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## XML DOM: DocumentType.systemId Property The `systemId` property of the `DocumentType` object returns the **system identifier** of an external Document Type Definition (DTD) associated with the XML or HTML document. Typically, this is a URI (such as a local file path or a web URL) pointing to the external DTD file. --- ## Syntax ```javascript documentObject.doctype.systemId ``` ### Return Value * **String**: A string representing the system identifier (URI) of the external DTD. * **Null**: Returns `null` if there is no external DTD or if the system identifier is not specified. --- ## Code Example The following example demonstrates how to retrieve the system identifier of an external DTD. Suppose we have an XML file named `note_external_dtd.xml` with the following DOCTYPE declaration: ```xml Tove Jani Reminder Don't forget me this weekend! ``` We can load this XML document and retrieve its `systemId` using JavaScript: ```javascript // Load the XML document var xmlDoc = loadXMLDoc("note_external_dtd.xml"); // Access the doctype and retrieve the systemId var systemIdentifier = xmlDoc.doctype.systemId; // Output the result console.log(systemIdentifier); // Output: "note.dtd" ``` --- ## Practical Considerations ### 1. Difference Between `systemId` and `publicId` * **`systemId`**: Specifies the system-dependent location of the DTD file (usually a URL or file path). It is intended for the XML parser to locate and download the DTD. * **`publicId`**: Specifies a globally unique, system-independent public identifier (e.g., `"-//W3C//DTD XHTML 1.0 Transitional//EN"`). If a public identifier is used, the parser may look up the DTD in a local catalog before attempting to download it via the `systemId`. ### 2. HTML5 Behavior In modern HTML5 documents, the standard doctype declaration is: ```html ``` Since HTML5 does not reference an external DTD, calling `document.doctype.systemId` in a standard HTML5 web page will return an empty string (`""`). ### 3. Browser Compatibility and Read-Only Status * **Read-Only**: The `systemId` property is read-only. You cannot modify the DTD's system identifier dynamically after the document has been parsed. * **Compatibility**: This property is supported across all major modern browsers and standard XML DOM parsers.