Steamの設定を適当にやって環境変数を適当にやれば動く

steamssshare.js

'use strict';

const fs = require('fs');
const path = require('path');
const Twitter = require('twitter');

const dir = process.env.STEAM_SCREENSHOT_DIR;

let client = new Twitter({
	consumer_key: process.env.IKR7GYAZO_CONSUMER_KEY, 
	consumer_secret: process.env.IKR7GYAZO_CONSUMER_SECRET, 
	access_token_key: process.env.IKR7GYAZO_ACCESS_TOKEN_KEY, 
	access_token_secret: process.env.IKR7GYAZO_ACCESS_TOKEN_SECRET
});

let uploaded = [];

fs.watch(dir, (event, filename) => {
	
	if (!filename) {
		return;
	}

	if (event !== 'change') {
		return;
	}

	if (uploaded.indexOf(filename) !== -1) {
		return;
	}

	uploaded.push(filename);

	let screenshot = fs.readFileSync(path.join(dir, filename));

	client.post('media/upload', {media: screenshot}, (error, media, response) => {
		
		if (error) {
			return;
		}

		let status = {
			status: '', 
			media_ids: media.media_id_string
		};

		client.post('statuses/update', status, (error, tweet, response) => {
			console.log(tweet.text);
		});

	});

});