conditional.md

Conditional Logic

A condition is a test for something. Conditions are very important for programming, in several ways:

First of all conditions can be used to ensure that your program works, regardless of what data you throw at it for processing. If you blindly trust data, you’ll get into trouble and your programs will fail. If you test that the thing you want to do is possible and has all the required information in the right format, that won’t happen, and your program will be a lot more stable. Taking such precautions is also known as programming defensively.

The other thing conditions can do for you is allow for branching. You might have encountered branching diagrams before, for example when filling out a form. Basically, this refers to executing different “branches” (parts) of code, depending on if the condition is met or not.

In this chapter, we'll learn the base of conditional logic in Javascript.

Comparators

Lets now focus on the conditional part:

if (country === "France") {
    ...
}

The conditional part is the variable country followed by the three equal signs (===). Three equal signs tests if the variable country has both the correct value (France) and also the correct type (String). You can test conditions with double equal signs, too, however a conditional such as if (x == 5) would then return true for both var x = 5; and var x = "5";. Depending on what your program is doing, this could make quite a difference. It is highly recommended as a best practice that you always compare equality with three equal signs (=== and !==) instead of two (== and !=).

Other conditional test:

• x > a: is x bigger than a?
• x < a: is x less than a?
• x <= a: is x less than or equal to a?
• x >=a: is x greater than or equal to a?
• x != a: is x not a?
• x: does x exist?

Condition If

The easiest condition is an if statement and its syntax is if(condition){ do this … }. The condition has to be true for the code inside the curly braces to be executed. You can for example test a string and set the value of another string dependent on its value:

var country = 'France';
var weather;
var food;
var currency;

if(country === 'England') {
    weather = 'horrible';
    food = 'filling';
    currency = 'pound sterling';
}

if(country === 'France') {
    weather = 'nice';
    food = 'stunning, but hardly ever vegetarian';
    currency = 'funny, small and colourful';
}

if(country === 'Germany') {
    weather = 'average';
    food = 'wurst thing ever';
    currency = 'funny, small and colourful';
}

var message = 'this is ' + country + ', the weather is ' +
            weather + ', the food is ' + food + ' and the ' +
            'currency is ' + currency;

Note: Conditions can also be nested.

Else

There is also an else clause that will be applied when the first condition isn’t true. This is very powerful if you want to react to any value, but single out one in particular for special treatment:

var umbrellaMandatory;

if(country === 'England'){
    umbrellaMandatory = true;
} else {
    umbrellaMandatory = false;
}

The else clause can be joined with another if. Lets remake the example from the previous article:

if(country === 'England') {
    ...
} else if(country === 'France') {
    ...
} else if(country === 'Germany') {
    ...
}