Some time developers need to call the javascript file from code behind or dynamically from content page then you may call javascript file with below given code.

HtmlGenericControl externalJScript = new HtmlGenericControl();
externalJScript.TagName = "script";
externalJScript.Attributes.Add("type", "javascript");
externalJScript.Attributes.Add("src","FolderName/MyJsFile.js");
this.Page.Header.Controls.Add(externalJScript);