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Conditional operators: if, '?'

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

There is the if statement for that and also the conditional (ternary) operator for conditional evaluation which we will be referring as the “question mark” operator ? for simplicity.

The "if" statement

The if statement 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 is more than one statement to be executed, we have to wrap our code block inside curly braces:

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

It is recommended to wrap your code block with curly braces {} every time with if, even if there is only one statement. That improves readability.

Boolean conversion

The if (…) statement evaluates the expression in parentheses and converts it to the boolean type.

Let's recall the conversion rules from the chapter info:type-conversions:

  • A number 0, an empty string "", null, undefined and NaN become false. Because of that they are called "falsy" values.
  • Other values become true, so they are called "truthy".

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 boolean value to if, like here:

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

if (cond) {
  ...
}

The "else" clause

The if statement 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 is 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 it then goes to the next condition year > 2015, and otherwise shows the last alert.

There can be more else if blocks. The ending else is optional.

Ternary operator '?'

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

For instance:

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

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

alert(accessAllowed);

The so-called "ternary" or "question mark" operator lets us do that shorter and simpler.

The operator is represented by a question mark ?. The formal term "ternary" means that the operator has three 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 accessAllowed = (age > 18) ? true : false;

Technically, we can omit parentheses around age > 18. The question mark operator has a low precedence. It executes after the comparison >, so that'll do the same:

// the comparison operator "age > 18" executes first anyway
// (no need to wrap it into parentheses)
let accessAllowed = age > 18 ? true : false;

But parentheses make the code more readable, so it's recommended to use them.

In the example above it's possible to evade the question mark operator, because the comparison by itself returns `true/false`:

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

Multiple '?'

A sequence of question mark ? operators allows returning a value that depends 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 after a closer look we can see that it's just an ordinary sequence of tests.

  1. The first question mark checks whether age < 3.
  2. If true -- returns 'Hi, baby!', otherwise -- goes after the colon ":" and checks for age < 18.
  3. If that's true -- returns 'Hello!', otherwise -- goes after the next colon ":" and checks for age < 100.
  4. If that's true -- returns 'Greetings!', otherwise -- goes after the last colon ":" and returns 'What an unusual age!'.

The same logic using if..else:

if (age < 3) {
  message = 'Hi, baby!';
} else if (age < 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. The idea is to execute different code depending on the condition.

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

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

Here is the same code 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 is if to execute different branches of the code.