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Substitute datetime placeholders in an API description

Authors:

What this does and why

When an API description includes datetime values, it's nice if they present soon or realistic date values. This decorator provides the functionality to replace a special datetime placeholder, $DateTimeNow, with the current date and time when bundling the API description. You can also use expressions to add a set number of days to the value, for example $DateTimeNow + 10 days.

Warning

Placeholders aren't supported by all OpenAPI tools. Using this approach, you should bundle your OpenAPI to apply the decorators before passing the API description to other tools.

Code

The dates-plugin plugin defines the decorator section and the plugin id:

const updateExampleDates = require("./decorator");

module.exports = {
  id: "dates-plugin",
  decorators: {
    oas3: {
      "update-example-dates": updateExampleDates,
    },
  },
};

Here's the main part of the decorator (from decorator.js):

function updateExampleDates() {
  return {
    // Covers the 'examples' keyword (including examples in the 'components' section)
    Example: {
      enter(example) {
        traverseAndReplaceWithComputedDateTime(example.value);
      },
    },
    // Covers the 'example' in media type objects
    MediaType: {
      enter(mediaTypeObject) {
        if (mediaTypeObject.example) {
          traverseAndReplaceWithComputedDateTime(mediaTypeObject.example);
        }
      },
    },
    // Covers the 'example' in schemas
    Schema: {
      enter(schema) {
        if (schema.example) {
          traverseAndReplaceWithComputedDateTime(schema);
        }
      },
    },
  };
}

The decorator operates on the Example, MediaType and Schema nodes to cover the different ways of specifying examples.

The traverseAndReplaceWithComputedDateTime function traverses the tree of nodes and replaces the values that contain the specific $DateTimeNow variable in examples with a computed datetime value:

function traverseAndReplaceWithComputedDateTime(value) {
  if (typeof value === "object" && value !== null) {
    for (const key in value) {
      if (typeof value[key] === "string" && /\$DateTimeNow/.test(value[key])) {
        // Replacing the template with a computed datetime value
        value[key] = computeDateTemplate(value[key]);
      } else {
        traverseAndReplaceWithComputedDateTime(value[key]);
      }
    }
  }
}

The computeDateTemplate function is a very basic datetime calculator that can only add days to the current date, but it illustrates the idea and you can extend it to meet your own specific needs:

function computeDateTemplate(template) {
  const substitutedExpression = template
    .replace("$DateTimeNow", Date.now()) // Replacing the variable with the current date in milliseconds
    .replace(/(\d+)\s*days?/g, (_, t) => t * 1000 * 60 * 60 * 24) // Replacing days with milliseconds
    .trim();
  const calculated = substitutedExpression // A basic calculator
    .split(/\s*\+\s*/)
    .reduce((sum, item) => sum + +item, 0);
  return new Date(calculated).toISOString();
}

Examples

Add the plugin to redocly.yaml and enable the decorator:

plugins:
  - ./plugin.js

decorators:
  dates-plugin/update-example-dates: on

Given the following API description, the decorator will replace $DateTimeNow with the current date and $DateTimeNow + 7d with the date a week from now:

openapi: 3.1.0
paths:
  /date-time:
    get:
      responses:
        200:
          content:
            application/json:
              example:
                somewhere-in-the-past: 2023-11-06T12:00:00.000Z
                today: $DateTimeNow # Will be replaced with the current date
                in-a-week: $DateTimeNow + 7 days # Will be replaced with the date in a week

Please note that API descriptions are static text documents, so the dates will get updated only when the description gets bundled.

References