➕dot notation (.)

`obj . prop` syntax.

• JS ⟩ value ⟩ object ⟩ property ⟩ access ⟩ dot notation

• JS ⟩ statement ⟩ expression ⟩ operator ⟩ left-hand side ⟩ property access ⟩ dot natation

(property access expression) (🌟 chaining rules | table of operators )

use obj . prop to evaluate object property's value.

obj . prop    // `prop` must be an "identifier".

🈯 synonyms： "chaining", "chaining operator", "dot notation"

🗺️ 圖表

getting property

setting property

• replit：dot notation

// ⭐️ 1. prop: invalid identifier
// ------------------------------
undeclared.1 = 'bad';  // ⛔ SyntaxError: Unexpected number
//        ^^

// ⭐️ 2. obj: "undeclared"
// -------------------------------
undeclared.prop;       // ⛔ ReferenceError: 'undeclared' is not defined

// ⭐️ 3. obj: "nullish"
// -------------------------------
null.prop              // ⛔ TypeError: Cannot read properties of null

// ⭐️ 4. obj: not "nullish"
// -------------------------------
(18).prop;            // ❗ undefined (no such `prop`)
(18).toString(16);    // ❓ "12" (property value, could be any value)

// object
let obj = {};

// ┌───────────────────┐
// │ valid identifiers │
// └───────────────────┘

obj.$1 = 'joe';       // ✅ can begin with $, _
obj.var = 'OK';       // ✅ reserved word is allowed❗

⭐️ 重點

⭐ the "prop" part must be an identifier❗

⭐ use ?. instead if obj may be nullish❗

the assignment of a property always creates/sets a property in the original object, it never modifies objects in the prototype chain.

🔴 主題

• assignment (=)

• ❗reserved word as property name is allowed❗️
• ⚖️dot notation vs. decimal point

👥 相關

• 👉optional chaining (?., ?.[])
• 👉bracket notation []
• ℹ️dot (.) punctuator is used.
• ⛔ possible errors：

  • ReferenceError：'xxx' is not defined❗ (👉undeclared identifier)
  • SyntaxError：unexpected number❗ (👉 needs valid identifier)
  • TypeError：cannot read properties of nullish❗ (👉nullish)

📗 參考

• JavaScript: The Definitive Guide ⟩

  • 6.3 Querying & Setting Properties

  • 14.2 Object Extensibility

📘 手冊

• Property accessors ⟩ dot notation
• Optional chaining (?.)