From server side to call javascript functions/Javascript at a particular point we use ClientScript.RegisterStartupScript .

Ex:
ScriptManager.RegisterClientScriptBlock(this, typeof(Page), "Hi", "alert('The Field is mandatory');", true);

And btn.Attributes.Add("onclick", "return sampleValidation)
On click of button if we want to execute any javascript function from server side we use this.
Both we use in code behind.