Making decisions

First, let’s learn how to do some basic comparison

The relational operators:

3 > 2    // true
8 < 5    // false
9 >= 13  // false
6 <= 6   // true

Use the strict equality operator to determine if two values are equal:

2 === 5 - 3           // true
89.0 === 89           // true
10 === '10'           // false
'hello' === 'Hello'   // false
true === false        // false

There’s a strict inequality counterpart:

2 !== 5 - 3           // false
89.0 !== 89           // false
10 !== '10'           // true
'hello' !== 'Hello'   // true
true !== false        // true

No two objects have the same value, even if they look alike:

const obj1 = { prop: "value" }
const obj2 = { prop: "value" }
console.log(obj1 === obj2) // false 😕

But, same object, same value:

const obj1 = { prop: "value" }
const obj2 = obj1
console.log(obj1 === obj2) // true

The if statement

if (expression) {
  statement1
  statement2
  ...
}

if (expression) {
  statement1
  statement2
  ...
}

If expression is true, execute the statements in the braces. Otherwise, ignore the statements.

Let’s consider the FutureLearn example.

We represented a course like so in the previous lesson:

const course = {
  title: 'The Museum as a Site and ...',
  rating: 4.6,
  reviewsCount: 75,
  isNew: false,
  isPartOfAnExpertTrack: false,
}

Let’s determine if the course has a rating:

const course = {
  rating: 4.6,
  // ...
}

course.rating !== 0 // true

Now we can act accordingly:

const course = {
  rating: 4.6,
  // ...
}

if (course.rating !== 0) {
  console.log(`Rating: ${course.rating}`)
}

Let’s add some logs around the if statement for clarity:

const course = {
  rating: 4.6,
  // ...
}

console.log('Before decision')
if (course.rating !== 0) {
  console.log(`Rating: ${course.rating}`)
}
console.log('After decision')

Here’s the output:

Before decision
Rating: 4.6
After decision

What if the rating is 0?

const course = {
  rating: 0,
  // ...
}

console.log('Before decision')
if (course.rating !== 0) {
  console.log(`Rating: ${course.rating}`)
}
console.log('After decision')

Then the output is just this:

Before decision
After decision

How do we print a different message?

else

const course = {
  rating: 0,
  // ...
}

console.log('Before decision')
if (course.rating !== 0) {
  console.log(`Rating: ${course.rating}`)
} else {
  console.log('No rating')
}
console.log('After decision')

The result:

Before decision
No rating
After decision

Let’s consider another example.

A USSD menu with different options to choose from.

How do we express the several alternatives?

else if

const choice = 1 // Could be any other number

if (choice === 1) {
  console.log('Open Account')
} else if (choice === 2) {
  console.log('Account Balance')
} else if (choice === 3) {
  console.log('Airtime/Data')
} // ...

You can use a final else to handle any other choice:

if (choice === 1) {
  console.log('Open Account')
} else if (choice === 2) {
  console.log('Account Balance')
} else if (choice === 3) {
  console.log('Airtime/Data')
} // ...
else {
  console.log('Invalid choice')
}