Rule - MD009

Property	Value
Aliases	md009, no-trailing-spaces
Autofix Available	Yes
Enabled By Default	Yes

Summary

Trailing spaces.

Reasoning

Simplicity

The primary reason for enabling this rule is simplicity. Two spaces at the end of a line within a paragraph produces a Hard Line Break element. Other than that, there is no other reason for trailing spaces on any line. Any other form of trailing spaces is ignored, with no effect on the final HTML document.

Examples

As the space character is not normally visible, each occurrence of the text {space} in the following example represents a single space character.

Failure Scenarios

This rule triggers when any line of the document ends with more than one space, except for the number of spaces specified by the br_spaces configuration value:

Markdown this line ends with one space character{space}

This rule can be triggered on any eligible line (non code block line) with trailing spaces, ignoring the br_spaces configuration value, by setting the strict configuration value to True. With the default value for br_spaces of 2 and strict set to True, all these lines will trigger the rule:

Markdown This line does not end with any spaces. This line ends with one space.{space} This line ends with two spaces.{space}{space} This line ends with three spaces.{space}{space}{space}

Correct Scenarios

This rule does not trigger if a line does not end with any spaces or if the line ends with the exact number of spaces specified by the br_spaces configuration value:

Markdown This line does not end with any spaces. This line ends with two spaces, which is okay.{space}{space}

As various programming languages allow spaces at the end of their lines, the Indented Code Blocks and Fenced Code Blocks do not trigger this rule for any of their lines.

In addition, various parsers may require spaces on blank lines within List elements to keep that List element open. Enabling the list_item_empty_lines configuration value will cause this sample to not trigger the rule:

Markdown - a list item {space}{space} still the same item, different paragraph

Note that the number of spaces needed to recognize the Blank Line within the List element is decided by the current List Item element that has the Blank Line.

Fix Description

When fixed, each eligible line will either have 0 whitespace characters or br_spaces whitespace characters at the end of that line.

Configuration

Prefixes
plugins.md009.
plugins.no-trailing-spaces.

Value Name	Type	Default	Description
enabled	boolean	True	Whether the Rule Plugin is enabled.
br_spaces	integer	2	Specifies the exact number of spaces allowed at the end of the line.
strict	boolean	False	Whether strict mode is enabled for the plugin.
list_item_empty_lines	boolean	False	Whether empty list item lines are exempt from this rule.

Origination of Rule

This rule is largely inspired by the MarkdownLint rule MD009.

Differences From MarkdownLint Rule

The main difference from the original rule is in how a HTML block is managed. According to the original rule:

Trailing space is allowed in indented and fenced code blocks because some languages require it.

However, in tests against the original rule, HTML blocks seemed to be immune to triggering this rule. This rule adheres to the text in the original specification by not triggering on indented code blocks and fenced code blocks but triggering on HTML blocks.

In addition, the original implementation used the br_spaces value when using the list_item_empty_lines configuration value to decide if that List Item Empty Line triggered the rule. That algorithm has been changed in this rule to instead use the number of spaces required to satisfy the indentation requirements of the List element.