apollo-link-context

---
title: apollo-link-context
description: Easily set a context on your operation, which is used by other links further down the chain.
---

The setContext function takes a function that returns either an object or a promise that returns an object to set the new context of a 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!

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

const setAuthorizationLink = setContext((request, previousContext) => ({
headers: {authorization: "1234"}
}));

const asyncAuthLink = 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:

`js
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.name ==='ServerError' && networkError.statusCode === 401) {
// remove cached token on 401 from the server
token = null;
}
});

const authFlowLink = withToken.concat(resetToken);
``