Making decisions

Video

Exercises

1. This exercise is to help you get familiar with reading and understanding code.

   Read the code below carefully and try to make sense of it, but don’t run it.

   const newMessages = [
     {
       from: 'Amal',
       body: 'When does it start?',
       time: '31 Dec 2021, 1:23 PM'
     },
     {
       from: 'Isa',
       body: "I'm just going there now",
       time: '31 Dec 2021, 10:16 AM'
     },
   ];
   
   const messageCount = newMessages.length;
   let notification;
   
   if (messageCount === 0) {
     notification = "You have no new messages.";
   } else if (messageCount === 1) {
     notification = "You have a new message.";
   } else {
     notification = `You have ${messageCount} new messages.`;
   }
   
   console.log(notification);
   

   Copy

   What does the code do? What output do you expect? Observe that the newMessages array has two objects in it. If you removed one of the objects or both, or you added a new one, how would it affect the output of the code?

   Now run the code to see if it works as you expect. If you’re correct, then well done; you’re starting to get a hold of the language. If you’re not, don’t worry; it’s all part of the learning process; try to figure out where you went wrong.

2. Identify as many expressions and statements as you can in the code of the previous question. (You may need to review Values and types for this.)

3. Language codes are codes used to identify human languages. They’re often used in websites to organise content into different languages. An example of such a website is Wikipedia. If you visit a Wikipedia page, you might notice that the URL domain begins with a two-letter code, as in “en.wikipedia.org”. The “en” indicates that the page is in English. Similarly, an Arabic page would have the domain “ar.wikipedia.org”.

   The following table lists some languages and their codes:

   Language	Code
   Arabic	ar
   English	en
   French	fr
   Hausa	ha
   Igbo	ig
   Turkish	tr
   Yoruba	yo

   Complete the following program so that it prints the language that the languageCode variable corresponds to. The variable is currently set to "en", so the program should print English. If you change the value to a different language code from the above table, like "ha", the program should still work correctly.

   const languageCode = 'en';
   
   // Your task: complete the program.
   

   Copy

   (Hint: use the if statement and its branches to decide what language to print.)

Extras

Loose equality

Many programming languages use the double equals symbol == to compare if two values are equal. We’ve seen that JavaScript uses the triple equals === instead, which we call the strict equality operator. However, JavaScript also supports the == operator for “loose” equality comparison.

The double equals operator works like the triple equals; it’s difference is that it converts the types of it’s operands, if they’re different, before comparing them. Let’s take an example:

// Strict equality
// Different types, different values
console.log(2 === '2') // false

// Loose equality
// The string will be converted to a number before comparing
console.log(2 == '2') // true

It’s discouraged to use this operator because of it’s confusing behavior:

console.log(false == null) // false
console.log(false == '')   // true
console.log([1] == '1')    // true
console.log([[]] == 0)     // true

There’s a != counterpart for loose inequality too.

A shorter if

You may omit the braces in the if statement if there’s only one statement within them. Take the following if statement for example:

if (messageCount === 0) {
  notification = "You have no new messages.";
}

There’s only one statement in the braces, so you may write it as:

if (messageCount === 0)
  notification = "You have no new messages.";

If you have more than one statement, then you must use the braces:

if (messageCount === 0) {
  notification = "You have no new messages.";
  console.log(notification)
}

Omitting the braces otherwise changes the meaning of the program:

if (messageCount === 0)
  notification = "You have no new messages.";
  console.log(notification)

// The above is equivalent to:
if (messageCount === 0) {
  notification = "You have no new messages.";
}
console.log(notification)

You may also omit the braces in the else if and else branches if they contain only one statement:

if (messageCount === 0)
  notification = "You have no new messages.";
else if (messageCount === 1)
  notification = "You have a new message.";
else
  notification = `You have ${messageCount} new messages.`;

The braces actually create something called a block statement (“block” for short), which is just a way to group other statements.

Anyway, when you’re not sure, always use the braces.