Why JavaScript returns empty string for "logical AND" operation?

Question

Why do logical AND comparison between an empty string and false boolean returns an empty string? and why do logical AND comparison between a string and false boolean returns false?

Example:

'' && true; --> returns ''
'string' && true --> returns true;
'' && false --> returns ''
'string' && false --> returns false;

Question is why javascript behaves this way?

it returns the first decision value for leaving the expression. — Nina Scholz, Oct 10 '17 at 06:17
You'll also get the same with `0` and `NaN`. It's one of "falsy" values in JavaScript. So your logical AND returns those values being same as `false` (since `true && false` is `false`). https://developer.mozilla.org/en-US/docs/Glossary/Falsy — Walk, Oct 10 '17 at 06:22

score 8 · Accepted Answer · answered Oct 10 '17 at 06:33

Javascript AND(expr1 && expr2) operator works by returning the expressions based on the logic:

if expr1 is falsy
  return expr1
else
  return expr2

Falsy values include your empty string(''), null, NaN, undefined, etc. You can read more about it at https://developer.mozilla.org/en-US/docs/Glossary/Falsy.

Also for more info on boolean operators, check out https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators.

score 0 · Answer 2 · answered Oct 10 '17 at 06:28

The && operator works very simply: if the first value is falsy, it returns the first value, otherwise it returns the second value.

The empty string '' is falsy, so '' && x returns '' for all x. On the other hand, 'string' is truthy, so 'string' && x returns x for all x.

Why JavaScript returns empty string for "logical AND" operation?

2 Answers2