object

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object#

Everything not primitive is an object. Plain objects are unordered (insertion-ordered for string keys in practice) key-value containers; keys are strings or symbols.

Object literal with two properties.

const obj = {name: "rk", port: 8080};

Dot access for a known key.

obj.name;             // "rk"

Bracket access with a string key.

obj["port"];          // 8080

Bracket access with a computed key.

const k = "name";
obj[k];               // "rk"

Symbol keys are invisible to for...in and JSON.stringify.

const sym = Symbol("token");
obj[sym] = "secret";

Arrays are objects with integer keys plus .length; their prototype carries iteration helpers (map, filter, reduce, find, flat). See Data Structures.

Functions are objects too; typeof fn === "function" is the historic exception.

typeof (() => {});    // "function"

References#

  • Data Structures for Array, Map, Set, WeakMap, TypedArray.

  • OOP for classes and prototypes.

  • symbol for symbol-keyed properties.

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