Wrapping Up
The newly introduced Symbol primitive type is useful to define properties and methods with unique names, like [Symbol.iterator], for example. The enhanced for loop provides an elegant, less noisy syntax, to iterate over collections of objects. To facilitate iteration over user-defined classes, JavaScript supports custom iterators and generators. The generators are lazy evaluators and may be used to create infinite streams of data.
So far we have used regular functions. In the next chapter you’ll learn about the pros and cons of arrow functions.
Exercises
You learned about Symbols, iterators, and generators, and these are some of the most important and highly used features of JavaScript. These practice exercises will help you to review these concepts before you move on to the next chapter. You can find answers to these exerciseshere.
Exercise 1
The code in this exercise prints every third letter, starting with the first, using the traditional for loop. Let’s convert the loop into an enhanced for loop:
| | const letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']; |
| | |
| | for(let i = 0; i < letters.length; i = i + 3) { |
| | console.log(letters[i]); |
| | } |
Exercise 2
Let’s complete the following code to explore the Symbol properties in an Array:
| | const numbers = [1, 2, 3]; |
| | |
| | console.log("The Symbol properties in arrays are:"); |
| | |
| | console.log(//...code goes here... |
| | ); |
Exercise 3
Let’s implement a method with the special name Symbol.replace to achieve the desired result in the following code:
| | class Message { |
| | constructor(text) { this.text = text; } |
| | |
| | //Your code goes here |
| | } |
| | |
| | const message = new Message('There are no stupid questions.'); |
| | |
| | console.log('stupid'.replace(message, 's*****')); |
| | //There are no s***** questions. |
| | |
| | console.log(''.replace(message, 'Yes, ')); |
| | //Yes, There are no stupid questions. |
Exercise 4
Let’s create a generator function to produce a Fibonacci series not exceeding the desired value:
| | //Your code goes here |
| | |
| | for(const value of fibonocciSeries()) { |
| | if(value > 25) break; |
| | process.stdout.write(value + ", "); |
| | } |
Exercise 5
In the previous exercise, we terminated the iteration when the value in the series exceeded 25. Let’s modify the fibonocciSeries() function so that we can terminate the series when a certain number of values has been obtained:
| | //Your code goes here |
| | |
| | for(const [index, value] of fibonocciSeries()) { |
| | if(index > 8) break; |
| | process.stdout.write(value + ", "); |
| | } |