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node_modules/apollo-link-context/README.md

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+---
+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);
+```