📦property access expression

use . / [] / ?. / ?.[] to evaluate object property / array element.

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

• JS ⟩ value ⟩ object ⟩ property ⟩ access ⟩ expression

(operator)

use ., [], ?., ?.[], ?.() to evaluate the value of object property / array element.

// ⭐️ (normal) chaining
obj . identifier    // dot notation
obj   [ expr ]      // bracket notation (no "dot"❗️)

// ⭐️ optional chaining
obj  ?. prop        // dot notation
obj  ?. [ expr ]    // bracket notation
func ?. ( args )    // optional invocation

🈯 synonyms： "property accessor", "property access expression", "chaining"

⭐️ 重點

the "obj" part of the property accessor：

• the expression before . / [ (the obj part) is first evaluated.

• if the value is null / undefined, TypeError is thrown.

• use (?. / ?.[]) to guard againt this case.

the [expr] part of bracket notation []：

• is evaluated and converted to a string❗

property access and invocation expressions have higher precedence than any of the operators.

🔴 主題

• 🌟chaining rules

• dot notation (.) - chaining

• bracket notation []

• optional chaining (?., ?.[])

• optional invocation ?.()

👥 相關

• 💡Symbol can be used in the [expr] part of bracket notation [].

• ⛔cannot read properties of nullish❗️ - occurs when obj is nullish.

• delete <expr> returns true if <expr> is not a property access expression.

📗 參考

• JavaScript: The Definitive Guide ⟩

  • 4.4 Property Access Expressions

📘 手冊

• Expressions and operators ⟩ Property accessors