🆔identifier

the name of a variable/function/property/label/parameter.

JS ⟩ grammar ⟩ token ⟩ identifier

(token) the name of a variable / function / property / label / parameter.

• case-sensitive ❗

• must begin with

  • letter (a...z, A...Z, or Unicode letters)

  • underscore (_)

  • dollar sign ($)

  • may not start with a digit (0-9)❗

• subsequent characters can be digits (0...9).

⭐️ 重點

identifiers

• registered to respective scope at compile-time.

• created at the beginning of the scope it belongs to, every time that scope is entered (at runtime).

avoid using reserved word as identifier❗ (at least for variable/function/label)

the property name in dot notation (.) can be a reserved word❗

there are 5 categories of identifier names:

• not keywords (always allowed as identifiers)： Math, window, toString, _ ...

• keywords：

  • never allowed as identifiers： reserved word (except await, yield)

  • contextually allowed as identifiers： namely await, yield.

  • contextually disallowed as identifiers, in strict mode code:： let, static, implements, interface, package, private, protected, public.

  • always allowed as identifiers, but also appear as keywords within certain syntactic productions, at places where identifier is not allowed: as, async, from, get, meta, of, set, target.

The term conditional keyword, or contextual keyword, is sometimes used to refer to the keywords that fall in the last 3 categories, and thus can be used as identifiers in some contexts and as keywords in others. 📘ECMA

NaN/Infinity/ undefined are

• read-only properties (identifier) of the global object❗

• not keywords.

the identifier of a function expression is in its own function name scope.

only object's configurable properties are "qualified" for "delete", other identifiers are "unqualified" ( can't delete them❗). 📘 MDN

🔴 主題

• types of identifiers

  • undeclared identifier

  • unqualified identifier for delete - can't be deleted❗
  • globalThis - identifier for the global object.

  • special identifier

  • label of a statement

  • function name

  • parameter(s) of a function

  • property name

• comparisons

  • identifier vs. string

  • identifier vs. keyword

• binding of a value

  • static binding (the value of an identifier is predetermined at compile-time)

  • dynamic binding (the value of an identifier is not determined until runtime)

👥 相關

• part of grammar

• the identifier of a function expression is in its own function name scope.

• ⛔delete of an unqualified identifier in strict mode❗️ - variables can't be deleted❗
• ⛔can't delete variable/function❗️
• ⛔ the "prop" part of dot notation (.) must be an identifier❗

📗 參考

• YDKJS: Scope & Closures (v.2) ⟩ Ch. 5 ⟩ When Can I Use a Variable?

📘 手冊

• Identifier
• ECMA ⟩ 13. Expressions ⟩ 13.1 Identifiers
• Statements and declarations ⟩ Others ⟩ label