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​​ Background

All HTTP request and response headers are available through the Headers API.

When a header name possesses multiple values, those values will be concatenated as a single, comma-delimited string value. This means that Headers.get will always return a string or a null value. This applies to all header names except for Set-Cookie, which requires Headers.getAll. This is documented below in Differences.

let headers = new Headers();
headers.get('x-foo'); //=> null
headers.set('x-foo', '123');
headers.get('x-foo'); //=> "123"
headers.set('x-foo', 'hello');
headers.get('x-foo'); //=> "hello"
headers.append('x-foo', 'world');
headers.get('x-foo'); //=> "hello, world"

​​ Differences

  • Despite the fact that the Headers.getAll method has been made obsolete, Cloudflare still offers this method but only for use with the Set-Cookie header. This is because cookies will often contain date strings, which include commas. This can make parsing multiple values in a Set-Cookie header more difficult. Any attempts to use Headers.getAll with other header names will throw an error. A brief history Headers.getAll is available in this GitHub issue.

  • Due to RFC 6265 prohibiting folding multiple Set-Cookie headers into a single header, the Headers.append method will allow you to set multiple Set-Cookie response headers instead of appending the value onto the existing header.

const headers = new Headers();
headers.append("Set-Cookie", "cookie1=value_for_cookie_1; Path=/; HttpOnly;");
headers.append("Set-Cookie", "cookie2=value_for_cookie_2; Path=/; HttpOnly;");
// Array(2) [ cookie1=value_for_cookie_1; Path=/; HttpOnly;, cookie2=value_for_cookie_2; Path=/; HttpOnly; ]

​​ Cloudflare headers

Cloudflare sets a number of its own custom headers on incoming requests and outgoing responses. While some may be used for its own tracking and bookkeeping, many of these can be useful to your own applications – or Workers – too.

For a list of documented Cloudflare request headers, refer to HTTP request headers.