Errors and control flow

if and switch, break and continue, and handling errors with try, catch and finally.

Choosing a path

temperature = 22;

if ( temperature > 30 ) {
    println( "hot" );
} else if ( temperature > 15 ) {
    println( "pleasant" );
} else {
    println( "cold" );
}

day = 3;
switch ( day ) {
    case 1:
        println( "Monday" );
        break;
    case 3:
        println( "Wednesday" );
        break;
    default:
        println( "some other day" );
}

Output:

pleasant
Wednesday

switch needs a break at the end of each case, otherwise execution falls through into the next one.

Skipping and stopping

continue skips to the next round of a loop, and break leaves the loop.

for ( i = 1; i <= 6; i++ ) {
    if ( i == 2 ) {
        continue;
    }
    if ( i == 5 ) {
        break;
    }
    println( i );
}

Output:

1
3
4

Handling errors

When something goes wrong, code throws an error. If nothing catches it, the script stops. A try block lets you catch the error and carry on.

try {
    println( "before" );
    throw "something broke";
    println( "never printed" );
} catch ( any e ) {
    println( "caught: " & e.message );
} finally {
    println( "always runs" );
}

Output:

before
caught: something broke
always runs

catch ( any e ) catches every kind of error. finally runs whether or not an error happened, which makes it the right place for cleanup.

Built-in errors

The runtime throws errors of its own, for example when you divide by zero.

try {
    println( 10 / 0 );
} catch ( any e ) {
    println( "caught: " & e.message );
}

Output:

caught: Division by zero

Your own error types

Give an error a type so callers can catch exactly the problems they know how to handle.

function parseAge( text ) {
    if ( !isNumeric( text ) ) {
        throw( type = "BadInput", message = "'" & text & "' is not a number" );
    }
    return text + 0;
}

try {
    println( parseAge( "36" ) );
    println( parseAge( "old" ) );
} catch ( BadInput e ) {
    println( e.type & ": " & e.message );
}

Output:

36
BadInput: 'old' is not a number

Try it

Change the second call to parseAge( "42" ) and run again. Then add a catch ( any e ) after the BadInput one and throw a different kind of error to see which handler runs.