Weblogs: Javascript

The most popular client-side scripting language - often overused and maligned. Unobtrustive Javascript is slowly gaining ground as a better way of enhancing html pages with dynamic features.

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Monday, October 02, 2006

Truthy, falsy and type-casting

JavaScript has keywords for true and false, but like many C-style derivative languages, it has concepts of truthy and falsy. These are non-boolean expressions that can be treated as a boolean value. The number zero is falsy, and any other number is truthy. Equally for strings, an empty string is falsy, and a non-empty string is truthy.

JavaScript also has the neat type-conversion functions. Strings containing numbers can be coerced into being just numbers. When combined with truthy and falsy logic, things can get a little surprising, and there are a few gotchas to avoid.

Expression	Evaluation	Reason
true	true	true is always true
false	false	false is always false
1	true	non-zero numerics are truthy
0	false	the numeric zero is falsy
'1'	true	a non-empty string is always truthy
'0'	true	a non-empty string is always truthy
1 - 1	false	a numeric value of zero is falsy
'1' - '1'	false	The minus coerces both strings into being integers, and so a numeric value of zero is falsy
'0' + '0'	true	The plus does string concatenation, so the end result is a two character string, which is truthy
0 + '0'	true	The plus does string concatenation, so the end result is a two character string, which is truthy
0 + 0	false	Because both operands a numerics, the plus does numerical addition, so the end result is zero, which is falsy

For a best practice approach, it makes sense to use the keywords true and false, and try to avoid code that uses 0 and 1 for the purposes of truthy and falsy. A common scenario this situation arises is when JavaScript is dynamically generated by a server-side language where the string '0' is treated as numeric in expressions.

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Older Posts:

• [24/01/2006] DOM Scripting by Jeremy Keith
• [03/01/2006] Fixing the back button that AJAX broke
• [27/09/2005] Web applications with Ajax, XUL and Flash
• [18/07/2005] Promoting modern JavaScripting
• [26/06/2005] @Media 2005: The behaviour layer by Jeremy Keith

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