Add files via upload

manifest.json

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+{
+	"manifest_version": 2,
+	"name": "RedditViewsHidder",
+	"version": "1.0",
+
+	"description": "Hide viewed reddit posts",
+
+	"icons": {
+	  "48": "icon.png"
+	},
+
+	"content_scripts": [
+	  {
+		"matches": ["*://*.reddit.com/*"],
+		"js": ["redditviewshidder.js"]
+	  }
+	]
+  }
+

redditviewshidder.js

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+let maxStoreCount = 10000;
+let articlesContainer = document.getElementsByTagName('shreddit-feed')[0];
+let localStorageSaveKey = "viewedRedditPosts";
+let viewedPostsById = new Set();
+
+function LoadSavedViews(){
+	let savedJson = localStorage.getItem(localStorageSaveKey);
+	if(savedJson){
+		const arrFromJson = JSON.parse(savedJson);
+		viewedPostsById = new Set(arrFromJson);
+		//trim size
+		if(viewedPostsById.size > maxStoreCount){
+			let arrToTrim = Array.from(viewedPostsById);
+			arrToTrim.splice(0, viewedPostsById.size - maxStoreCount)
+			viewedPostsById = new Set(arrToTrim);
+		}
+	}
+}
+
+function SaveViews(){
+	localStorage.setItem(localStorageSaveKey, JSON.stringify(Array.from(viewedPostsById)));
+}
+
+function ForeachPosts(callback){
+	let articles = articlesContainer.getElementsByTagName('article');
+	for(let article of articles){
+		let post = article.getElementsByTagName('shreddit-post')[0];
+		callback(article, post, post.id);
+	}
+}
+
+function RemoveViewedPosts(){
+	ForeachPosts((articleEl, postEl, postId)=>{
+		if(viewedPostsById.has(postId))
+		{
+			articleEl.nextSibling.remove(); //remove hr
+			articleEl.remove();
+		}
+	});
+}
+
+//START
+
+LoadSavedViews();
+
+//Hide viewedPosts
+let newpostObserver = new MutationObserver(function(mutations) {
+	mutations.forEach(function(mutation) {
+		if(mutation.addedNodes.length > 0){
+			RemoveViewedPosts();
+		}
+	});
+  });
+
+newpostObserver.observe(articlesContainer, {childList: true, subtree: true, characterData: false});
+
+RemoveViewedPosts();
+
+//Catch viewed posts
+addEventListener("scroll", (event) => {
+	ForeachPosts((articleEl, postEl, postId)=>{
+		let bounds = articleEl.getBoundingClientRect();
+		if(bounds.bottom < 0) {
+			viewedPostsById.add(postId);
+		}
+	});
+});
+
+addEventListener("beforeunload", (event) => {
+	SaveViews();
+});