README.md

title
SetContext Link

Purpose

An Apollo Link to easily set context including support for async actions!

Installation

npm install apollo-link-context --save

Usage

The setContext function takes a function that returns either an object or a promise that returns an object to set the new context of the request. It receives two arguments: the GraphQL request being executed, and the previous context. This link makes it easy to perform async look up of things like authentication tokens and more!

import { setContext } from "apollo-link-context";

const middleware = setContext((request, previousContext) => ({
  authorization: '1234'
}))


const asyncMiddleware = setContext((request) => new Promise(success, fail) => {
  // do some async lookup here
  setTimeout(() => {
    success({ token: 'async found token' })
  }, 10);
});

Caching lookups

Typically async actions can be expensive and may not need to be called for every request, especially when a lot of request are happening at once. You can setup your own caching and invalidation outside of the link to make it faster but still flexible!

Take for example a user auth token being found, cached, then removed on a 401 response:

import { setContext } from 'apollo-link-context';
import { onError } from 'apollo-link-error';

// cached storage for the user token
let token;
const withToken = setContext(() => {
  // if you have a cached value, return it immediately
  if (token) return { token };

  return AsyncTokenLookup()
    .then((userToken) => {
      token = userToken;
      return { token };
    })
});

const resetToken = onError(({ networkError }) => {
  if (networkError && networkError.statusCode === 401) {
    // remove cached token on 401 from the server
    token = null;
  }
});

const authFlowLink = withToken.concat(resetToken);