➕bracket notation []

`obj[prop]` syntax.

JS ⟩ value ⟩ object ⟩ property ⟩ access ⟩ bracket notation
JS ⟩ statement ⟩ expression ⟩ operator ⟩ left-hand side ⟩ property accessor ⟩ bracket notation

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

use obj [ prop ] to evaluate an object property / array element.

// ⭐️ bracket notation
obj [ prop ]    // obj: non-nullish, prop: string | symbol (any other value coerced to string)

🈯 synonyms： "computed property access"

element index is string❗️

replit：bracket notation

// object
let obj = { name: 'Joe' };

// ⭐️ bracket notaton  
// -------------------------------------------------------------------
//    • syntax: `obj [ prop ]`
//    • obj : expression (❗ value of null/undefined raises TypeError)
//    • prop: expression (❗ evaluated and "converted" to string)
// -------------------------------------------------------------------

obj[''] = 'joe';     // ✅ "empty string" is OK
obj[' '] = 'OK';     // ✅ "space" is OK
obj[true] = true;    // ✅ boolean is OK


const key = "name";          // variable
const getKey = () => "name"; // function

// ⭐️ `prop` can be any expression.
obj["name"],     // 'Joe'    (prop: string literal)
obj[key],        // 'Joe'    (prop: variable)
obj[getKey()],   // 'Joe'    (prop: function return value)

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Last updated 3 years ago