[]是Array

{}是Object

[]的key是int，{}的key是string

var a= Array(); 

a[a.length] = '3'; 

a[a.length] = '4'; 

a[a.length] = '5'; 

a toJSON 后 ='["3", "4", "5"]' 

var a = new Object(); 

a['test1'] = '3'; 

a['test2'] = '4'; 

a['test3'] = '5'; 

a toJSON 后 = '{"test1":"3", "test2":"4", "test3":"5"}'