HTTP Headers

Headers allow you to set custom HTTP headers for an incoming request path.

To set custom HTTP headers you can use the headers key in blitz.config.js:

module.exports = {
  async headers() {
    return [
      {
        source: "/about",
        headers: [
          {
            key: "x-custom-header",
            value: "my custom header value",
          },
          {
            key: "x-another-custom-header",
            value: "my other custom header value",
          },
        ],
      },
    ]
  },
}

headers is an async function that expects an array to be returned holding objects with source and headers properties:

source is the incoming request path pattern.
headers is an array of header objects with the key and value properties.
basePath: false or undefined - if false the basePath won't be included when matching, can be used for external rewrites only.
locale: false or undefined - whether the locale should not be included when matching.
has is an array of has objects with the type, key and value properties.

Headers are checked before the filesystem which includes pages and /public files.

Header Overriding Behavior

If two headers match the same path and set the same header key, the last header key will override the first. Using the below headers, the path /hello will result in the header x-hello being world due to the last header value set being world.

module.exports = {
  async headers() {
    return [
      {
        source: '/:path*',
        headers: [
          {
            key: 'x-hello',
            value: 'there',
          },
        ],
      },
      {
        source: '/hello',
        headers: [
          {
            key: 'x-hello',
            value: 'world',
          },
        ],
      },
    ],
  },
}

Path Matching

Path matches are allowed, for example /blog/:slug will match /blog/hello-world (no nested paths):

module.exports = {
  async headers() {
    return [
      {
        source: '/blog/:slug',
        headers: [
          {
            key: 'x-slug',
            value: ':slug', // Matched parameters can be used in the value
          },
          {
            key: 'x-slug-:slug', // Matched parameters can be used in the key
            value: 'my other custom header value',
          },
        ],
      },
    ],
  },
}

Wildcard Path Matching

To match a wildcard path you can use * after a parameter, for example /blog/:slug* will match /blog/a/b/c/d/hello-world:

module.exports = {
  async headers() {
    return [
      {
        source: '/blog/:slug*',
        headers: [
          {
            key: 'x-slug',
            value: ':slug*', // Matched parameters can be used in the value
          },
          {
            key: 'x-slug-:slug*', // Matched parameters can be used in the key
            value: 'my other custom header value',
          },
        ],
      },
    ],
  },
}

Regex Path Matching

To match a regex path you can wrap the regex in parenthesis after a parameter, for example /blog/:slug(\\d{1,}) will match /blog/123 but not /blog/abc:

module.exports = {
  async headers() {
    return [
      {
        source: '/blog/:post(\\d{1,})',
        headers: [
          {
            key: 'x-post',
            value: ':post',
          },
        ],
      },
    ],
  },
}

The following characters (, ), {, }, :, *, +, ? are used for regex path matching, so when used in the source as non-special values they must be escaped by adding \\ before them:

module.exports = {
  async redirects() {
    return [
      {
        // this will match `/english(default)/something` being requested
        source: "/english\\(default\\)/:slug",
        destination: "/en-us/:slug",
        permanent: false,
      },
    ]
  },
}

To only apply a header when either header, cookie, or query values also match the has field can be used. Both the source and all has items must match for the header to be applied.

has items have the following fields:

type: String - must be either header, cookie, host, or query.
key: String - the key from the selected type to match against.
value: String or undefined - the value to check for, if undefined any value will match. A regex like string can be used to capture a specific part of the value, e.g. if the value first-(?<paramName>.*) is used for first-second then second will be usable in the destination with :paramName.

module.exports = {
  async headers() {
    return [
      // if the header `x-add-header` is present,
      // the `x-another-header` header will be applied
      {
        source: "/:path*",
        has: [
          {
            type: "header",
            key: "x-add-header",
          },
        ],
        headers: [
          {
            key: "x-another-header",
            value: "hello",
          },
        ],
      },
      // if the source, query, and cookie are matched,
      // the `x-authorized` header will be applied
      {
        source: "/specific/:path*",
        has: [
          {
            type: "query",
            key: "page",
            // the page value will not be available in the
            // header key/values since value is provided and
            // doesn't use a named capture group e.g. (?<page>home)
            value: "home",
          },
          {
            type: "cookie",
            key: "authorized",
            value: "true",
          },
        ],
        headers: [
          {
            key: "x-authorized",
            value: ":authorized",
          },
        ],
      },
      // if the header `x-authorized` is present and
      // contains a matching value, the `x-another-header` will be applied
      {
        source: "/:path*",
        has: [
          {
            type: "header",
            key: "x-authorized",
            value: "(?<authorized>yes|true)",
          },
        ],
        headers: [
          {
            key: "x-another-header",
            value: ":authorized",
          },
        ],
      },
      // if the host is `example.com`,
      // this header will be applied
      {
        source: "/:path*",
        has: [
          {
            type: "host",
            value: "example.com",
          },
        ],
        headers: [
          {
            key: "x-another-header",
            value: ":authorized",
          },
        ],
      },
    ]
  },
}

Headers with basePath support

When leveraging basePath support with headers each source is automatically prefixed with the basePath unless you add basePath: false to the header:

module.exports = {
  basePath: "/docs",

  async headers() {
    return [
      {
        source: "/with-basePath", // becomes /docs/with-basePath
        headers: [
          {
            key: "x-hello",
            value: "world",
          },
        ],
      },
      {
        source: "/without-basePath", // is not modified since basePath: false is set
        headers: [
          {
            key: "x-hello",
            value: "world",
          },
        ],
        basePath: false,
      },
    ]
  },
}

Headers with i18n support

When leveraging i18n support with headers each source is automatically prefixed to handle the configured locales unless you add locale: false to the header. If locale: false is used you must prefix the source with a locale for it to be matched correctly.

module.exports = {
  i18n: {
    locales: ["en", "fr", "de"],
    defaultLocale: "en",
  },

  async headers() {
    return [
      {
        source: "/with-locale", // automatically handles all locales
        headers: [
          {
            key: "x-hello",
            value: "world",
          },
        ],
      },
      {
        // does not handle locales automatically since locale: false is set
        source: "/nl/with-locale-manual",
        locale: false,
        headers: [
          {
            key: "x-hello",
            value: "world",
          },
        ],
      },
      {
        // this matches '/' since `en` is the defaultLocale
        source: "/en",
        locale: false,
        headers: [
          {
            key: "x-hello",
            value: "world",
          },
        ],
      },
      {
        // this gets converted to /(en|fr|de)/(.*) so will not match the top-level
        // `/` or `/fr` routes like /:path* would
        source: "/(.*)",
        headers: [
          {
            key: "x-hello",
            value: "worlld",
          },
        ],
      },
    ]
  },
}

Cache-Control

Cache-Control headers set in blitz.config.js will be overwritten in production to ensure that static assets can be cached effectively. If you need to revalidate the cache of a page that has been statically generated, you can do so by setting revalidate in the page's getStaticProps function.

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