vize:croquis/cf/array-mutation

An array is mutated by index, which a reactive array does not track.

Default severity: Not emitted
Applies to: Analyzed component graph and the supported facts described below
Automatic fix: None; review related files and apply the repair
Options: No per-code options; supported CLI findings accept severity overrides

This is a published diagnostic contract without a current producer. The Bad/Good scenario below explains the risk and repair; no flag currently makes this code trigger.

Historical Vue 2.7 only: use matching Vue 2.7 and SFC compiler dependencies for this scenario. Vue 3 proxies track array index assignment, so items[0] = next is reactive in Vue 3 and is not a Vue 3 defect. This published code has no current producer.

Shared project files

Use these unchanged files in both Bad and Good. Install the imported packages in the project: Vue, plus vue-router or Pinia where shown. Follow any version-specific support note. The entry root makes the component relationship explicit.

main.ts

import Vue from 'vue';
import App from './App.vue';
new Vue({ render: h => h(App) }).$mount('#app');

index.html

<!doctype html>
<html lang="en"><head><meta charset="UTF-8"><title>Contract scenario</title></head>
<body><div id="app"></div><script type="module" src="/main.ts"></script></body></html>

App.vue

<script lang="ts">
import Vue from 'vue';
import { replaceFirst } from './replace-first';
export default Vue.extend({
  data() { return { items: ['Before'] }; },
  methods: { replace() { replaceFirst(this.items, 'After'); } },
});
</script>
<template><section><p>{{ items[0] }}</p><button @click="replace">Replace</button></section></template>

Bad

In this historical Vue 2.7 project, items[0] = next changes the array without notifying Vue 2’s array observer, so the displayed first item need not update.

replace-first.ts

export function replaceFirst(items: string[], next: string): void {
  items[0] = next;
}

Good

splice(0, 1, next) uses the array mutation method observed by Vue 2, allowing the same replacement to update the view.

replace-first.ts

export function replaceFirst(items: string[], next: string): void {
  items.splice(0, 1, next);
}

The Good files demonstrate the change described above; other diagnostics can still apply to the complete project.

Public explanation

Cross-file index