空判断
//空判断
if (variable1==null || variable1==undefined || variable1=='') {
var variable2 = variable1;
}
if (variable1 !== null || variable1 !== undefined || variable1 !== '') {
var variable2 = variable1;
}
if (variable1 !== null && variable1 !== undefined && variable1 !== '') {
var variable2 = variable1;
}
//空判断(简写)
var variable2 = variable1 || '';