en.javascript.info/1-js/02-first-steps/10-ifelse/article.md

Conditional operators: if, '?'

Sometimes we need to perform different actions basing on a condition.

There's an if operator for that and also the "question mark" operator: "?" for conditional evaluation.

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The "if" operator

The "if" operator gets a condition, evaluates it and -- if the result is true -- executes the code.

For example:

let year = prompt('In which year was ECMAScript-2015 specification published?', '');

*!*
if (year == 2015) alert( 'You are right!' );
*/!*

In the example above, the condition is a simple equality check: year == 2015, but it can be much more complex.

If there's more than one command to execute -- we can use a code block in figure brackets:

if (year == 2015) {
  alert( "That's correct!" );
  alert( "You're so smart!" );
}

It is recommended to use figure brackets every time with if, even if there's only one command. That improves readability.

Boolean conversion

The if (…) operator evaluates the condition in brackets and converts it to boolean type.

Let's recall the rules. In the logical context:

• A number 0, an empty string "", null, undefined and NaN are false,
• Other values -- true.

So, the code under this condition would never execute:

if (0) { // 0 is falsy
  ...
}

...And inside this condition -- always works:

if (1) { // 1 is truthy
  ...
}

We can also pass a pre-evaluated logical value to if, like here:

let cond = (year == 2015); // equality evaluates to true or false

if (cond) {
  ...
}

The "else" clause

The if operator may contain an optional "else" block. It executes when the condition is wrong.

For example:

let year = prompt('In which year was ECMAScript-2015 specification published?', '');

if (year == 2015) {
  alert( 'You guessed it right!' );
} else {
  alert( 'How can you be so wrong?' ); // any value except 2015
}

Several conditions: "else if"

Sometimes we'd like to test several variants of a condition. There's an else if clause for that.

For example:

let year = prompt('In which year was ECMAScript-2015 specification published?', '');

if (year < 2015) {
  alert( 'Too early...' );
} else if (year > 2015) {
  alert( 'Too late' );
} else {
  alert( 'Exactly!' );
}

In the code above JavaScript first checks year < 2015, if it is falsy then goes to the next condition year > 2015. Any number of else if may follow with an optional last else.

Ternary operator '?'

Sometimes we need to assign a variable depending on a condition.

For instance:

let hasAccess;
let age = prompt('How old are you?', '');

*!*
if (age > 18) {
  hasAccess = true;
} else {
  hasAccess = false;
}
*/!*

alert(hasAccess);

The so called "ternary" or "question mark" operator allows to do that shorter and simpler.

The operator is represented by a question mark "?". The formal term "ternary" means that the operator has 3 operands. It is actually the one and only operator in JavaScript which has that many.

The syntax is:

let result = condition ? value1 : value2

The condition is evaluated, if it's truthy then value1 is returned, otherwise -- value2.

For example:

let hasAccess = (age > 18) ? true : false;

We can omit brackets around age > 14, because the question mark operator has a low precedence. It executes after comparisons, so:

// the same
let hasAccess = age > 18 ? true : false;

...But brackets make the code more readable. So it's recommended to put them.

In the described case it is possible to evade the question mark operator, because the comparison by itself returns `true/false`:

```js
// the same
let hasAccess = age > 18;
```

Multiple '?'

A sequence of question mark "?" operators allows to return a value depending on more than one condition.

For instance:

let age = prompt('age?', 18);

let message = (age < 3) ? 'Hi, baby!' :
  (age < 18) ? 'Hello!' :
  (age < 100) ? 'Greetings!' :
  'What an unusual age!';

alert( message );

It may be difficult at first to grasp what's going on. But looking more carefully we note that it's just an ordinary sequence of tests.

1. The first question mark checks for age < 3.
2. If true -- returns 'Hi, baby!', otherwise -- goes to the right side of the colon ":" and checks for age < 18.
3. If that's true -- returns 'Hello!', otherwise checks for age < 100 and returns 'Greetings!' if that is so...
4. At last, if all checks are falsy, the message becomes 'What an unusual age!'.

The same logic using if..else:

if (age < 3) {
  message = 'Hi, baby!';
} else if (a < 18) {
  message = 'Hello!';
} else if (age < 100) {
  message = 'Greetings!';
} else {
  message = 'What an unusual age!';
}

Non-traditional use of '?'

Sometimes the question mark '?' is used as a replacement for if:

let company = prompt('Which company created JavaScript?', '');

*!*
(company == 'Netscape') ?
   alert('Right!') : alert('Wrong.');
*/!*

Depending on the condition company == 'Netscape', either the first or the second part after "?" gets executed and shows the alert.

We don't assign a result to a variable here, cause the alert doesn't return anything anyway, our purpose is only to execute it.

It is not recommended to use the question mark operator in this way.

The notation seem to be shorter than if, that appeals to some programmers. But it is less readable.

Here's the same with if for comparison:

let company = prompt('Which company created JavaScript?', '');

*!*
if (company == 'Netscape') {
  alert('Right!');
} else {
  alert('Wrong.');
}
*/!*

Our eyes scan the code vertically. The constructs which span several lines are easier to understand than a long horizontal instruction set.

The idea of a question mark '?' is to return one or another value depending on the condition. Please use it for exactly that. There's if to execute different branches of the code.