Is there a way in Vue2 to do this:
....,
computed {
...mapState('myModule', {
myVal: (state, vm) => state.someVar.filter((_) => { return vm.someVar })
})
},
....
The actual function I'm working on is filtering the state.someVar in a more complex scenerio, so this is simplified. The point being that this is undefined.
Anyone know how to reference the vue in such a function?
The 2nd argument to the mapState function is supposed to be a state mapper (array or object)
If you are using an object (as you are), you can only set the properties of the object to the keys in your "myModule" state object. You cannot set them as functions
e.g {customName: moduleStateKey}
As for passing component instance, you can only do that in an action or mutation. However, that would make your store impure. It would be better to pass someVar to state via a mutation fired from the component, then use a getter for the filter logic and use the someVar passed to state there
I have just started working with Vue 3 and Composition API.
I was wondering what are the differences between ref, toRef and toRefs?
Vue 3 ref
A ref is a mechanism for reactivity in Vue 3. The idea is to wrap a non-object inside a reactive object:
Takes an inner value and returns a reactive and mutable ref object. The ref object has a single property .value that points to the inner value.
Hmm.. Why?
Vue 3 relies on JavaScript proxies to detect changes to your reactive data and implement the reactivity. Proxies are essentially event listeners for objects: any reading or writing on a proxied object triggers the proxy, which can then do something with the values. This is convenient for reactivity since the variable changing will provide a trigger from which Vue can update any dependencies.
But proxies require objects to work. So Vue provides the ref method to convert your non-object variables into objects and then to grant reactive capabilities through a proxy. (Objects are reference variables, hence the name ref.)
(And Vue automatically unwraps your refs in the template, which is an added benefit of ref that you wouldn't get if you wrapped your value variables in an object manually.)
reactive
If your original variable is already an object (or array), a ref wrapping is not needed because it is already a reference type. It only needs Vue's reactive functionality (which a ref also has):
const state = reactive({
foo: 1,
bar: 2
})
But now consider copying a property from the object above to a new variable, for example foo which contains the number 1. If you copied this to a new variable, the copy would of course be a regular, non-reactive variable having no connection to the reactive object it was copied from. If foo changed later, the copy would not, meaning the copy is not reactive. This is where toRef is useful.
toRef
toRef converts a single reactive object property to a ref that maintains its connection with the parent object:
const state = reactive({
foo: 1,
bar: 2
})
const fooRef = toRef(state, 'foo')
/*
fooRef: Ref<number>,
*/
Now if state.foo changes, fooRef.value will change as well. So toRef has enabled copying a value property in such a way that the copy also has a reactive connection to the original parent object.
toRefs
toRefs converts all of the properties, to a plain object with properties that are refs:
const state = reactive({
foo: 1,
bar: 2
})
const stateAsRefs = toRefs(state)
/*
{
foo: Ref<number>,
bar: Ref<number>
}
*/
When would I ever use toRef or toRefs?
The most likely time would be when importing a reactive object, say from a composable, and destructuring it. The act of destructuring will pull the current property value from the object into a new local variable that will not be reactive without toRef. You may notice this when importing and destructuring a Pinia store into your component, for example.
reactive
reactive creates a deeply reactive proxy object based on a given object. The proxy object will look exactly the same as the given, plain object, but any mutation, no matter how deep it is, will be reactive - this includes all kinds of mutations including property additions and deletions. The important thing is that reactive can only work with objects, not primitives.
For example, const state = reactive({foo: {bar: 1}}) means:
state.foo is reactive (it can be used in template, computed and watch)
state.foo.bar is reactive
state.baz, state.foo.baz, state.foo.bar.baz are also reactive even though baz does not yet exist anywhere. This might look surprising (especially when you start to dig how reactivity in vue works). By state.baz being reactive, I mean within your template/computed properties/watches, you can write state.baz literally and expect your logic to be executed again when state.baz becomes available. In fact, even if you write something like {{ state.baz ? state.baz.qux : "default value" }} in your template, it will also work. The final string displayed will reactively reflect state.baz.qux.
This can happen because reactive not only creates a single top level proxy object, it also recursively converts all the nested objects into reactive proxies, and this process continues to happen at runtime even for the sub objects created on the fly. Dependencies on properties of reactive objects are continuously discovered and tracked at runtime whenever a property access attempt is made against a reactive object. With this in mind, you can work out this expression {{ state.baz ? state.baz.qux : "default value" }} step by step:
the first time it is evaluated, the expression will read baz off state (in other words, a property access is attempted on state for property baz). Being a proxy object, state will remember that your expression depends on its property baz, even though baz does not exist yet. Reactivity off baz is provided by the state object that owns the property.
since state.baz returns undefined, the expression evaluates to "default value" without bothering looking at state.baz.qux. There is no dependency recorded on state.baz.qux in this round, but this is fine. Because you cannot mutate qux without mutating baz first.
somewhere in your code you assign a value to state.baz: state.baz = { qux: "hello" }. This mutation qualifies as a mutation to the baz property of state, hence your expression is scheduled for re-evaluation. Meanwhile, what gets assigned to state.baz is a sub proxy created on the fly for { qux: "hello" }
your expression is evaluated again, this time state.baz is not undefined so the expression progresses to state.baz.qux. "hello" is returned, and a dependency on qux property is recorded off the proxy object state.baz. This is what I mean by dependencies are discovered and recorded at runtime as they happen.
some time later you change state.baz.qux = "hi". This is a mutation to the qux property and hence your expression will be evaluated again.
With the above in mind, you should be able understand this as well: you can store state.foo in a separate variable: const foo = state.foo. Reactivity works off your variable foo just fine. foo points to the same thing that state.foo is pointing to - a reactive proxy object. The power of reactivity comes from the proxy object. By the way, const baz = state.baz wouldn't work the same, more on this later.
However, there are always edge cases to watch for:
the recursive creation of nested proxies can only happen if there is a nested object. If a given property does not exist, or it exists but it is not an object, no proxy can be created at that property. E.g. this is why reactivity does not work off the baz variable created by const baz = state.baz, nor the bar variable of const bar = state.foo.bar. To make it clear, what it means is that you can use state.baz and state.foo.bar in your template/computed/watch, but not baz or bar created above.
if you extract a nest proxy out to a variable, it is detached from its original parent. This can be made clearer with an example. The second assignment below (state.foo = {bar: 3}) does not destroy the reactivity of foo, but state.foo will be a new proxy object while the foo variable still points the to original proxy object.
const state = reactive({foo: {bar: 1}});
const foo = state.foo;
state.foo.bar = 2;
foo.bar === 2; // true, because foo and state.foo are the same
state.foo = {bar: 3};
foo.bar === 3; // false, foo.bar will still be 2
ref and toRef solve some of these edge cases.
ref
ref is pretty much the reactive that works also with primitives. We still cannot turn JS primitives into Proxy objects, so ref always wraps the provided argument X into an object of shape {value: X}. It does not matter if X is primitive or not, the "boxing" always happens. If an object is given to ref, ref internally calls reactive after the boxing so the result is also deeply reactive. The major difference in practice is that you need to keep in mind to call .value in your js code when working with ref. In your template you dont have to call .value because Vue automatically unwraps ref in template.
const count = ref(1);
const objCount = ref({count: 1});
count.value === 1; // true
objCount.value.count === 1; // true
toRef
toRef is meant to convert a property of a reactive object into a ref. You might be wondering why this is necessary since reactive object is already deeply reactive. toRef is here to handle the two edge cases mentioned for reactive. In summary, toRef can convert any property of a reactive object into a ref that is linked to its original parent. The property can be one that does not exist initially, or whose value is primitive.
In the same example where state is defined as const state = reactive({foo: {bar: 1}}):
const foo = toRef(state, 'foo') will be very similar to const foo = state.foo but with two differences:
foo is a ref so you need to do foo.value in js;
foo is linked to its parent, so reassigning state.foo = {bar: 2} will get reflected in foo.value
const baz = toRef(state, 'baz') now works.
toRefs
toRefs is a utility method used for destructing a reactive object and convert all its properties to ref:
const state = reactive({...});
return {...state}; // will not work, destruction removes reactivity
return toRefs(state); // works
I'm creating a new Audio() object inside a Vue component's data() { ... } function, but I got concerned about if this is unnecessarily creating more Audio objects.
I can't seem to find when or how the data() function is called, but I also couldn't find any examples that crates a new object in the data function.
Is it ok to create new objects inside data() ? Or should I leave those initializations to the created() function?
Read the documentation about data() : https://v2.vuejs.org/v2/guide/components.html#data-Must-Be-a-Function
a component’s data option must be a function, so that each instance can maintain an independent copy of the returned data object.
It is the right way to do it.
Your new object will be instantiated like so :
data() {
// new object returned
return {
audio: new Audio()
}
}
If you want to have a global object to avoid multiple instance of audio, think about attaching it to the global Vue instance or using Vuex depending on what you need.
It's completely fine to create new objects inside the returned object of the data() function, that's what is meant for. More informations here.
I'm not new to Vue.js, but I'm going through the docs again, trying to pick up on anything I missed the first time. I came across this statement in basic example section of using computed properties:
You can data-bind to computed properties in templates just like a normal property. Vue is aware that vm.reversedMessage depends on vm.message, so it will update any bindings that depend on vm.reversedMessage when vm.message changes. And the best part is that we’ve created this dependency relationship declaratively: the computed getter function has no side effects, which makes it easier to test and understand.
The part about we’ve created this dependency relationship declaratively: the computed getter function has no side effects, isn't clear to me. I do understand that a side effect is some action happening that is not directly related to the pure intentions of the function, but I'm not clear how it's being used in this statement.
Could someone explain further what the opposite could be? What are the potential side effects that could be happening?
The term side effect here refers to any data mutations performed in the computed property getter. For example:
export default {
data() {
return {
firstName: 'john',
lastName: 'doe',
array: [1,2,3]
}
},
computed: {
fullName() {
this.firstName = 'jane'; // SIDE EFFECT - mutates a data property
return `${this.firstName} ${this.lastName}`
},
reversedArray() {
return this.array.reverse(); // SIDE EFFECT - mutates a data property
}
}
}
Notice how fullName mutates firstName, and reversedArray mutates array. If using ESLint (e.g., from Vue CLI generated project), you'd see a warning:
[eslint] Unexpected side effect in "fullName" computed property. (vue/no-side-effects-in-computed-properties)
[eslint] Unexpected side effect in "reversedArray" computed property. (vue/no-side-effects-in-computed-properties)
demo
What are the differences between defining the data object in the following ways:
1. Using Object Literal
data: {
title: ‘Helly VueJS’
}
2. Function returning object
data() {
return {
title: ‘Helly VueJS’
};
}
From the documentation:
When defining a component, data must be declared as a function that returns the initial data object, because there will be many instances created using the same definition. If we still use a plain object for data, that same object will be shared by reference across all instances created! By providing a data function, every time a new instance is created, we can simply call it to return a fresh copy of the initial data.
The documentation is excellent.