as far as the clientside Javascript is concerned, the textbox in the masterpage or on a content page should appear to be a part of the page itself and you could use
document.getElementById("text1").value;
// if its in a form, then you can get it these ways...
document.formname.textboxname.value
// or
document.forms[0].textboxname.value;
Its different of course if you are using .NET, because the code will know that the textbox isn't a part of the page, and you would need to use something like this to get at a textbox on a masterpage..
((TextBox)this.Master.FindControl("TextBox1")).Visible = false;
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