Javascript - multiple complex if statements

Refactor complex code to function

If you have complex if statements try and wrap them up in functions. So

(flightDestinations[i] == yourDestination &&
yourAirline == 'Any airline' &&
yourFare == 'Any price')

could become

function YourDestinationIsTheSameForAnyAirlineOrPrice(flightDestination, yourDestination, yourAirline, yourFare){
    return flightDestination == yourDestination &&
        yourAirline == 'Any airline' &&
        yourFare == 'Any price';
}

// And called from if
if (YourDestinationIsTheSameForAnyAirlineOrPrice(flightDestinations[i], yourDestination, yourAirline, yourFare)) {}

Rather than trying to decipher the if statement you have a function name telling you what it does.

Use an object over multiple arrays

Specific to your example I would also try and create a single flight object that contains the destination, time and airline. eg:

var flight = {
    destination = "London",
    operator = "BA",
    time = "18:00 UTC",
    fare = "£239829"
}

This should make the code more readable than using multiple arrays. Example:

destinationTime = flightTimes[i];
destinationOperator = flightOperators[i];
destinationFare = flightFares[i];

message += destinationTime + ' ' + destinationOperator + '. £' + destinationFare + '<BR>';

// Using an object
message += flight.time + ' ' + flight.operator + '. £' + flight.fare + '<br />';

Return early

Finally I would break out of the function as soon as possible. So use:

if (yourDestination == 'Choose your destination') {
    displayMessage('<B>Please choose a destination city from the Destination menu and then click Find flights again.</B>');
    return;
}

instead of an if...else. I personally find this more readable, so feel free to ignore this.

You've mismatched your parenthesis here:

((flightDestinations[i] == yourDestination &&                // destination = selected destination &
 yourAirline == 'Any airline' &&                             // airline = any airline &
 yourFare == 'Any price'))                                    // fare <= chosen fare
 ||                                                          // OR
(flightDestinations[i] == yourDestination &&                // destination = selected destination &
yourAirline == flightOperators[i] &&                        // airline = chosen airline &
yourFare <= flightFares[i]))                                // fare <= chosen fare

Change yourFare == 'Any price')) to yourFare == 'Any price').

I'm not sure what the question is, but you should use more consistent indentation and the complex if statements will definitely become clearer.

My rules:

• use tabs to indent your lines of code according to scope (e.g. +1 tab while within curly braces, if statements, etc.)
• do not use tabs for anything else; use spaces for alignment, multiple spaces if necessary (so, use spaces to line up your conditions in the if statement if the statement spans multiple lines)

Although I think that this is not a necessary tuning, you can write the code as follows:

if (yourDestination == 'Choose your destination') {
    displayMessage('<B>Please choose a destination city from the Destination menu and then click Find flights again.</B>');
    return;
}

This helps you remove the else and its brackets {...}.

You can change the for loop to reduce the code size:

if(flightDestinations[i] != yourDestination) continue;

So that, you don't need to write this condition repeatedly.