9/14/2011 Clarifying Boolean Parameters, part 2

Part 1 is here

Some languages have better ways to pass boolean parameters.  C# 4.0, and all versions of VB, allow parameters to be passed by name.  This allows us to write much clearer code:

//C# 4.0:
UpdateLayout(doFullLayout: false) 

'VB.Net:
UpdateLayout(doFullLayout:=False) 

Without requiring any changes to the function definition, this makes the meaning of the true / false abundantly clear at the call-site.

Javascript offers another interesting alternative.  In Javascript, booleans conditions actually check for “truthyness”.  The statement if(x) will trigger  not just if x is true, but also if x is any “truthy” value, including any object, non-empty string, or non-zero number. Similarly, the expression !x will return false if x is “truthy” and true if x “falsy”.

This means that we can actually use any non-empty string instead of true in Javascript.  Note that this will only work if the function checks the value for “truthyness”; it won’t work for code like if (x === true).

Thus, instead of passing true as a boolean, you can pass a string that describes what you’re actually indicating.

For example:

function updatePosition(animate) {
    //Calculate position
    if (animate)
        //...
    else
        //...
}

$(window).resize(function() {
    updatePosition();
});

updatePosition("With animation");

Although this results in much more readable code, it can be difficult to understand for people who aren’t familiar with this trick.  If the meaning of the parameter changes, you’ll need to hunt down every place that the function is called and change the string to reflect the new meaning.

Finally, unlike an enum, this does not scale to multiple options.  If you need to have more than two options, you should use global variables or objects to simulate an enum, not strings.