I don't understand the syntax of ES2015 for loop <Introducing ES2015 - Object Property Shorthand>

Question

use strict';

function submit(name, comments, rating = 5) {
  let data = { name, comments, rating };

  for (let key in data) {
    console.log(key + ':', data[key]);
  }
  // … do ajax request
}

submit('English', 'Great course!');

From the above code, the syntax for "for-loop" has never been explained along the course. Could anyone please explain the syntax?

Answer 1 · 2018-02-01T14:24:54Z

February 1, 2018 2:24pm

a for loop is generally used to loop through a list of items. In some languages the idea is sometimes know as a "foreach" loop, and that makes the idea clearer. consider in C#

int[] nums = { 1, 2, 3, 4, 5 };
foreach(int num in nums)
{
  Console.WriteLine(num); 
}

The idea is similar in javascript, given a collection, the loop will iterate over the collection, and complete the block of code one time, for each value in it. so in javascript

let words = [ 'hello', 'how', 'are', 'you?' ];
for(let word in words) {
  console.log(word);
}
// output:
// hello
// how
// are
// you?

When javascript does a for loop on an object "{}" instead of an array "[]", it loops over each key or propertyname in the object. so the loop in your example is run 3 times, once with key = 'name', once with key='comments', and finally with key='rating'.

Welcome to the Treehouse Community

Looking to learn something new?

HanJoon Kim

HanJoon Kim

I don't understand the syntax of ES2015 for loop <Introducing ES2015 - Object Property Shorthand>

1 Answer

Samuel Ferree

Samuel Ferree