!!variant will return boolean type according to given value. 1) !!a will return true when the value is given non-empty string or non-zero number var a = " "; alert(!!a); var a = "s"; alert(!!a); var a = true; alert(!!a); var a = 1; alert(!!a); var a = -1; alert(!!a); var a = -2; alert(!!a); 2) !!a will return false when the value is empty string or zero number or null var a = 0; alert(!!a); var a = ""; alert(!!a); var a = false; alert(!!a); var a = null; alert(!!a); 原作: http://yyq448.blog.163.com/blog/static/1276004422007111163528588/