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2.0.0 • Public • Published

libcors, of course

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CORS handling, 100% transportation agnostic. Is bundled with TypeScript type information.

This package is heavily influenced by corser for its logic. It is more or less hand-translated from corser line by line into TypeScript and by removing the dependencies on req/res.


Of the other libraries out there handling CORS, cors, corser, koa-cors etc, none of them are pure CORS related, but are coupled with express or koa or Node.js' http modules, and they potentially alter the corresponding req and res objects.

This package provides purely the logic for CORS.

There is no mutable global state being stored within this module. Each instance you create contains its own state. Expect no magic.

It's written in TypeScript. Exported as JavaScript with separate typings.


The package exports one function; setup.

This function initializes a new CORS context given a set of options (if any). The return value is a function that can be called with a method and a set of headers to return (a promise to) CORS properties.

import { setup as setupCors } from 'libcors'

const corsFn = setupCors( /* optional options object */ );

// We get these from somewhere:
const method;  // 'GET', 'POST', etc
const headers; // key-value of strings
const corsResult = await corsFn( method, headers );

Don't forget that corsFn returns a promise which needs to be awaited.


The options which can be provided to setup are

    origins: Origins;              // see below
    methods: string[];             // ['GET', 'HEAD', 'POST']
    requestHeaders: string[];      // see below
    responseHeaders: string[];     // see below
    supportsCredentials: boolean;  // default: false
    maxAge: number;                // default: null
    endPreflightRequests: boolean; // default: true

The origins is either an array of strings or a function taking the Origin header as argument (a string) and returns a boolean (or a promise to a boolean).

The requestHeaders defaults to [ "accept", "accept-language", "content-language", "content-type" ] and responseHeaders defaults to [ "cache-control", "content-language", "content-type", "expires", "last-modified", "pragma" ].


The corsResult above is defined by the TypeScript interface CorsResult:

interface CorsResult
    headers: { [ key: string ]: string; };
    vary: string[];
    status?: number; // Response code, if the request should be ended

The headers is a key-value lookup of headers that should be sent back in the response and the vary is a list of fields that should be appended to the Vary header.

If status is defined, this means the response should be sent immediately (without allowing further middlewares/routes) and the HTTP response code should be set to this value. if ( !status ), the normal route flow should continue.

Usage as a middleware

The package is 100% transport/framework agnostic, so to use it as a middleware in a framework, a wrapping package should be used instead where this provides the pure logic.

For Express, use express-libcors.

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  • grantila