Conditionals {

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Summary

Conditionals allow us to make decisions in our code. They are most obviously represented by the classic if-statement.

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Contents

• if statements
• Conditional operators
• else
• else if
• Logic operators
• switch statements

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if statements

An if statement lets us do something only if a logical expression is true.

let x = 10;
if (x < 20) {
  console.log(`x is less than 20`);
}

The key is the conditional expression inside the parentheses after the word if. This expression has to be something that evaluates to either true or false.

If it’s true the code inside the curly brackets will run!

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Conditional operators

The kinds of conditions you can check are all premised on simple math:

let x = 10;
let y = 20;
let z = 30;

x + y === z // equality, true
x - y !== z // inequality, true
x < y // less than, true
z > y // greater than, true
x <= z // less than or equal, true
z >= y // greater-than or equal, true

Note we always prefer === to check for equality and !== to check for inequality. Do not use == and != please.

Why? Because == and != can lead to weird results:

let iAmHappy = true;
let numberOfNosesOnMyFace = 1;

console.log(iAmHappy === numberOfNosesOnMyFace); // false (quite right too!)

console.log(iAmHappy == numberOfNosesOnMyFace); // true! (terrible)

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else

We can make conditionals more flexible by specifying what to do if the conditional expression is false using an else:

let r = Math.random(); // Math.random() returns a random number between 0 and 1

if (r < 0.5) {
  console.log(`I'm happy!`);
}
else {
  console.log(`I'm sad!`);
}

The else code here is run when the condition of the if is false (that is, when r >= 0.5)

And now we have a highly realistic simulation of life.

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else if

We can create more complex conditions by checking a series of conditions:

let r = Math.random();

if (r < 0.2) {
  console.log(`I'm happy!`);
}
else if (r < 0.4) {
  console.log(`I'm sad!`);
}
else if (r < 0.6) {
  console.log(`I'm confused!`);
}
else {
  console.log(`I feel nothing anymore.`);
}

Each subsequent else if and the final else are only checked if the previous condition(s) were false.

Is life really this complex? Surely not!

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Logic operators

We can make more efficient conditional expressions with the three logic operators

a && b // true if both a AND b are true, false otherwise

a || b // true if one or both of a and b are true, false if both are false

!a // true if a is false, false if a is true

a and b both need to be something that is true or false (i.e. conditional expressions themselves)

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switch statement

A useful way of writing a set of conditions where you want to check the value in a variable against a list of possibilities is a switch statement, which works as follows:

let mood = `HAPPY`;

switch (mood) {
  case `SAD`:
  console.log(`*sob*`);
  break;

  case `AMBIVALENT`:
  console.log(`*shrug*`);
  break;

  case `HAPPY`:
  console.log(`*wheeeeee!*`);
  break;

  default:
  console.log(`*hmm*`);
}

The switch statement checks the value in the variable provided (mood in this example) and compares it with each case listed. If it matches the value specified by a case it executes the code inside the case.

We include break; after a case so that code doesn’t keep executing into the next case, which is usually not what we want.

We use the default case at the end to have a response if the variable doesn’t match any of our specific cases.

So the above program would print "*wheeeeee!*", because mood is HAPPY.

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}