True but not true

Written by Ian Elliot

Article Index
True but not true
Solution

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Solution

The key to the problem is to realize that not all things that are true in JavaScript are equal to true.

The original programmer of the code had something slightly different in mind to flag being a Boolean. They were working with strings and discovered that if flag was "true" or any non-null string then

flag="true";
if(flag)

evaluated to true. On the other hand if flag was the null string then

flag="";
if(flag)

evaluated to false. This worked perfectly in this situation as the test was about what to do when the string was null or not.

JavaScript allows any type to be treated as if it was a Boolean - so called truthy and falsey values. Any zero, null, undefined or null string are falsey and behave in logical expressions as if they were false and everything else is truthy and behave in logical expressions as if they were true.

However our refactoring programmer was unaware of the intent and assumed that flag was simply a Boolean and true or false rather than truthy or falsy.

So why didn't ==true or ===true work if flag was truthy?

The simple answer is that for == JavaScript attempts an automatic conversion. The way that this works is that to evaluate x==y any Booleans are first converted to numbers with true as 1 and false as 0. So flag==true is converted to flag==1 which it isn't so the result is false.

For the strict equality operator the result is even simpler. If the two values are of different types then the result is false - and flag is a string and true is a Boolean.

Pattern

The pattern that you need to follow to make sure that hoisting never causes you any problem is simple:

Don't ever use or rely on truthy and false values.

Always use a Boolean if you need a two state indicator.

This said there are problems even in this approach as there are two distinct ways to create Boolean as:

var b=new Boolean(true);

and

var b="true";

The bad news is that these two don't always behave in the same way - but that's for another Programmer Puzzle.

For more on this topic:

Javascript Jems - active logic, truthy and falsey

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