- [Conditional execution](../fundamentals/conditional_execution.md)

console.log(InstanceMovie.getStars().map(actor => `${actor.getName()} ${actor.getAge()}`));

The normal order of execution of statements in a computer program is in straight-line order, from top to bottom. However, sometimes it is desirable to execute one or more statements _conditionally_, i.e. depending on whether some condition – determined by the state of your program – holds true.

In its simplest form the `if` statement looks like this:

if (condition) {

// one or more statements that will be executed

// if, and only if the condition holds true

}

Here, `condition` is a boolean expression that resolves to either `true` or `false` (or, more precisely, any expression that is 'truthy' or 'falsy', as will be explained later).

The statements within the curly braces `{` and `}` will be executed if the condition holds true, otherwise these statements will be skipped (i.e. ignored).

An example:

if (distance < 10) {

console.log('I will take the bike.');

}

Here, the condition is the boolean expression `distance > 10`, which is either `true` or `false`.

It is also possible to add a statement block to be executed if (and only if) the condition does **not** hold true, using an `else` clause.

if (distance < 10) {

console.log('I will take the bike.');

} else {

console.log('I will go by car.');

}

A condition can take more complex forms, using `&&` (logival AND) and `||` (logical OR) operators:

if (distance < 10 && !raining) {

console.log('I will take the bike.');

} else {

console.log('I will go by car.');

}

In the example above `raining` is a boolean variable (either `true` or `false`), and the exclamation mark is the logical NOT operator that negates the boolean value (if it was `true` the result after negation is false and vice versa).

For more complex decisions you can concatenate multiple conditions using `else if` clauses.

if (distance < 1) {

console.log('I will walk.');

} else if (distance < 10) {

console.log('I will take the bike.');

} else if (distance < 50) {

console.log('I will go by car.');

} else {

console.log('I will take the train.');

}

The statement block inside an `if`, `else` or `else if` may itself contain nested `if` statements, as in this example:

if (distance < 10) {

if (raining) {

console.log('I will go public transportation.');

} else {

console.log('I will walk.');

}

} else {

console.log('I will go by car.');

}

This operator can be used as a shortcut for an `if` statement when dealing with expressions.

The general format is:

condition ? expr1 : expr2

('ternary' means: _composed of three parts_)

It is often used in combination with an assignment, as in this example:

const conditionOfCar = age < 1 ? 'new' : 'used';

The `conditionOfCar` variable will be assigned the string `'new'` if the `age < 1` condition holds true, otherwise it is assigned the string `'used'`.

It is **not** recommended to use the conditional operator if you do not intend to use its value:

age < 1 ? console.log('new') : console.log('used');

The `switch` statement can sometimes be a useful alternative to a concatenation of `if` statements. This is the case when the condition is an expression that can be decomposed into a number of distinct values or _cases_, as shown in the example below.

const hyfModule = 'JavaScript-1';

switch (hyfModule) {

case 'HTML/CSS':

console.log('In this module you will learn HTML and CSS.');

break;

case 'JavaScript-1':

console.log('In this module you will learn Git and JavaScript basics.');

break;

case 'JavaScript-2':

console.log('In this module you will learn about JavaScript in the browser with HTML and CSS.');

break;

case 'JavaScript-3':

console.log('In this module you will learn about Async and API calls.');

break;

case 'Node':

console.log('This module is about building server and CLI applications using Node.');

break;

case 'Database':

console.log('In this module is about Relational and Non-Relational Data and Database Systems.');

break;

case 'React':

console.log('In this module you will to build Single Page Applications using React.');

break;

case 'Project':

console.log('In this final module you will do your graduation project.');

break;

default:

console.log('This module is unknown: ' + hyfModule);

}}

Depending on the value of the expression specified in the `switch` clause, one of the `case` statement blocks is executed. Each statement block should end with a `break` statement to ensure that a `case` doesn't 'fall through' into the next `case`.

The `default` statement at the end is executed when none of the preceding cases hold true. The `default` statement is not strictly required, but is a best practice to always specify one.

**truthy**: 'sort of' `true`

**falsy**: 'sort of' `false`

From MDN:

In JavaScript, a **truthy** value is a value that is considered true when evaluated in a Boolean context. All values are truthy unless they are defined as **falsy**.

**falsy** values are:

- `false`

- `0`

- `""`

- `null`

- `undefined`

- `NaN`

The example below will print `x is undefined` because `undefined` is **falsy**.

let x;

if (x) {

console.log('x is defined');

} else {

console.log('x is undefined');

}