The Object.entries() Method

The Object.entries() method in JavaScript is a powerful and versatile tool that allows you to convert objects into arrays of key-value pairs. This can be incredibly useful for various tasks such as iterating over an object's properties, converting an object to a Map, or simply making object property manipulation more straightforward.

Syntax

Object.entries(obj)

Parameters

• obj: The object whose enumerable string-keyed property key-value pairs are to be returned.

Return Value

An array of the given object's own enumerable string-keyed property key-value pairs.

Basic Usage

Let's start with a simple example to see how Object.entries() works:

const obj = { foo: "bar", baz: 42 };
console.log(Object.entries(obj)); // [ ['foo', 'bar'], ['baz', 42] ]

The method converts the object into an array of key-value pairs. Each pair is represented as an array of two elements: the key and the value.

Working with Array-like Objects

Object.entries() can also be used with array-like objects:

const arrayLike = { 0: "a", 1: "b", 2: "c" };
console.log(Object.entries(arrayLike)); // [ ['0', 'a'], ['1', 'b'], ['2', 'c'] ]

Even if the keys are numeric, they will still be treated as strings within the key-value pairs.

Key Order

Keys in JavaScript objects do not always maintain the order you expect. However, Object.entries() respects the order:

const unordered = { 100: "a", 2: "b", 7: "c" };
console.log(Object.entries(unordered)); // [ ['2', 'b'], ['7', 'c'], ['100', 'a'] ]

Handling Non-Enumerable Properties

Non-enumerable properties will be skipped:

const myObj = Object.create(
  {},
  {
    getFoo: {
      value() {
        return this.foo;
      },
    },
  }
);
myObj.foo = "bar";
console.log(Object.entries(myObj)); // [ ['foo', 'bar'] ]

Primitives and Errors

Non-object arguments are coerced to objects, but undefined and null throw a TypeError:

console.log(Object.entries("foo")); // [ ['0', 'f'], ['1', 'o'], ['2', 'o'] ]
console.log(Object.entries(100)); // []
// console.log(Object.entries(null)); // TypeError: Cannot convert undefined or null to object

Converting to a Map

Objects can be easily converted to a Map:

const obj = { foo: "bar", baz: 42 };
const map = new Map(Object.entries(obj));
console.log(map); // Map(2) {"foo" => "bar", "baz" => 42}

Iterating Through Objects

You can iterate through an object's properties easily using Object.entries() with for...of loop or array methods:

const obj = { a: 5, b: 7, c: 9 };

// Using for...of loop
for (const [key, value] of Object.entries(obj)) {
  console.log(`${key} ${value}`); // "a 5", "b 7", "c 9"
}

// Using .forEach method
Object.entries(obj).forEach(([key, value]) => {
  console.log(`${key} ${value}`); // "a 5", "b 7", "c 9"
});