if var a = 'value',Text1 is a obj, How to Make Text1.value = 'bb' in javascript;

if var a = 'value',Text1 is a obj, How to Make Text1.value = 'bb' in javascript;but Text1.a='bb' is not right.
sorry   I   have   no   Chinese   input   mathod,I   want   to   use   a   instead   of   value,
eval(Text1.a   =   'bb '),also   not   right.

[解决办法]
eval( "Text1. "+a+ "= 'bb ' ")