Object.keys() / values() / entries() Since: ES5（ECMAScript 2009）

// Returns an array of the object's key names.
Object.keys(object)

// Returns an array of the object's values.
Object.values(object)

// Returns an array of the object's key-value pairs.
Object.entries(object)

var user = {
	name: "Kuu",
	age: 25,
	city: "Tokyo"
};

// Get an array of key names.
console.log(Object.keys(user)); // Outputs ["name", "age", "city"].

// Get an array of values.
console.log(Object.values(user)); // Outputs ["Kuu", 25, "Tokyo"].

// Get an array of key-value pairs.
console.log(Object.entries(user)); // Outputs [["name", "Kuu"], ["age", 25], ["city", "Tokyo"]].

// You can loop over keys using Object.keys with forEach.
Object.keys(user).forEach(function(key) {
	console.log(key + ": " + user[key]);
});

// You can also loop using Object.entries with destructuring.
Object.entries(user).forEach(function([key, value]) {
	console.log(key + " = " + value);
});

// You can get the number of properties.
console.log(Object.keys(user).length); // Outputs 3.

Object.keys(), Object.values(), and Object.entries() are methods that retrieve an object's property information as arrays. Unlike a for...in loop, they do not include properties from the prototype chain, so you can safely retrieve only the object's own properties.

Because the return value is an array, you can combine these methods with array methods such as forEach(), map(), and filter() for flexible processing. To count the number of properties, Object.keys(obj).length is convenient and can also be used to check whether an object is empty.

Object.entries() works well with for...of loops and destructuring assignment to process both keys and values efficiently. To convert an array of key-value pairs back into an object, use Object.fromEntries().