點

scopes are

• determined at compile-time.

• but (lexical environment) created at runtime, each time a scope is entered.

• but in sloppy mode, you can modify scope (I think the term "scope" is not quite accurate, maybe lexical environment❓) at runtime. ( 👉 modify current scope at runtime❗️❗)

The scope of a code block can reach all its parents lexical environment so it can use its parent’s variables. In this way, we have a scope chain, chain of all parent’s scope.

context vs. scope

• scope：visibility of variable.

• context：(this) object to which a function refers.

top-level scope vs. global scope

• top-level scope： relative to each JS file.

• global scope： global absolutely.

in Node.js, each JS file is treated as a module.

inner scopes have access to outer scopes, but not vice versa.

a closure (or IIFE) can be used to create a scope to hide variables/ functions.

if the code you need to wrap a scope around has return, this, break, or continue in it, don't use a function/IIFE(which has its own function boundary), use a block instead❗️

📗 You Don't Know JS Yet: Scopes & Closrues

雖說似乎有一個奇怪的 "for-init" scope，但裡面 let 變數的表現其實更像是 block-scoped。👉 scopes matter with closures ⟩ ❓

👥 相關

• scopes are determined at compile-time.

• scopes are created at runtime.

• in strict mode, script in eval has its own scope.

• closure ... 🚧

• lexical environment

📗 參考

• Kevin Chisholm ⟩ What is the Difference Between Scope and Context in JavaScript?
• YDKJS: Scope & Closures (v.2)

• JS.info ⟩ Variable scope, closure
• itHome ⟩ 你不可不知的 JavaScript 二三事 ⟩

  • #Day5：變數作用域 (Scope) (1)
  • #Day6：變數作用域 (Scope) (2)
  • #Day9：圖解變數作用域 (Scope)

📘 手冊

• Scope

🗣 討論

• What is the 'global' object in NodeJS
• Lexical environment and function scope