JavaScript Document writeln() Method

What You’ll Learn

document.writeln() is a Deprecated instance method that writes one or more HTML strings (or TrustedHTML values) into a document stream opened by document.open(), then appends a newline (see MDN Document: writeln()). Learn how that newline only shows in whitespace-preserving elements, how it compares to write(), XSS / Trusted Types notes, safer alternatives, and five iframe-based try-it labs.

Introduction

writeln() belongs to the same legacy document stream family as write():

1. document.open() — start (and often clear) the stream.
2. document.writeln(...) — push markup, then a newline character.
3. document.close() — finish writing so parsing can complete.

MDN: the method is essentially the same as document.write() but adds a newline. That newline is only visible if it lands inside an element where newlines are displayed (MDN highlights <pre>).

Related tutorials: write(), open(), close(), createElement().

Understanding document.writeln()

An instance method on Document (dynamic markup insertion with a trailing newline).

• Parameters — one or more markup strings or TrustedHTML values (MDN).
• Return value — undefined (MDN).
• vs write() — same stream behavior, plus a newline after the written text (MDN).
• Newline visibility — only noticeable in elements that preserve whitespace (for example <pre>) (MDN).
• After load — same wipe risk as write() via auto-open().
• Injection sink — parses HTML; XSS risk if input is untrusted (MDN).
• Trusted Types — when enforced, strings can throw TypeError unless a policy creates TrustedHTML (MDN).
• Status — Deprecated (MDN).

📝 Syntax

General forms of Document.writeln (MDN):

writeln(markup)
writeln(markup, markup2)
writeln(markup, markup2, /* …, */ markupN)

Parameters

• markup, …, markupN — TrustedHTML or string objects containing the text to write to the document (MDN).

Return value

None (undefined) (MDN).

Exceptions

• InvalidStateError-style cases on MDN include calling on an XML document, or while a custom element constructor is running (see MDN).
• TypeError — a string is passed when Trusted Types are enforced and no default policy exists for creating TrustedHTML (MDN).

Classic stream pattern

const doc = iframe.contentDocument;
doc.open();
doc.writeln("<pre>Line A");
doc.writeln("Line B</pre>");
doc.close();

⚖️ writeln() vs write()

Examples Gallery

Examples follow MDN Document: writeln() and the related write/open/close stream. Prefer iframe demos.

const doc = document.getElementById("preview").contentDocument;
doc.open();
doc.writeln("<pre>Hello world!");
doc.writeln("I am a fish");
doc.writeln("The number is 42</pre>");
doc.close();
console.log("pre text:", doc.querySelector("pre").textContent);

pre shows three lines (newlines from writeln)

const doc = document.getElementById("preview").contentDocument;
doc.open();
doc.write("<pre id='a'>");
doc.write("write:");
doc.write("same-line");
doc.write("</pre><pre id='b'>");
doc.writeln("writeln:");
doc.writeln("next-line");
doc.write("</pre>");
doc.close();

write block stays denser; writeln block shows extra breaks

const doc = document.getElementById("preview").contentDocument;
const one = "<h1>Out with";
const two = "the old</h1>";
const three = "<pre>in with";
const four = "the new!</pre>";

doc.open();
doc.writeln(one);
doc.writeln(two, three);
doc.writeln(four);
doc.close();

iframe shows heading + pre; newlines appear inside the pre

const doc = document.getElementById("preview").contentDocument;
doc.open();
doc.writeln("<p id='keep'>Original iframe content</p>");
doc.close();

document.getElementById("wipe").addEventListener("click", () => {
  // Late writeln on a loaded document can open() and replace it.
  doc.writeln("<p>Replaced via late writeln()</p>");
  doc.close();
});

Original content disappears; only the late writeln remains

const host = document.getElementById("host");
host.replaceChildren();

const pre = document.createElement("pre");
pre.textContent = "Out with the old\nin with the new!";
host.append(pre);

host shows a pre with two lines — no document.writeln

🚀 Common Use Cases

• Legacy literacy — recognize Deprecated writeln() next to write() (MDN).
• Teaching newlines — show why a trailing \n only matters in whitespace-preserving elements.
• Iframe previews — rebuild a sandbox document with open/writeln/close for demos.
• Explaining auto-open — teach why late stream writes wipe pages.
• Trusted Types training — show writeln as an HTML injection sink (MDN).
• Not for production UI — migrate to modern DOM APIs.

📝 Notes

• MDN: marked Deprecated.
• Not marked Experimental or Non-standard on MDN, but still unsuitable for new products.
• Essentially write() + newline; newline visibility depends on the target element (MDN).
• Late writeln() can auto-open() and clear the document.
• HTML injection sink — XSS risk; prefer TrustedHTML when Trusted Types are enforced (MDN).
• Practice on iframe.contentDocument, never on the live tutorial page.
• Related: write(), open(), close(), createElement().

💡 Best Practices