Objects in JavaScript

If you come from Java or C#, think of a JavaScript object as an open-ended map: keys are strings or symbols, values are any type, and the shape can change at runtime. There is no compile-time field list for plain objects (TypeScript interfaces are erased at run time).

Object literals

The usual syntax builds an object with properties in one expression.

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const user = {
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  id: 1,
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  name: 'Grace',
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  active: true,
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};

Shorthand when the property name matches a variable:

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const name = 'Grace';
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const user = { name };

Computed keys use an expression in brackets:

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const field = 'id';
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const row = { [field]: 42 };

Property access and existence

Dot notation works for valid identifiers; bracket notation accepts variables and special names.

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user.name;
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user['name'];
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'toString' in user; // inherited methods count
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Object.hasOwn(user, 'name'); // own property only

undefined means the key is missing or explicitly set to undefined; use hasOwn or in when the distinction matters.

References, mutation, and copying

Objects are assigned by reference. Two variables can point at the same object; mutating through one is visible through the other.

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const a = { x: 1 };
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const b = a;
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b.x = 2;
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console.log(a.x); // 2

Shallow copy: Object.assign({}, src), spread { ...src }, or structuredClone (modern engines) duplicate top-level properties; nested objects are still shared unless you deep-clone. See Pass by value and reference in JavaScript for primitives versus objects.

Symbols and non-string keys

Symbol values are unique opaque keys, useful for avoiding name clashes on shared objects.

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const id = Symbol('id');
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const item = { [id]: 99 };

Only Object.getOwnPropertySymbols enumerates symbol keys alongside Object.keys for strings.

When to use plain objects

Use literals for records, options bags, and JSON-shaped data. Reach for Map when keys are not strings/symbols or when you need stable iteration order without prototype noise. Classes and constructors (later in this series) fit when many instances share behavior on a prototype.

Next: Defining object properties in JavaScript.