Watch on Arrays

breaking

Overview

• BREAKING: When watching an array, the callback will only trigger when the array is replaced. If you need to trigger on mutation, the deep option must be specified.
  • In Vue 3.5+, deep: 1 should be used, which will trigger on array replacement and array mutation.
  • Prior to Vue 3.5, deep: true can be used, but it will also trigger on deep changes in array elements.

3.x Syntax

When using the watch option to watch an array, the callback will only trigger when the array is replaced. In other words, the watch callback will no longer be triggered on array mutation. To trigger on mutation, the deep option must be specified.

watch: {
  bookList: {
    handler(val, oldVal) {
      console.log('book list changed')
    },
    deep: 1 // Vue 3.5+
    deep: true // Vue 3.0 - 3.4
  },
}

WARNING

In versions prior to Vue 3.5, watches don't allow setting the max traversal depth, so you're stuck with deep: true, which will also trigger the callback on deep modification of array elements.

Migration Strategy

If you rely on watching array mutations, add the deep option to ensure that your callback is triggered correctly.

Migration build flag: WATCH_ARRAY