In JavaScript (and many other C-style languages) you can do this:

if (condition)
    foo();

While this code is perfectly valid, and will run as expected, you should never omit an if statement’s curly braces like this. Why? Take a look at this code:

if (condition)
    foo();
    bar();

This code is also perfectly valid, but it’s incredibly deceptive. In this example, only foo(); is attached to the if statement. Despite appearances, bar(); will run no matter what.

This code is functionally identical, but much clearer:

if (condition) {
    foo();
}
bar();

Leaving out curly braces might save you a few keystrokes, but it’s a nightmare for code readability and maintenance. Save Future You from confusion and hassles: always use curly braces!