Nutritious Pumpkin Meal


    2.0.0 • Public • Published


    JavaScript Style Guide Build Status

    A general purpose find-my-way custom constraint strategy.

    Tested for Fastify !

    This module let you to drive the incoming HTTP request into a route based on the header's strict content. Doing so, if a request has a specific string header, it can reach a route hide behind a constraint. Go to the Usage section to get a complete overview of this feature!


    npm install header-constraint-strategy

    Usage with Fastify

    Here all the constraint types you can define with this module!
    This setup shows you all the settings header-constraint-strategy provides to you.

    const headerConstraintStrategy = require('header-constraint-strategy')
    const Fastify = require('fastify')
    // STEP 1: setup the constraints into your fastify instance
    const app = Fastify({
      constraints: {
        // basic usage
        foo: headerConstraintStrategy('foo'),
        // strict usage
        mustBeIn: headerConstraintStrategy({ header: 'mustBeIn', mustMatchWhenDerived: true }),
        // custom header usage
        appOption: headerConstraintStrategy({ name: 'appOption', header: 'x-my-app' })
    // STEP 2: use the constraint where you need them
    app.get('/', {
      handler: reply('no constraint')
    app.get('/', {
      handler: reply('foo'),
      constraints: {
        foo: 'bar'
    app.get('/', {
      handler: reply('mustBeIn'),
      constraints: {
        mustBeIn: '123'
    app.get('/', {
      handler: reply('appOption'),
      constraints: {
        appOption: 'ABC'
    app.get('/', {
      handler: reply('mustBeIn and appOption'),
      constraints: {
        mustBeIn: '123',
        appOption: 'ABC'

    The routes can be reached via an HTTP request with these headers.

    # foo header mustBeIn header x-my-app header response
    1 - - - 200 - no constraint
    2 bar - - 200 - foo
    3 hello - - 200 - no constraint
    4 - 123 - 200 - mustBeIn
    5 - 456 - 404
    6 - - ABC 200 - appOption
    7 - 123 ABC 200 - mustBeIn and appOption
    8 - ops ABC 404
    9 bar 123 ABC 200 - mustBeIn and appOption
    10 bar ops ABC 404

    Cases explanation

    1. When the is not headers that meets the constraint, the route without constraint will be used if set. Otherwise 404
    2. The foo constraint is matched
    3. The foo constraint is not matched so the route without constraint is used
    4. The mustBeIn constraint is matched
    5. Like the 4), but this time the route without constraint is no used because of the flag mustMatchWhenDerived: true
    6. The appOption constraint is matched
    7. Multiple constraint matches
    8. Regardless the appOption constraint is matched, the mustBeIn constraint with mustMatchWhenDerived=true forces the handler to be used
    9. When there are multiple matches (the route with foo and route with mustBeIn and appOption), the route with more fulfilled constraint wins!
    10. As the 8), the mustBeIn constraint is not fulfilled


    You can pass the following options during the registration:

    Option Default Description
    name as the header if not set The name of the JSON property that you will set in the route's constraints option
    header as the name if not set The HTTP header where read the input to match the constraint
    mustMatchWhenDerived false Define if the same route without constraint must be evaluated for the routing. You can translate it as: is this constraint mandatory?


    Copyright Manuel Spigolon, Licensed under MIT.


    npm i header-constraint-strategy

    DownloadsWeekly Downloads






    Unpacked Size

    14.9 kB

    Total Files


    Last publish


    • eomm