Develop Reference

vue computed property not able to get data

I am currently experiencing an issue where the computed() property is not able to get data. Although data was already initiated in created() property. Am I doing it wrong? Please advise how I can fix this issue.
const randomPlayers = {
template:
`
<input type="text" v-model="search_player">
<div v-for="player in modPlayers" v-if="list_of_random_players!=null">
<p>{{player.firstname}}</p>
<p>{{player.lastname}}</p>
<div>
`,
props: ['data'],
data (){
return{
list_of_random_players: null,
search_player: null
}
},
created(){
this.get_random_players()
},
computed: {
modPlayers(){
return this.list_of_random_players.filter( person => {
return !this.search_player ||
( person.firstname.toLowerCase().indexOf(this.search_player.toLowerCase()) > -1 || person.lastname.toLowerCase().indexOf(this.search_player.toLowerCase()) > -1)
})
}
},
methods: {
get_random_players: function(){
$.post(
url:'random_url'
data: {
players: data
}
).done((success)=>{
this.list_of_random_players: JSON.parse(success)
})fail((err)=>{
console.log(err)
})
}
}
}
I get the following two errors:
(1) TypeError: Cannot read property 'filter' of null
(2) TypeError: this.list_of_random_players.filter is not a function

From Vue: "When a Vue instance is created, it adds all the properties found in its data object to Vue’s reactivity system. When the values of those properties change, the view will “react”, updating to match the new values."
So data is a function that returns an object but as mentioned by #Sovalina you are not returning it so you cannot access its properties. You need to add return and change null to []:
data() {
return {
list_of_random_players: [],
search_player: []
}
},
or you can do without return and like a regular object:
data: {
list_of_random_players: [],
search_player: []
}
When your Vue component is used multiple times, it is better to use it like a function(first case).
"When defining a component, data must be declared as a function that returns the initial data object. Why? 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 instance created! By providing a data function, every time a new instance is created we can call it to return a fresh copy of the initial data."
Reference:link
It might be just a typo but you need to add : to methods as well.

Vue.js directive v-html not updating if the model is overwritten

By running the following code (a Vue.js component), I expect that, after the AJAX call returns, both the v-html directive and the console.log() display the same value.
On the contrary, v-html is stuck with "loading...(1)" even though obj.html has a different value, as console.log() confirms.
The behaviour is caused by getObject overwriting obj, and being afterwards obj.html undefined for a short time before getHTML returns (all this happens in function created).
Can please someone explain whether this is Vue's desired behavior (doc links are welcome), or whether should I submit a bug report, or finally whether I am simply structuring my code in a bad way?
Thanks in advance
<template>
<main v-html="obj.html || 'loading... (1)'">
</main>
</template>
<script>
export default {
name: 'Post',
data: function () {
return {
obj: {
html: 'loading... (2)'
}
}
},
created: async function () {
this.obj = await this.getObject()
this.obj.html = await this.getHtml()
console.log(this.obj.html)
},
methods: {
getObject: async function () {
const resp = await this.$http.get('https://jsonplaceholder.typicode.com/todos')
return resp.body[0]
},
getHtml: async function () {
const resp = await this.$http.get('https://jsonplaceholder.typicode.com/todos')
return resp.body[0].title
},
}
}
</script>

The function getObject returns a String so at the first line of created hook
this.obj = await this.getObject()
you change the reference of the obj and you make it pointing to a string and then you try to put a property on a string, which does not work ;)
it's like you would do
this.obj = 'test'
then console.log(this.obj);
// test
and then this.obj.abc = 'whatever'
console.log(this.obj.abc);
// undefined
You would need to parse the object before, see JSON.parse(string)
Update:
If this is not the case i.e you somehow have an object coming from that service.
Then the only problem I can think is that you lose the reference of your original obj and v-html is still pointing to the old one. In that case you have to avoid modification of the root obj or you can use the vue $set method: https://v2.vuejs.org/v2/guide/reactivity.html#Change-Detection-Caveats

It seems vue data objects are not deeply reactive, which means that altering a property will not trigger change detection in the template.
Try rearranging the created hook to compose the full object before assigning it to the data property. That way when the template reacts it will see the html property of obj.
Ref CodeSandbox
created: async function () {
const fetchedObj = await this.getObject()
fetchedObj.html = await this.getHtml()
this.obj = fetchedObj;
console.log(this.obj.html)
},

VueJS observe plugged parameter

I'm starting with VueJS 2 and I created a simple plugin which adds parameter to Vue instance.
I have problem because when I update this value my computed properties are still same.
My example plugin's code:
export default function (Vue) {
Vue.MyProperty = "test"
Object.defineProperties(Vue.prototype, {
"$myProperty": {
"get": function () {
return Vue.MyProperty
},
"set": function (value) {
Vue.MyProperty = value
return this
}
}
})
}
And my component's code
export default {
"computed": {
"test": function () {
return this.$myProperty
}
}
}
When I changed this.$myProperty in other component my component returns vaid value (in example when I changed from "test" into "newvalue" I can see "newvalue") but computed property test is still old value ("test" in my example).
I tried to use this.$set(this, "$myProperty", value) but this still not working.
How can I use or declare this property to use it in computed or watched properties?

The reason the data value is not automatically updated in the computed is because the property you added to Vue, MyProperty is not an observed property. Fundamentally, Vue's reactivity works because all values added to data are converted into observed properties; under the hood they are converted into getter/setter pairs with some additional code so that when one of those properties changes, Vue knows to propagate the changes to all the things that depend on it's value.
The code in the question, however, just adds a normal property to the Vue object. You can change it, but it's not reactive.
That said, it's relatively easy to make it reactive. I cover how to do this in the comments to my answer here. Basically, instead of adding your property to Vue, just create a new Vue object (which has very low overhead) and make the property you want to be reactive a property of that Vue. Here is a working example.
console.clear()
function MyPlugin(Vue) {
let store = new Vue({data:{MyProperty: "some value"}})
Object.defineProperties(Vue.prototype, {
"$myProperty": {
"get": function () {
return store.MyProperty
},
"set": function (value) {
store.MyProperty = value
return this
}
}
})
}
Vue.use(MyPlugin)
const MyComponent = {
template:`<div>{{test}}</div>`,
"computed": {
"test": function () {
return this.$myProperty
}
}
}
new Vue({
el: "#app",
components:{
MyComponent
}
})
<script src="https://unpkg.com/vue#2.4.2"></script>
<div id="app">
<my-component></my-component>
<button #click="$myProperty = 'new value'">Change</button>
</div>

VueJS: Passing arguments to a computed property from methods object [duplicate]

is this possible to pass parameter in computed properties in Vue.Js. I can see when having getters/setter using computed, they can take a parameter and assign it to a variable. like here from documentation:
computed: {
fullName: {
// getter
get: function () {
return this.firstName + ' ' + this.lastName
},
// setter
set: function (newValue) {
var names = newValue.split(' ')
this.firstName = names[0]
this.lastName = names[names.length - 1]
}
}
}
Is this also possible:
computed: {
fullName: function (salut) {
return salut + ' ' + this.firstName + ' ' + this.lastName
}
}
Where computed property takes an argument and returns the desired output. However, when I try this, I am getting this error:
vue.common.js:2250 Uncaught TypeError: fullName is not a function(…)
Should I be using methods for such cases?

Most probably you want to use a method
<span>{{ fullName('Hi') }}</span>
methods: {
fullName(salut) {
return `${salut} ${this.firstName} ${this.lastName}`
}
}
Longer explanation
Technically you can use a computed property with a parameter like this:
computed: {
fullName() {
return salut => `${salut} ${this.firstName} ${this.lastName}`
}
}
(Thanks Unirgy for the base code for this.)
The difference between a computed property and a method is that computed properties are cached and change only when their dependencies change. A method will evaluate every time it's called.
If you need parameters, there are usually no benefits of using a computed property function over a method in such a case. Though it allows you to have a parametrized getter function bound to the Vue instance, you lose caching so not really any gain there, in fact, you may break reactivity (AFAIU). You can read more about this in Vue documentation https://v2.vuejs.org/v2/guide/computed.html#Computed-Caching-vs-Methods
The only useful situation is when you have to use a getter and need to have it parametrized. For instance, this situation happens in Vuex. In Vuex it's the only way to synchronously get parametrized result from the store (actions are async). Thus this approach is listed by official Vuex documentation for its getters
https://vuex.vuejs.org/guide/getters.html#method-style-access

You can use methods, but I prefer still to use computed properties instead of methods, if they're not mutating data or do not have external effects.
You can pass arguments to computed properties this way (not documented, but suggested by maintainers, don't remember where):
computed: {
fullName: function () {
var vm = this;
return function (salut) {
return salut + ' ' + vm.firstName + ' ' + vm.lastName;
};
}
}
EDIT: Please do not use this solution, it only complicates code without any benefits.

Well, technically speaking we can pass a parameter to a computed function, the same way we can pass a parameter to a getter function in vuex. Such a function is a function that returns a function.
For instance, in the getters of a store:
{
itemById: function(state) {
return (id) => state.itemPool[id];
}
}
This getter can be mapped to the computed functions of a component:
computed: {
...mapGetters([
'ids',
'itemById'
])
}
And we can use this computed function in our template as follows:
<div v-for="id in ids" :key="id">{{itemById(id).description}}</div>
We can apply the same approach to create a computed method that takes a parameter.
computed: {
...mapGetters([
'ids',
'itemById'
]),
descriptionById: function() {
return (id) => this.itemById(id).description;
}
}
And use it in our template:
<div v-for="id in ids" :key="id">{{descriptionById(id)}}</div>
This being said, I'm not saying here that it's the right way of doing things with Vue.
However, I could observe that when the item with the specified ID is mutated in the store, the view does refresh its contents automatically with the new properties of this item (the binding seems to be working just fine).

computed: {
fullName: (app)=> (salut)=> {
return salut + ' ' + this.firstName + ' ' + this.lastName
}
}
when you want use
<p>{{fullName('your salut')}}</p>

[Vue2] Filters are a functionality provided by Vue components that let you apply formatting and transformations to any part of your template dynamic data.
They don’t change a component’s data or anything, but they only affect the output.
Say you are printing a name:
new Vue({
el: '#container',
data() {
return {
name: 'Maria',
lastname: 'Silva'
}
},
filters: {
prepend: (name, lastname, prefix) => {
return `${prefix} ${name} ${lastname}`
}
}
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<div id="container">
<p>{{ name, lastname | prepend('Hello') }}!</p>
</div>
Notice the syntax to apply a filter, which is | filterName. If you're familiar with Unix, that's the Unix pipe operator, which is used to pass the output of an operation as an input to the next one.
The filters property of the component is an object.
A single filter is a function that accepts a value and returns another value.
The returned value is the one that’s actually printed in the Vue.js template.
Filters were removed in Vue3

You can pass parameters but either it is not a vue.js way or the way you are doing is wrong.
However there are cases when you need to do so.I am going to show you a simple example passing value to computed property using getter and setter.
<template>
<div>
Your name is {{get_name}} <!-- John Doe at the beginning -->
<button #click="name = 'Roland'">Change it</button>
</div>
</template>
And the script
export default {
data: () => ({
name: 'John Doe'
}),
computed:{
get_name: {
get () {
return this.name
},
set (new_name) {
this.name = new_name
}
},
}
}
When the button clicked we are passing to computed property the name 'Roland' and in set() we are changing the name from 'John Doe' to 'Roland'.
Below there is a common use case when computed is used with getter and setter.
Say you have the follow vuex store:
export default new Vuex.Store({
state: {
name: 'John Doe'
},
getters: {
get_name: state => state.name
},
mutations: {
set_name: (state, payload) => state.name = payload
},
})
And in your component you want to add v-model to an input but using the vuex store.
<template>
<div>
<input type="text" v-model="get_name">
{{get_name}}
</div>
</template>
<script>
export default {
computed:{
get_name: {
get () {
return this.$store.getters.get_name
},
set (new_name) {
this.$store.commit('set_name', new_name)
}
},
}
}
</script>

You can also pass arguments to getters by returning a function. This is particularly useful when you want to query an array in the store:
getters: {
// ...
getTodoById: (state) => (id) => {
return state.todos.find(todo => todo.id === id)
}
}
store.getters.getTodoById(2) // -> { id: 2, text: '...', done: false }
Note that getters accessed via methods will run each time you call them, and the result is not cached.
That is called Method-Style Access and it is documented on the Vue.js docs.

I'd like to first reiterate the previous caveats that using computed (which is cached) with a parameter simply makes the computed not cached, effectively just making it a method.
However, that being said, here are all the variations that I can think of which may have edge-cases for use. If you cut & paste this into a demo app it should be clear what is going on:
<template>
<div>
<div style="background: violet;"> someData, regularComputed: {{ someData }}, {{ regularComputed }} </div>
<div style="background: cornflowerblue;"> someComputedWithParameterOneLine: {{ someComputedWithParameterOneLine('hello') }} </div>
<div style="background: lightgreen;"> someComputedWithParameterMultiLine: {{ someComputedWithParameterMultiLine('Yo') }} </div>
<div style="background: yellow"> someComputedUsingGetterSetterWithParameterMultiLine: {{ someComputedUsingGetterSetterWithParameterMultiLine('Tadah!') }} </div>
<div>
<div style="background: orangered;"> inputData: {{ inputData }} </div>
<input v-model="inputData" />
<button #click="someComputedUsingGetterSetterWithParameterMultiLine = inputData">
Update 'someComputedUsingGetterSetterWithParameterMultiLine' with 'inputData'.
</button>
</div>
<div style="background: red"> newConcatenatedString: {{ newConcatenatedString }} </div>
</div>
</template>
<script>
export default {
data() {
return {
someData: 'yo',
inputData: '',
newConcatenatedString: ''
}
},
computed: {
regularComputed(){
return 'dude.'
},
someComputedWithParameterOneLine(){
return (theParam) => `The following is the Parameter from *One* Line Arrow Function >>> ${theParam}`
},
someComputedWithParameterMultiLine(){
return (theParam) => {
return `The following is the Parameter from *Multi* Line Arrow Function >>> ${theParam}`
}
},
// NOTICE that Computed with GETTER/SETTER is now an Object, that has 2 methods, get() and set(), so after the name of the computed we use : instead of ()
// thus we do: "someComputedUsingGetterSetterWithParameterMultiLine: {...}" NOT "someComputedUsingGetterSetterWithParameterMultiLine(){...}"
someComputedUsingGetterSetterWithParameterMultiLine: {
get () {
return (theParam) => {
return `As part of the computed GETTER/SETTER, the following is inside get() which receives a Parameter (using a multi-line Arrow Function) >>> ${theParam}`
}
},
set(newSetValue) {
console.log('Accessing get() from within the set()', this.someComputedUsingGetterSetterWithParameterMultiLine('hello from inside the Setter, using the Getter.'))
console.log('Accessing newSetValue in set() >>>>', JSON.stringify(newSetValue))
this.newConcatenatedString = `**(1)${this.someComputedUsingGetterSetterWithParameterMultiLine('hello from inside the Setter, using the Getter.')}** This is a concatenation of get() value that had a Parameter, with newSetValue **(2)${newSetValue}** that came into the set().`
}
},
},
}
</script>

Computed could be considered as a function. So for an example on validation you could clearly do something like :
methods: {
validation(attr){
switch(attr) {
case 'email':
const re = /^(([^<>()\[\]\.,;:\s#\"]+(\.[^<>()\[\]\.,;:\s#\"]+)*)|(\".+\"))#(([^<>()[\]\.,;:\s#\"]+\.)+[^<>()[\]\.,;:\s#\"]{2,})$/i;
return re.test(this.form.email);
case 'password':
return this.form.password.length > 4
}
},
...
}
Which you'll be using like :
<b-form-input
id="email"
v-model="form.email"
type="email"
:state="validation('email')"
required
placeholder="Enter email"
></b-form-input>
Just keep in mind that you will still miss the caching specific to computed.

Yes methods are there for using params. Like answers stated above, in your example it's best to use methods since execution is very light.
Only for reference, in a situation where the method is complex and cost is high, you can cache the results like so:
data() {
return {
fullNameCache:{}
};
}
methods: {
fullName(salut) {
if (!this.fullNameCache[salut]) {
this.fullNameCache[salut] = salut + ' ' + this.firstName + ' ' + this.lastName;
}
return this.fullNameCache[salut];
}
}
note: When using this, watchout for memory if dealing with thousands

You need to be careful with the vue/no-side-effects-in-computed-properties ESlint rule and not making any inside of computed.
Meanwhile, if you're looking towards having a memoization kind of approach, you can give a read to this article or useMemoize from Vueuse.
Or even v-memo since Vue3.2.

I don't see the an answer for the Vue3 and/or using composition API, so here is my bit (,because I always forget how to do that).
You can make computed to accept arguments by wrapping it in another function, like this:
const firstName = ref("John");
const lastName = ref("Doe");
const fullName = (salut: string) =>
computed(() =>
`${salut} ${firstName.value} ${lastName.value}`);
If you want to use Pinia (Passing arguments to getters) for state management, you can use this computed as a getter:
// In component's setup function
const store = useStore();
const fullName = store.fullName("Hello"); // ComputedRef<string>
console.log(fullName) // "Hello John Doe"

I didn't see a clear Vue 3 example so I am adding one from an app I worked on. You call a method first that then returns a computed value. So the method will be called when Vue re-renders but then the cached computed value is returned and only executed if the reactive input variables change.
<script setup>
import { computed, ref } from 'vue'
const itemOne = ref(1);
const itemTwo = ref(2);
const getItemDoubled: (key) => {
return computed(()=> item[key].value * 2);
}
</script>
<template>
<p>get dynamic name computed value: {{ getItemDoubled('One') }}
<p>get dynamic name computed value: {{ getItemDoubled('Two') }}
</template

What's the correct way to pass props as initial data in Vue.js 2?

So I want to pass props to an Vue component, but I expect these props to change in future from inside that component e.g. when I update that Vue component from inside using AJAX. So they are only for initialization of component.
My cars-list Vue component element where I pass props with initial properties to single-car:
// cars-list.vue
<script>
export default {
data: function() {
return {
cars: [
{
color: 'red',
maxSpeed: 200,
},
{
color: 'blue',
maxSpeed: 195,
},
]
}
},
}
</script>
<template>
<div>
<template v-for="car in cars">
<single-car :initial-properties="car"></single-car>
</template>
</div>
</template>
The way I do it right now it that inside my single-car component I'm assigning this.initialProperties to my this.data.properties on created() initialization hook. And it works and is reactive.
// single-car.vue
<script>
export default {
data: function() {
return {
properties: {},
}
},
created: function(){
this.data.properties = this.initialProperties;
},
}
</script>
<template>
<div>Car is in {{properties.color}} and has a max speed of {{properties.maxSpeed}}</div>
</template>
But my problem with that is that I don't know if that's a correct way to do it? Won't it cause me some troubles along the road? Or is there a better way to do it?

Thanks to this https://github.com/vuejs/vuejs.org/pull/567 I know the answer now.
Method 1
Pass initial prop directly to the data. Like the example in updated docs:
props: ['initialCounter'],
data: function () {
return {
counter: this.initialCounter
}
}
But have in mind if the passed prop is an object or array that is used in the parent component state any modification to that prop will result in the change in that parent component state.
Warning: this method is not recommended. It will make your components unpredictable. If you need to set parent data from child components either use state management like Vuex or use "v-model". https://v2.vuejs.org/v2/guide/components.html#Using-v-model-on-Components
Method 2
If your initial prop is an object or array and if you don't want changes in children state propagate to parent state then just use e.g. Vue.util.extend [1] to make a copy of the props instead pointing it directly to children data, like this:
props: ['initialCounter'],
data: function () {
return {
counter: Vue.util.extend({}, this.initialCounter)
}
}
Method 3
You can also use spread operator to clone the props. More details in the Igor answer: https://stackoverflow.com/a/51911118/3143704
But have in mind that spread operators are not supported in older browsers and for better compatibility you'll need to transpile the code e.g. using babel.
Footnotes
[1] Have in mind this is an internal Vue utility and it may change with new versions. You might want to use other methods to copy that prop, see How do I correctly clone a JavaScript object?.
My fiddle where I was testing it:
https://jsfiddle.net/sm4kx7p9/3/

In companion to #dominik-serafin's answer:
In case you are passing an object, you can easily clone it using spread operator(ES6 Syntax):
props: {
record: {
type: Object,
required: true
}
},
data () { // opt. 1
return {
recordLocal: {...this.record}
}
},
computed: { // opt. 2
recordLocal () {
return {...this.record}
}
},
But the most important is to remember to use opt. 2 in case you are passing a computed value, or more than that an asynchronous value. Otherwise the local value will not update.
Demo:
Vue.component('card', {
template: '#app2',
props: {
test1: null,
test2: null
},
data () { // opt. 1
return {
test1AsData: {...this.test1}
}
},
computed: { // opt. 2
test2AsComputed () {
return {...this.test2}
}
}
})
new Vue({
el: "#app1",
data () {
return {
test1: {1: 'will not update'},
test2: {2: 'will update after 1 second'}
}
},
mounted () {
setTimeout(() => {
this.test1 = {1: 'updated!'}
this.test2 = {2: 'updated!'}
}, 1000)
}
})
<script src="https://cdn.jsdelivr.net/npm/vue#2.5.17/dist/vue.js"></script>
<div id="app1">
<card :test1="test1" :test2="test2"></card>
</div>
<template id="app2">
<div>
test1 as data: {{test1AsData}}
<hr />
test2 as computed: {{test2AsComputed}}
</div>
</template>
https://jsfiddle.net/nomikos3/eywraw8t/281070/

I believe you are doing it right because it is what's stated in the docs.
Define a local data property that uses the prop’s initial value as its initial value
https://vuejs.org/guide/components.html#One-Way-Data-Flow

Second or third time I run into that problem coming back to an old vue project.
Not sure why it is so complicated in vue, but it can we done via watch:
export default {
props: ["username"],
data () {
return {
usernameForLabel: "",
}
},
watch: {
username: {
immediate: true,
handler (newVal, oldVal) {
this.usernameForLabel = newVal;
}
},
},

Just as another approach, I did it through watchers in the child component.
This way is useful, specially when you're passing an asynchronous value, and in your child component you want to bind the passed value to v-model.
Also, to make it reactive, I emit the local value to the parent in another watcher.
Example:
data() {
return {
properties: {},
};
},
props: {
initial-properties: {
type: Object,
default: {},
},
},
watch: {
initial-properties: function(newVal) {
this.properties = {...newVal};
},
properties: function(newVal) {
this.$emit('propertiesUpdated', newVal);
},
},
This way I have more control and also less unexpected behaviour. For example, when props that passed by the parent is asynchronous, it may not be available at the time of created or mounted lifecycle. So you can use computed property as #Igor-Parra mentioned, or watch the prop and then emit it.

Following up on Cindy's comment on another answer:
Be carful. The spread operator only shallow clones, so for objects
that contain objects or arrays you will still copy pointers instead of
getting a new copy.
Indeed this is the case. Changes within objects inside arrays will still propagate to your components even when a spread operator is employed.
Here was my solution (using Composition API):
setup() {
properties = ref([])
onMounted(() => {
properties.value = props.initialProperties.map((obj) => ({ ...obj }));
})
}
This worked to set the values and prevent them from getting changed, even if the data was changed in the parent component.