What sets declaring variables apart in Vue?

Question

What sets declaring variables apart in Vue?

Can you clarify the distinctions among various methods of declaring variables? When should I employ each of these declaration methods?

<script>
const someVariable = 12345

export default {
  name: 'App',
}
</script>

<script>

export default {
  name: 'App',
  data() {
    return {
      someVariable: 12345
    }
  }
}
</script>

javascript vue.js vuejs2

Answer 1

Answer №1

Initially, using the someVariable in your template is not possible.

<script>
const someVariable = 12345

export default {
  name: 'App',
}
</script>
<template> <p> {{someVariable}} </p> </template> //does not work

In Vue3:

To resolve this issue, you need to include a setup keyword in your script and wrap your variable value with either ref(...) or reactive(...) for it to be reactive to changes. For more information, refer to this link.

<script setup>
const someVariable = 12345

export default {
  name: 'App',
}
</script>
<template> <p> {{someVariable}} </p> </template> //now works (data is displayed)

Answer 2

Initially, using the someVariable in your template is not possible.

<script>
const someVariable = 12345

export default {
  name: 'App',
}
</script>
<template> <p> {{someVariable}} </p> </template> //does not work

In Vue3:

To resolve this issue, you need to include a setup keyword in your script and wrap your variable value with either ref(...) or reactive(...) for it to be reactive to changes. For more information, refer to this link.

<script setup>
const someVariable = 12345

export default {
  name: 'App',
}
</script>
<template> <p> {{someVariable}} </p> </template> //now works (data is displayed)

Answer 3

Answer №2

An example of a standard Single File Component looks like this:

<template>
...
</template>

<script>
...
</script>

When you declare a variable outside the export statement, it functions as a regular JavaScript variable that is accessible anywhere within the script tag. It is not tied to the component or Vue in any way. Advantages:
- The variable exists within the scope of the entire <script> element.
- You can utilize it in inner functions without needing a binding to this.
If you define a variable inside the data function, essentially creating a property for the component instance's data object, then it becomes associated with the component and thus usable within the <template> tags. Advantages:
- The variable can be accessed from <template>.
- You can leverage Vue's reactivity by defining computed properties for this variable. Any changes to the variable are automatically reflected in the HTML template.
- You can pass it as props to child components.
- Easier debugging using Vue devtools, allowing you to monitor changes to the variable. You can also log the variable in the console like $vm.data.someVarData, indicating that the variable has been added to the Vue component's instance.

<template>
    <div>
        <div :class="someVarData"/> <!-- This is correct -->
        <div :class="someVar"/> <!-- This is incorrect  -->
    </div>
<template>
<script>
const someVar = "blah";
export default {
    data() {
        return {
            someVarData: "blahData",
        };
    },
    mounted() {
        const el = document.getElementById('myId');
        el.addEventListener('mouseenter', function () {
            console.log(someVar); // This is correct
            console.log(this.someVarData); // This is incorrect
        });
    },
    beforeRouteEnter() { 
        console.log(someVar); // This is correct
        console.log(this.someVarData); // This is incorrect
    },
</script>

It is best practice to avoid defining variables outside of export since it can complicate understanding the code flow. There is usually a way to redesign your approach to avoid using variables outside of export.

For instance, in the above scenario, you can still access your data variable within the mounted hook and the navigation guard with some modifications:

    mounted() {
        const el = document.getElementById('myId');
        el.addEventListener('mouseenter', () => {
            console.log(someVar); // This is correct
            console.log(this.someVarData); // Works because we changed the function to an arrow function, ensuring it is bound to the instance's `this`
        });
        el.addEventListener('mouseenter', function () {
            console.log(someVar); // This is correct
            console.log(this.someVarData); // Works because we manually bound the function to the instance's `this`
        }.bind(this));
    },
    beforeRouteEnter(to, from, next) { 
        console.log(someVar); // This is correct
        console.log(this.someVarData); // This is incorrect
        next((vm) => {
            console.log(vm.someVarData); // Works - Navigation guard's next function provides the instance's context as a callback parameter
        });
    },

Answer 4

An example of a standard Single File Component looks like this:

<template>
...
</template>

<script>
...
</script>

When you declare a variable outside the export statement, it functions as a regular JavaScript variable that is accessible anywhere within the script tag. It is not tied to the component or Vue in any way. Advantages:
- The variable exists within the scope of the entire <script> element.
- You can utilize it in inner functions without needing a binding to this.
If you define a variable inside the data function, essentially creating a property for the component instance's data object, then it becomes associated with the component and thus usable within the <template> tags. Advantages:
- The variable can be accessed from <template>.
- You can leverage Vue's reactivity by defining computed properties for this variable. Any changes to the variable are automatically reflected in the HTML template.
- You can pass it as props to child components.
- Easier debugging using Vue devtools, allowing you to monitor changes to the variable. You can also log the variable in the console like $vm.data.someVarData, indicating that the variable has been added to the Vue component's instance.

<template>
    <div>
        <div :class="someVarData"/> <!-- This is correct -->
        <div :class="someVar"/> <!-- This is incorrect  -->
    </div>
<template>
<script>
const someVar = "blah";
export default {
    data() {
        return {
            someVarData: "blahData",
        };
    },
    mounted() {
        const el = document.getElementById('myId');
        el.addEventListener('mouseenter', function () {
            console.log(someVar); // This is correct
            console.log(this.someVarData); // This is incorrect
        });
    },
    beforeRouteEnter() { 
        console.log(someVar); // This is correct
        console.log(this.someVarData); // This is incorrect
    },
</script>

It is best practice to avoid defining variables outside of export since it can complicate understanding the code flow. There is usually a way to redesign your approach to avoid using variables outside of export.

For instance, in the above scenario, you can still access your data variable within the mounted hook and the navigation guard with some modifications:

    mounted() {
        const el = document.getElementById('myId');
        el.addEventListener('mouseenter', () => {
            console.log(someVar); // This is correct
            console.log(this.someVarData); // Works because we changed the function to an arrow function, ensuring it is bound to the instance's `this`
        });
        el.addEventListener('mouseenter', function () {
            console.log(someVar); // This is correct
            console.log(this.someVarData); // Works because we manually bound the function to the instance's `this`
        }.bind(this));
    },
    beforeRouteEnter(to, from, next) { 
        console.log(someVar); // This is correct
        console.log(this.someVarData); // This is incorrect
        next((vm) => {
            console.log(vm.someVarData); // Works - Navigation guard's next function provides the instance's context as a callback parameter
        });
    },

What sets declaring variables apart in Vue?

Answer №1

In Vue3:

Answer №2

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