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no-duplicate-definitions

Disallow duplicate definitions.

Background

In Markdown, it's possible to define the same definition identifier multiple times. However, this is usually a mistake, as it can lead to unintended or incorrect link, image, and footnote references.

Please note that this rule does not report definition-style comments. For example:

[//]: # (This is a comment 1)
[//]: <> (This is a comment 2)

Rule Details

Important

The footnotes are only supported when using language mode markdown/gfm.

This rule warns when Definition and FootnoteDefinition type identifiers are defined multiple times. Please note that this rule is case-insensitive, meaning earth and Earth are treated as the same identifier.

Examples of incorrect code:

<!-- eslint markdown/no-duplicate-definitions: "error" -->

<!-- definition -->

[mercury]: https://example.com/mercury/
[mercury]: https://example.com/venus/

[earth]: https://example.com/earth/
[Earth]: https://example.com/mars/ 

<!-- footnote definition -->

[^mercury]: Hello, Mercury!
[^mercury]: Hello, Venus!

[^earth]: Hello, Earth!
[^Earth]: Hello, Mars!

Examples of correct code:

<!-- eslint markdown/no-duplicate-definitions: "error" -->

<!-- definition -->

[mercury]: https://example.com/mercury/
[venus]: https://example.com/venus/

<!-- footnote definition -->

[^mercury]: Hello, Mercury!
[^venus]: Hello, Venus!

<!-- definition-style comment -->

[//]: # (This is a comment 1)
[//]: <> (This is a comment 2)

Options

The following options are available on this rule:

  • allowDefinitions: Array<string> - when specified, duplicate definitions are allowed if they match one of the identifiers in this array. This is useful for ignoring definitions that are intentionally duplicated. (default: ["//"])

    Examples of correct code when configured as "no-duplicate-definitions": ["error", { allowDefinitions: ["mercury"] }]:

    <!-- eslint markdown/no-duplicate-definitions: ["error", { allowDefinitions: ["mercury"] }] -->

[mercury]: https://example.com/mercury/
[mercury]: https://example.com/venus/

  • allowFootnoteDefinitions: Array<string> - when specified, duplicate footnote definitions are allowed if they match one of the identifiers in this array. This is useful for ignoring footnote definitions that are intentionally duplicated. (default: [])

    Examples of correct code when configured as "no-duplicate-definitions": ["error", { allowFootnoteDefinitions: ["mercury"] }]:

    <!-- eslint markdown/no-duplicate-definitions: ["error", { allowFootnoteDefinitions: ["mercury"] }] -->

[^mercury]: Hello, Mercury!
[^mercury]: Hello, Venus!

  • checkFootnoteDefinitions: boolean - When set to false, the rule will not report duplicate footnote definitions. (default: true)

    Examples of correct code when configured as "no-duplicate-definitions": ["error", { checkFootnoteDefinitions: false }]:

    <!-- eslint markdown/no-duplicate-definitions: ["error", { checkFootnoteDefinitions: false }] -->

[^mercury]: Hello, Mercury!
[^mercury]: Hello, Venus!

When Not to Use It

If you are using a different style of definition comments, or not concerned with duplicate definitions, you can safely disable this rule.