Docs
Try Apollo Studio

Context Link

Easily set a context on your operation, which is used by other links further down the chain.


Overview

The setContext function accepts a function that returns either an object or a promise, which then 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 the asynchronous lookup of things like authentication tokens and more.

import { setContext } from "@apollo/client/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:

import { setContext } from "@apollo/client/link/context";
import { onError } from "@apollo/client/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);
Edit on GitHub
Previous
HTTP Batch
Next
Error