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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