Shailen Tuli's blog

Thoughts on coding in Dart, Ruby, Python and Javascript

Booleans in Dart

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We’ve all written code that looks something like this:

if (x) {
  // do something
} else {
  // do something else

In Javascript, the if (x) would evaluate to false if x was false, null, undefined, the number 0 (or 0.0), an '' or NaN (these are all falsey in Javascript); otherwise it would evaluate to true.

If this code were rewritten in Python, the if (x) would evaluate to false if x was None, false, zero (0, 0.0, 0L, etc.), an empty string (''), list ([]), tuple (()) or dict({}); otherwise it would evaluate to true.

So, what are the truthy and falsey lists for Dart? To answer that, let’s look at what the language spec says about booleans:

Boolean conversion maps any object o into a boolean. Boolean conversion is defined  by the function
  (bool v){
    assert(v != null);
    return identical(v, true);

In other words, anything that is not explicitly true, is false in Dart. So, in our example, if if (x) would evaluate to true if and only if x equalled the the boolean literal true (or an expression that evaluated to true (3 % 2 == 1, say)),; otherwise it would evaluate to false.

This is very simple. There are no truthy/falsey lists to memorize; it is all very clear, quite correct and ….

likely to get you quickly into trouble. Wait, what?

This is legal Dart code, right? Well, that depends if you are in checked mode or not. In unchecked mode, the code works exactly as described above. However, in checked mode, any if (x) type constructs will generate an exception unless x is a boolean. Let’s look at that function from the language spec again:

(bool v) {


In checked mode, v must be a bool. If it isn’t, the Dart editor will complain and throw an exception. We will never get to the stage of figuring out whether our if (x) evaluates to true or false.

Going back to other languages for a moment: I always felt that Javascript and Python had too many falsey values. I liked that in Ruby, only false and nil evaluated to false; everything else was true. So, instead of writing if myList ..., you would write if myList.empty? .. in Ruby, making your intent quite clear. Dart actually goes beyond Ruby in this regard and really simplifies things: false (and boolean expressions that return false) are false; true (and boolean expressions that return true) are true. Everything else is neither false nor true.

So, as a programmer, how should you handle the reality that the Dart’s boolean semantics change based on whether you are in checked or unchecked mode? My recommendation: always code in checked mode and be quite explicit about boolean expressions.

Avoid code like this (you will get an exception in checked mode):

String s = '';
if (s) {...}

This can be rewritten more clearly:

if (s.isEmpty) {...}

The following:

List myList = [1, 2, 3];
if(myList.indexOf(1)) {...}

should become:

if (myList.indexOf(1) != 0) {...}

and this:

int num;
if (num) {...}

is better written as:

if (num == null) {...}

The real take home lesson is a) listen to the static warnings b) only use booleans for boolean operations (Do not rely on implicit boolean conversions) c) checked mode is there to stop you immediately if you do bad things during development. Dart works pretty hard to force you to do boolean stuff explicitly and there are good reasons for this. Follow the warnings from the editor and your code will be clearer, more maintainable and you will avoid a huge class of bugs.