Does this add the prefix once, or to every line?

It adds the prefix to every line of the input, splitting on newlines first. For a single prefix added once to the whole text, use Prefix Adder instead.

What happens with an empty line?

Empty lines still get the prefix prepended, since the tool maps over every line produced by splitting on '\n', including blank ones.

Can I use it to comment out a block of code?

Yes, that is one of its most common uses. Set the prefix to '// ' or '# ' to match your language and every line, blank ones included, is commented out in a single pass.

Line Prefix Adder Online - Prepend Text to Every Line

Overview

Introduction

Turning a plain list of lines into bullet points, Markdown list items, or a block of shell flags usually means prefixing every single line the same way, a tedious edit to do by hand in a text editor.

It runs entirely client-side, so nothing you paste is ever uploaded to a server.

What Is Line Prefix Adder?

A line-based prefixing tool that prepends your chosen prefix to the start of every line in the input, rather than just once at the very beginning of the whole text.

It runs entirely client-side as part of this site's String Tools collection, so nothing you paste is ever uploaded to a server.

How Line Prefix Adder Works

The tool splits the input on newline characters, prepends the prefix to each resulting line, and joins the lines back together with newlines.

The transformation happens synchronously in JavaScript the moment you type, with no network round trip involved.

When To Use Line Prefix Adder

Use it to turn a plain list into Markdown bullets ('- '), a numbered-looking list, a comment block, or a set of CLI flags, all prefixed consistently.

It's a fast way to get the answer without opening a code editor, a REPL, or writing a one-off script just to check.

Often used alongside String Prefix Adder, Line Suffix Adder and Text Splitter.

Features

Advantages

Applies the prefix to every line in one pass, not just the start of the text.
Preserves blank lines and line order exactly.
Adds the prefix without trimming anything, so existing indentation survives intact and the prefix simply sits in front of it.

Limitations

Adds the same fixed prefix to every line; it can't generate an incrementing numbered list.
Does not trim or otherwise alter existing line content.

Examples

Turning lines into a bullet list

Input

apple
banana
cherry

Output

- apple
- banana
- cherry

With prefix '- ', each of the three lines gets the same marker prepended.

Best Practices & Notes

Best Practices

Use a trailing space in the prefix (like '- ' or '> ') so the marker doesn't run into the line content.
Include a trailing space in the prefix so markers do not run into the content, except when prefixing code where the exact column matters.
Think about whether blank lines should be prefixed, since a comment marker on an empty line is usually harmless but a bullet is not.

Developer Notes

The implementation is `input.split("\n").map((line) => prefix + line).join("\n")`, a straightforward per-line map with no trimming or filtering of the resulting lines.

Line Prefix Adder Use Cases

Converting a plain list into Markdown bullet points
Prefixing every line of a diff or patch snippet
Building a list of CLI flags from a list of bare values

Common Mistakes

Expecting the prefix to be added only once, at the top of the text, rather than to every line.
Prefixing indented code and expecting the marker to follow the indentation; it is placed at the very start of the line, ahead of any leading spaces.

Tips

Use Line Suffix Adder alongside this tool to wrap each line with both a prefix and a suffix.
Use '> ' to quote text for markdown or an email reply, where every line of the quoted block needs to carry the marker.

Line Prefix Adder