Module: middleware-builders

Middleware builders for data fetching, conditional execution, and template rendering.

This file provides a set of reusable middleware functions and utility functions that can be composed together to create custom workflows for a web application. Includes functions for fetching data from a dataset, conditionally executing middleware, and rendering templates with validation.

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Members

(static, constant) FetchOneFallbackPolicy

Collection of fallback policies for the fetchOneFn middleware. The policy is enacted when zero records is returned from the data source.

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(static, constant) handleRejections

Express 4.x does not handle promise rejections in middleware on its own. The {@fetchOne} and {@fetchMany} middleware handle that case but for any other async code you can wrap it in this middleware to ensure rejections don't end up unhandled.

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Methods

(static) fetchIf(condition, fetchFn, elseFnopt) → {function}

Returns a middleware that will fetch data if condition is satisfied, invoke elseFn otherwise.

Parameters:
Name Type Attributes Description
condition function

Predicate function that takes a request object and returns a boolean
fetchFn function

Fetch middleware function
elseFn function <optional>

Optional function to call if condition is not met
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Returns:

Middleware function

Type
function

(static) fetchMany(context) → {function}

Fetches a collection of records and stores them in req under key specified by result entry.

Parameters:
Name Type Description
context module:middleware~QueryContext

Configuration object
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Returns:

Middleware function

Type
function

(static) fetchManyFn(req, res, next)

Middleware. Attempts to fetch a collection of data from datasette.

this needs { query( {req, params } ) => any, result: string, dataset?: FetchParams | (req) => string }

Parameters:
Name Type Description
req *
res *
next *
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(static) fetchManyFromAllDatasets(context) → {function}

Fetches a collection of records from all dataset databases and stores them in req under key specified by result entry.

Parameters:
Name Type Description
context module:middleware~QueryContext

Configuration object
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Returns:

Middleware function

Type
function

(static) fetchOne(context) → {function}

Fetches a single entity and stores it in req under key specified by result entry.

Use fallbackPolicy to handle zero record responses differently than the default 404 response. See FetchOneFallbackPolicy

Parameters:
Name Type Description
context module:middleware~QueryContext

Configuration object
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Returns:

Middleware function

Type
function

(static) fetchOneFromAllDatasets(context) → {function}

Fetches a single record from each dataset databases and stores them in req under key specified by result entry.

Parameters:
Name Type Description
context module:middleware~QueryContext

Configuration object
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Returns:

Middleware function

Type
function

(static) parallel(middlewares) → {function}

Returns a middleware that invokes the passed middlewares in parallel.

Usage: when all sub-middlewre can be fetched independently, but we can't accept a partial success (e.g. we require all middlewares to succeed).

Parameters:
Name Type Description
middlewares Array.<function()>

Array of middleware functions
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Returns:

Express middleware function that returns a Promise

Type
function

(static) renderTemplate(context) → {function}

Validates and renders the template.

Parameters:
Name Type Description
context Object

Configuration object

Properties
Name Type Description
templateParams function

Function that returns template parameters
template string

Template name
handlerName string

Handler name
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Returns:

Express middleware function

Type
function

(static) renderTemplateFn(req, res, next) → {void}

Middleware. Validates and renders the template.

this needs: { templateParams(req), template, handlerName }

Parameters:
Name Type Description
req Object

Express request object
res Object

Express response object
next function

Express next function
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Returns:
Type
void

(static) validateAndRender(res, name, params) → {string}

Looks up schema for name in templateSchema (defaults to any()), validates and renders the template.

Parameters:
Name Type Description
res module:express~Response

response object
name string
params *

template params
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Returns:
Type
string

(async, inner) fetchIfFn(req, res, next)

Middleware. Does a conditional fetch. Optionally invokes else if condition is false.

this needs: { fetchFn, condition: (req) => boolean, else?: (req) => void }

fetchFn should be a middleware fn. Can be async.

Parameters:
Name Type Description
req *
res *
next *
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(async, inner) fetchManyFromAllDatasetsFn(req, res, next) → {Promise.<*>}

Fetches data from all available datasets and stores the result in the request object.

Parameters:
Name Type Description
req Object

The request object.

Properties
Name Type Attributes Description
params string

The URL parameters for the request.
originalUrl string

original URL
handlerName string <optional>

value set in this fn
res Object

The response object.
next function

The next middleware function in the chain.
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Throws:

If an error occurs while fetching data from any of the datasets.

Type
Error
Returns:
Type
Promise.<*>

(async, inner) fetchOneFn(req, res, next)

Middleware. Attempts to fetch data from datasette and short-circuits with 404 when data for given query does not exist. Meant to be used to fetch singular records.

this needs { query({ req, params }) => any, result: string, dataset?: FetchParams | (req) => string, fallbackPolicy: (req, res, next) => void }

where the result is the key under which result of the query will be stored in req

Parameters:
Name Type Description
req *
res *
next *
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(async, inner) fetchOneFromAllDatasetsFn(req, res, next)

Middleware. Fetches one result from all available datasets.

This function runs a query on each available dataset, catches any errors that may occur, and then compiles the results into a single object. The result object is then attached to the request object.

Parameters:
Name Type Description
req Object

The request object.

Properties
Name Type Description
params Object

Route parameters.
originalUrl string

The original URL of the request.
handlerName string

A property to store the name of the handler.
query function

A function to construct a query based on the request parameters.
res Object

The response object.
next function

The next middleware function in the stack.
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Throws:

If any of the queries fail.

Type
Error

(async, inner) parallelFn(req, res, next) → {Promise.<undefined>}

Returns a middleware that executes the given sub-middlewares in parallel and waits for all of them to complete.

Parameters:
Name Type Description
req Object

Express request object
res Object

Express response object
next function

Express next function
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Returns:
Type
Promise.<undefined>

Type Definitions

QueryContext

Type:
  • Object
Properties:
Name Type Attributes Description
query function

Function that takes req and params and returns query object
result string

Key to store result under in req
dataset Symbol <optional>

how to get the dataset name, see FetchOptions
context.fallbackPolicy function <optional>

Custom fallback policy for zero results
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