The value obtained by vue is not the latest

Vue is a front-end framework based on the MVVM pattern, used to build interactive single-page applications. During the development of Vue, we sometimes encounter a problem: the value obtained is not the latest value.

For example, we have a variable in the Vue component, and we want to perform a certain operation when its value changes:

<template>
  <div>
    <p>{{ message }}</p>
    <button @click="changeMessage">更改</button>
  </div>
</template>

<script>
export default {
  data() {
    return {
      message: 'hello world'
    }
  },
  methods: {
    changeMessage() {
      this.message = 'hello Vue'
      console.log(this.message)
      this.doSomething() // doSomething方法需要使用最新的message值
    },
    doSomething() {
      console.log(this.message) // 这里输出的是旧的message值
    }
  }
}
</script>

In the above code, when "Change" is clicked button, we change the value of message and print the latest message. However, when we call the doSomething method, we find that the output message value is not the latest value "hello Vue", but the old value "hello world".

Why is this?

In fact, updating data in Vue is asynchronous. When we change the value of a variable, Vue will not respond to the update immediately. Instead, it will first add the update to a queue and update the DOM in the next event loop. Therefore, when we call the doSomething method, Vue is still waiting for the next event loop before responding to the update.

So, how to get the latest value?

Vue provides the $nextTick method for executing the callback function after the DOM is updated. We can write the doSomething method asynchronously and get the latest message value through the $nextTick method:

<template>
  <div>
    <p>{{ message }}</p>
    <button @click="changeMessage">更改</button>
  </div>
</template>

<script>
export default {
  data() {
    return {
      message: 'hello world'
    }
  },
  methods: {
    changeMessage() {
      this.message = 'hello Vue'
      console.log(this.message)
      this.$nextTick(() => {
        this.doSomething() // 等到DOM更新后再执行doSomething方法
      })
    },
    doSomething() {
      console.log(this.message) // 这里输出的是最新的message值"hello Vue"
    }
  }
}
</script>

In the above code, we wrap the doSomething method with $nextTick. When the DOM is updated, $nextTick will Call the callback function to execute our method so that we can get the latest message value.

Summary:

Updating data in Vue is asynchronous. If we want to get the latest value, we need to use the $nextTick method to wait for the DOM to be updated before executing the corresponding method. In actual development, we need to use $nextTick at the appropriate time to avoid getting the old value.

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