first setup: Medium library

.babelrc

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+{
+  "presets": ["babel-preset-expo"],
+  "env": {
+    "development": {
+      "plugins": ["transform-react-jsx-source"]
+    }
+  }
+}

.gitignore

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+node_modules/**/*
+.expo/*
+npm-debug.*

.watchmanconfig

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+{}

App.js

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+import React from 'react';
+import { StyleSheet, Text, View, Button } from 'react-native';
+
+import { Medium } from "./src";
+
+export default class App extends React.Component {
+
+  handlePress = async () => {
+    const result = await Medium.logInAsync({
+      'client_id': '70eaa8fd5369',
+      'scope': Medium.getScopes(),
+      'state': 'my arbitary state'
+    })
+
+    console.log(result);
+  }
+
+  render() {
+    return (
+      <View style={styles.container}>
+        <Button 
+          title="Login with Medium"
+          onPress={this.handlePress}
+        />
+      </View>
+    );
+  }
+}
+
+const styles = StyleSheet.create({
+  container: {
+    flex: 1,
+    backgroundColor: '#fff',
+    alignItems: 'center',
+    justifyContent: 'center',
+  },
+});

app.json

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+{
+  "expo": {
+    "name": "react-native-expo-medium-login",
+    "description": "This project is really great.",
+    "slug": "react-native-expo-medium-login",
+    "privacy": "public",
+    "sdkVersion": "28.0.0",
+    "platforms": ["ios", "android"],
+    "version": "1.0.0",
+    "orientation": "portrait",
+    "icon": "./assets/icon.png",
+    "splash": {
+      "image": "./assets/splash.png",
+      "resizeMode": "contain",
+      "backgroundColor": "#ffffff"
+    },
+    "updates": {
+      "fallbackToCacheTimeout": 0
+    },
+    "assetBundlePatterns": [
+      "**/*"
+    ],
+    "ios": {
+      "supportsTablet": true
+    }
+  }
+}

package.json

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+{
+  "main": "node_modules/expo/AppEntry.js",
+  "private": true,
+  "dependencies": {
+    "expo": "^28.0.0",
+    "lodash": "^4.17.10",
+    "react": "16.3.1",
+    "react-native": "https://github.com/expo/react-native/archive/sdk-28.0.0.tar.gz"
+  }
+}

src/Medium.js

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+/**
+ * Import Dependencies
+ * 
+ * We're use AuthSession
+ * @see https://docs.expo.io/versions/v28.0.0/sdk/auth-session
+ * 
+ */
+import { AuthSession } from "expo";
+import isPlainObject from 'lodash/isPlainObject';
+
+/**
+ * BASE Constant
+ * This is used for base setting
+ * 
+ */
+export const AUTH_URL = 'https://medium.com/m/oauth/authorize';
+
+/**
+ * SCOPE Constant
+ * This is for default constant scope used by medium
+ * @see https://github.com/Medium/medium-api-docs#21-browser-based-authentication
+ * 
+ */
+export const SCOPES = [
+    "basicProfile", "listPublications", "publishPost"
+];
+
+/**
+ * Get the redirect url, and copy given url to your medium application project `Callback URLs`
+ * @see https://docs.expo.io/versions/v28.0.0/sdk/auth-session#authsessiongetredirecturl
+ * 
+ */
+export function getRedirectUrl() {
+    return AuthSession.getRedirectUrl();
+}
+
+/**
+ * Get all scopes
+ */
+export function getScopes() {
+    return SCOPES.join();
+}
+
+/**
+ * Converts an object to a query string.
+ * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries
+ * 
+ * @param {Object} params An object to convert
+ * @returns {String} A query string with key value pair
+ */
+function objectToQueryString(params) {
+    return '?' + Object.entries(params)
+      .map(([key, value]) => `${encodeURIComponent(key)}=${encodeURIComponent(value)}`)
+      .join('&');
+}
+
+/**
+ * Get code information from login with medium
+ * @see https://github.com/Medium/medium-api-docs#2-authentication
+ * 
+ * @param {Object} credentials Object of credentials you need to access information
+ */
+export async function logInAsync(credentials) {
+    if (!isPlainObject(credentials)) {
+        throw new Error('Credentials must be plain objects.');
+    }
+
+    if (typeof credentials.client_id === 'undefined' 
+        || typeof credentials.scope === 'undefined' 
+        || typeof credentials.state === 'undefined'
+    ) {
+        throw new Error(
+            'Credentials may not have an undefined "type" property. ' +
+            'Have you missing a value?'
+        );
+    }
+
+    // get the redirect uri
+    let redirectUrl = getRedirectUrl();
+
+    // append new object
+    credentials.response_type = 'code'; // always be `code`
+    credentials.redirect_uri = redirectUrl;
+
+    // get the auth url
+    let authUrl = `${AUTH_URL}` + objectToQueryString(credentials);
+
+    // start async
+    const result = await AuthSession.startAsync({
+        authUrl: authUrl
+    })
+
+    // give the result of start async
+    return result;
+}

src/index.js

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+/**
+ * We're use expo SDK 28.0.0
+ */
+
+import * as Medium from "./Medium";
+import { getScopes } from "./Medium";
+
+export {
+    Medium,
+    getScopes
+}