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Custom Tags

INFO:
It’s unlikely that you want this feature. You probably want shortcodes instead, Eleventy’s custom tags sugar (it’s easier to use).

Various template engines can be extended with custom tags.

Custom Tags are unrelated to Eleventy’s Collections using Tags feature. Unfortunately we’ve inherited this name from various upstream template languages.

But, after all that, you can still add a Custom Tag using the Configuration API.

Liquid

eleventy.config.js
export default function (eleventyConfig) {
  // Usage: {% uppercase myVar %} where myVar has a value of "alice"
  // Usage: {% uppercase "alice" %}
  eleventyConfig.addLiquidTag("uppercase", function (liquidEngine) {
    return {
      parse: function (tagToken, remainingTokens) {
        this.str = tagToken.args; // myVar or "alice"
      },
      render: async function (scope, hash) {
        // Resolve variables
        var str = await this.liquid.evalValue(this.str, scope); // "alice"

        // Do the uppercasing
        return str.toUpperCase(); // "ALICE"
      },
    };
  });
};
module.exports = function (eleventyConfig) {
  // Usage: {% uppercase myVar %} where myVar has a value of "alice"
  // Usage: {% uppercase "alice" %}
  eleventyConfig.addLiquidTag("uppercase", function (liquidEngine) {
    return {
      parse: function (tagToken, remainingTokens) {
        this.str = tagToken.args; // myVar or "alice"
      },
      render: async function (scope, hash) {
        // Resolve variables
        var str = await this.liquid.evalValue(this.str, scope); // "alice"

        // Do the uppercasing
        return str.toUpperCase(); // "ALICE"
      },
    };
  });
};

See all of the built-in tag implementations for LiquidJS.

Nunjucks

eleventy.config.js
export default function (eleventyConfig) {
  // Usage: {% uppercase myVar %} where myVar has a value of "alice"
  // Usage: {% uppercase "alice" %}
  eleventyConfig.addNunjucksTag("uppercase", function (nunjucksEngine) {
    return new (function () {
      this.tags = ["uppercase"];

      this.parse = function (parser, nodes, lexer) {
        var tok = parser.nextToken();

        var args = parser.parseSignature(null, true);
        parser.advanceAfterBlockEnd(tok.value);

        return new nodes.CallExtensionAsync(this, "run", args);
      };

      this.run = function (context, myStringArg, callback) {
        let ret = new nunjucksEngine.runtime.SafeString(
          myStringArg.toUpperCase()
        );
        callback(null, ret);
      };
    })();
  });
};
module.exports = function (eleventyConfig) {
  // Usage: {% uppercase myVar %} where myVar has a value of "alice"
  // Usage: {% uppercase "alice" %}
  eleventyConfig.addNunjucksTag("uppercase", function (nunjucksEngine) {
    return new (function () {
      this.tags = ["uppercase"];

      this.parse = function (parser, nodes, lexer) {
        var tok = parser.nextToken();

        var args = parser.parseSignature(null, true);
        parser.advanceAfterBlockEnd(tok.value);

        return new nodes.CallExtensionAsync(this, "run", args);
      };

      this.run = function (context, myStringArg, callback) {
        let ret = new nunjucksEngine.runtime.SafeString(
          myStringArg.toUpperCase()
        );
        callback(null, ret);
      };
    })();
  });
};

Other pages in Configuration: