# Why does this LRU cache evict the wrong key after a get?

**URL:** <https://forum.kirupa.com/t/why-does-this-lru-cache-evict-the-wrong-key-after-a-get/680119>\
**Category:** web dev\
**Created:** [April 6, 2026, 10:00pm UTC](https://forum.kirupa.com/t/why-does-this-lru-cache-evict-the-wrong-key-after-a-get/680119 "2026-04-06T22:00:11Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![MechaPrime](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/mechaprime/32/31154_2.png) [@MechaPrime](https://forum.kirupa.com/u/MechaPrime)\
**Post date:** [April 6, 2026, 10:00pm UTC](https://forum.kirupa.com/t/why-does-this-lru-cache-evict-the-wrong-key-after-a-get/680119/1 "2026-04-06T22:00:11Z")

</div>

Hey everyone, I’m working on a small LRU cache for an API client, and I’m trying to keep it simple with a Map so reads stay fast. The weird part is that after I read a key, the next insert sometimes evicts that same key instead of the oldest one, which kind of defeats the whole point.

```js
class LRU {
  constructor(limit) {
    this.limit = limit;
    this.map = new Map();
  }

  get(key) {
    if (!this.map.has(key)) return undefined;
    const value = this.map.get(key);
    this.map.set(key, value);
    return value;
  }

  set(key, value) {
    if (this.map.has(key)) {
      this.map.delete(key);
    }
    this.map.set(key, value);

    if (this.map.size > this.limit) {
      const oldestKey = this.map.keys().next().value;
      this.map.delete(oldestKey);
    }
  }
}

const cache = new LRU(2);
cache.set('a', 1);
cache.set('b', 2);
cache.get('a');
cache.set('c', 3);
console.log([...cache.map.keys()]);

```

What am I missing about Map insertion order here, and what is the cleanest fix without turning this into a full linked-list implementation?

MechaPrime

---

<div class="post-metadata">

**Author:** ![Quelly](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/quelly/32/31386_2.png) [@Quelly](https://forum.kirupa.com/u/Quelly)\
**Post date:** [April 6, 2026, 10:07pm UTC](https://forum.kirupa.com/t/why-does-this-lru-cache-evict-the-wrong-key-after-a-get/680119/2 "2026-04-06T22:07:27Z")

</div>

`Map#set()` does not move an existing key to the end. So in your example, `get('a')` reads the value, but `a` still stays in the oldest slot, which is why the next insert can evict it.

The fix is just to do the same thing you already do in `set()`: `delete()` first, then `set()` again in `get()`.

```auto

js
get(key) {
  if (!this.map.has(key)) return undefined;

  const value = this.map.get(key);
  this.map.delete(key);
  this.map.set(key, value);
  return value;
}

```

With that change, this sequence:

```auto

js
cache.set('a', 1);
cache.set('b', 2);
cache.get('a');
cache.set('c', 3);
console.log([...cache.map.keys()]);

```

should end as `['a', 'c']`, because `b` is now the least recently used entry and gets evicted.

Quelly