Codectionary / Developer documentation / JavaScript

Object.keys(), values(), entries(), assign(), freeze()

These static Object methods work with any plain object. keys(), values(), and entries() return arrays of an object's own enumerable property names, values, or [key, value] pairs respectively - useful for iterating over objects. assign() copies properties from source objects into a target. freeze() prevents any further modification to an object.

Syntax

Object.keys(obj)
Object.values(obj)
Object.entries(obj)

Examples

keys(), values(), entries()

The three ways to extract data from an object for iteration.

const user = { name: "Alice", age: 25, city: "London" };

console.log(Object.keys(user));   // ["name", "age", "city"]
console.log(Object.values(user)); // ["Alice", 25, "London"]
console.log(Object.entries(user)); // [["name","Alice"],["age",25],["city","London"]]

// entries() pairs perfectly with for...of
for (const [key, value] of Object.entries(user)) {
  console.log(`${key}: ${value}`);
}

Object.assign() - Merging Objects

Copying properties from one or more source objects into a target.

const defaults = { theme: "light", fontSize: 14 };
const userPrefs = { fontSize: 18 };

const merged = Object.assign({}, defaults, userPrefs);
console.log(merged); // { theme: "light", fontSize: 18 }

// Note: the spread operator ({...defaults, ...userPrefs}) does the same thing and is more common today

Object.freeze() - Immutability

Preventing an object from being modified after creation.

const config = Object.freeze({ apiUrl: "https://api.example.com", timeout: 5000 });

config.timeout = 10000; // silently fails in non-strict mode, throws in strict mode
console.log(config.timeout); // still 5000 - the object is frozen

console.log(Object.isFrozen(config)); // true

Best practices

  • Use Object.entries() combined with for...of or map() when you need both the key and value while iterating
  • Prefer spread syntax ({...obj}) over Object.assign() for merging objects in modern code - it is more concise and equally capable
  • Use Object.freeze() for configuration objects or constants that should never be accidentally mutated
  • Remember Object.freeze() is shallow - nested objects inside a frozen object can still be modified unless they are frozen too

Runtime & versions

Browsers and JavaScript runtimes. Feature availability depends on the language and runtime version used by your project; consult the official reference below.

ECMAScript specification

Specifications & further reading