watch.md

title	badges
Watch on Arrays	breaking

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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.

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: true
  },
}

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