# Weblogs: Javascript

## Truthy, falsy and type-casting

Monday, October 02, 2006JavaScript 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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