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>
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
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)
An example of a standard Single File Component looks like this:
<template>
...
</template>
<script>
...
</script>
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:
<script> element.this.<template> tags.
Advantages:
<template>.$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
});
},
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