What is an iterable?

An iterable is an object implements the iterable protocol. To be iterable, an object must implement the Symbol.iterator function. This function returns an iterator object, which is used to obtain values to be iterated.

An object is an iterator when it implements the next() method(iterator protocol). This method returns an object implementing IteratorResult. It has done(whether iteration is complete) and value(next value in iteration) properties.

The iterable object can be iterated over by for...of or other syntaxes expecting iterables.

• Some Built-in iterables are: String, Array, Map, Set, NodeList, and they can be iterated over using for...of. For example:

const monthMap = new Map();
monthMap.set("Jan", 1);
monthMap.set("Feb", 2);
monthMap.set("Mar", 3);

for (let [key, value] of monthMap) {
  // Jan 1 
  // Feb 2
  // Mar 3
  console.log(key, value) 
}

What is an enumerable?

An enumerable object has enumerable properties(internal enumerable flag set to true). By default properties created via simple assignment and propery initializer are enumerable.

They can be iterated over by for...in. For example:

const car = {
  "make": "honda",
  "year": 2016,
  "model": "civic"
};

for (let prop in car) {
  console.log(prop) // make year model
}

This is like iterating over object keys.

How to iterate over object values?

• Method 1: Use Object.values()

const car = {
  "make": "honda",
  "year": 2016,
  "model": "civic"
};

const values = Object.values(car);
for (let value of values) {
  console.log(value); // honda 2016 civic
}

• Method 2: Use Object.keys() and forEach on the result key array

const car = {
  "make": "honda",
  "year": 2016,
  "model": "civic"
};

const keys = Object.keys(car);
keys.forEach(k => {
  console.log(car[k]) // honda 2016 civic
})

How to iterate over object key/value pairs?

Object.entries() returns an array of key/value pairs(as array). Then iterate over the result array using for...of

const car = {
  "make": "honda",
  "year": 2016,
  "model": "civic"
};

for (let [key, value] of Object.entries(car)) {
  // make honda
  // year 2016
  // model civic
  console.log(key, value) 
}