Returning && expression from JS function

Question

Here is the function I am working with. I actually found it in React Native documentation :

var testFunction = function(word) {
  return word && '';
}

Here is how I am using this function :

var testWord = testFunction("Alex");

The final value of testWord, returned by testFunction, is "".

I would have expected the returned value to be either true or false, as the result of the && expression. However the value is a string of value "".

Could someone explain the logic behind this ?

please see related topic [http://stackoverflow.com/questions/17200315/logical-operator-and-two-strings-in-javascript](http://stackoverflow.com/questions/17200315/logical-operator-and-two-strings-in-javascript). You can see also a clear explanation [here](https://en.wikipedia.org/wiki/Short-circuit_evaluation) — morels, Jul 14 '16 at 13:43
@morels Short-circuiting and returning of an operand instead of a boolean are rather unrelated. — deceze, Jul 14 '16 at 13:56
`return word && '';` is a sc evaluation and in linked wiki page you can exactly read that the expected value is the one is got by OP `JavaScript`, operands: `&, |`, expected vaue: `&&, || Last value` — morels, Jul 14 '16 at 13:58

score 5 · Answer 1 · answered Jul 14 '16 at 13:39

5

The && evaluates as the right hand side if the LHS is true, otherwise it evaluates as the LHS. It doesn't evaluate as a boolean.

0 && 1 // 0 because 0 is not true
1 && 0 // 0 because 1 is true
1 && 2 // 2 because 1 is true

answered Jul 14 '16 at 13:39

Quentin

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2

Is this not a duplicate of http://stackoverflow.com/questions/5417969/why-do-logical-operators-and-return-object-and-not-boolean-result? – evolutionxbox Jul 14 '16 at 13:41
Many thanks, I didn't know that – Alexandre Bourlier Jul 14 '16 at 13:49

Returning && expression from JS function

1 Answers1