How to access a control using Id in JavaScript on ContentPage in ASP.Net

I am using a Master Page in my project and in the content page i want to access a control using document.getElementById but it is providing me null but when i do the same thing in a project which does not contail Master Page it works fine.

I have found a solution for this as i have to view the page source of my project and copy the id of the control generated by the ASP.NET and then it works fine.

I want to know if there is any other solution for this.

Thanks