How to deal with '[Vue warn]: Constant expressions should contain' error

How to deal with the "[Vue warn]: Constant expressions should contain" error

When developing applications using Vue.js, you may encounter an error message : "[Vue warn]: Constant expressions should contain". This error is usually caused by using code in a Vue template that does not meet the requirements for constant expressions. In this article, we will explore the causes of this error and how to fix it.

The reason for this error is that Vue.js requires that the expression used in the template must be a constant expression. A constant expression is an expression whose value can be determined at compile time, rather than at run time. For example, the following expressions are all constant expressions:

1 + 1
"hello" + "world"
Math.sqrt(4)

However, the following expressions are not constant expressions:

count + 1
getTotal()

When we use in Vue templates that do not meet the requirements of constant expressions When coding, Vue.js will issue a warning prompt. This is to avoid undefined behavior when rendering templates, since the result of the expression cannot be predicted in advance.

Next, we will introduce how to solve this error. Here are some code examples that may go wrong and their corresponding solutions:

1. Error example: Using a function call as an expression

<template>
  <div>
    {{ getTime() }}
  </div>
</template>

Solution: Replace the function call with Move to computed properties

<template>
  <div>
    {{ time }}
  </div>
</template>

<script>
export default {
  computed: {
    time() {
      return this.getTime()
    }
  },
  methods: {
    getTime() {
      // 执行相关的操作并返回一个值
    }
  }
}
</script>

2. Error example: Using object properties as expressions

<template>
  <div>
    {{ user.name }}
  </div>
</template>

Solution: Move object property access to computed properties

<template>
  <div>
    {{ userName }}
  </div>
</template>

<script>
export default {
  computed: {
    userName() {
      return this.user.name
    }
  },
  data() {
    return {
      user: {
        name: 'John Doe'
      }
    }
  }
}
</script>

3. Error example: Using code in a v-for loop that does not meet the requirements for constant expressions

<template>
  <div>
    <ul>
      <li v-for="item in getItems()" :key="item.id">
        {{ item.title }}
      </li>
    </ul>
  </div>
</template>

Solution: Move the function call into a calculated property and use the calculation Property to get the traversed list

<template>
  <div>
    <ul>
      <li v-for="item in items" :key="item.id">
        {{ item.title }}
      </li>
    </ul>
  </div>
</template>

<script>
export default {
  computed: {
    items() {
      return this.getItems()
    }
  },
  methods: {
    getItems() {
      // 执行相关的操作并返回一个数组
    }
  }
}
</script>

We can solve the "[Vue warn]: Constant expressions should contain" error by moving the code that does not meet the constant expression requirements into a computed property. Computed properties are calculated before the template is rendered and return a constant, which ensures the stability and predictability of the template.

When developing Vue applications, it is a good practice to follow the requirements for constant expressions. It helps us avoid unexpected behavior and makes our code more maintainable and readable.

I hope this article can help you solve constant expression errors in Vue.js!

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