为什么JavaScript为“逻辑AND”运算返回空字符串？

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?

Answer 1

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 .

Answer 2

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 .