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If arrays are ordered lists, objects are key→value maps: you look values up by name, not by position. They're how you group related data into a single value. In modern JS, objects are everywhere: configuration, JSON-decoded data, return values, instance state.

Creating objects

Keys are strings (or Symbols). Values can be ANY type — primitives, arrays, other objects, even functions.

Accessing properties

Two syntaxes:

Use dot when you know the key at write-time. Use brackets when the key is in a variable.

Adding, updating, removing

Objects are mutable even when declared with const — const locks the binding, not the contents.

Property shorthand

When variable name matches the key, you can omit the value:

Destructuring — pull out properties

Spread — copy and merge

The spread is the modern way to make shallow copies. Mutation-free updates: instead of obj.x = 5, write { ...obj, x: 5 }.

Iteration

Membership

Methods — functions stored as properties

Method shorthand { name() {} } is preferred over { name: function() {} }.

Common mistakes

• Using === to compare objects — checks reference, not contents. {a:1} === {a:1} is false.
• Forgetting that obj is passed by reference — modifying inside a function affects the caller.
• Using for...in and forgetting it includes inherited properties — use Object.keys() for own properties.
• Spreading deep objects expecting deep copy — ... is shallow. Use structuredClone(obj) for deep copies.