Conditional branching: if, '?'

Sometimes, we need to perform different actions based on different conditions.

To do that, we can use the if statement and the conditional operator ?, that’s also called a “question mark” operator.

The “if” statement

The if(...) statement evaluates a condition in parentheses and, if the result is true, executes a block of 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 we want to execute more than one statement, we have to wrap our code block inside curly braces:

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

We recommend wrapping your code block with curly braces {} every time you use an if statement, even if there is only one statement to execute. Doing so improves readability.

Boolean conversion

The if (…) statement evaluates the expression in its parentheses and converts the result to a boolean.

Let’s recall the conversion rules from the chapter Type Conversions:

• A number 0, an empty string "", null, undefined, and NaN all 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 – it always will:

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

We can also pass a pre-evaluated boolean value to if, like this:

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 falsy.

For example: