Prefer that
for-in loops should include an if statement JS-0051
Anti-pattern
Minor
2 months ago
—
2 years old
Wrap the body of a for-in loop in an if statement with a hasOwnProperty guard
84 if(!(watched[board] instanceof Array))
85 watched[board] = [];
86
87 for(const t in watched[board]) { 88 let thread = watched[board][t]; 89 if(typeof thread === "number") { 90 thread = watched[board][t] = {id: thread}; 91 } 92 if(thread.id === threadID) return; // thread is already in the watched list 93 } 94 getThreadJSON(threadID, board).then(data => {
95 const op = data.posts[0];
96 const threadObj: WatchedThreadJSON = {Wrap the body of a for-in loop in an if statement with a hasOwnProperty guard
20 continue;
21 }
22 $(document).trigger("beginNewPostsCheck");
23 for(const t in watched[board]) { 24 const thread = watched[board][t]; 25 if(thread.err !== undefined) continue; 26 getThreadJSON(thread.id, board).then(data => { 27 if(data.posts.length > thread.posts) { 28 // watched thread has new posts, trigger a menu update 29 if(currentPage.board === board && currentPage.id === thread.id) { 30 // we're currently in the thread, update the cookie 31 watched[board][t].posts = data.posts.length; 32 watched[board][t].latest = data.posts[data.posts.length - 1].no; 33 setStorageVal("watched", watched, true); 34 } 35 $(document).trigger("watcherNewPosts", { 36 newPosts: data.posts.slice(thread.posts), 37 newNumPosts: data.posts.length, 38 op: thread.id, 39 board: board 40 }); 41 } 42 }).catch(e => { 43 if(e.status === 404) { 44 watched[board][t].err = e.statusText; 45 setStorageVal("watched", watched, true); 46 } 47 }); 48 } 49 }
50}
51
Description
Looping over objects with a for in loop will include properties that are inherited through the prototype chain.
This behavior can lead to unexpected keys in your for loop.
Bad Practice
for (const key in foo) {
doSomething(key);
}
Recommended
for (const key in foo) {
if (Object.prototype.hasOwnProperty.call(foo, key)) {
doSomething(key);
}
}
for (const key of Object.keys(foo)) {
doSomething(key);
}