Can't acces prop inside method - vue.js

I'm creating a single file component in Vue which takes an object as a prop called data. I want to access this prop inside a custom function that I'm creating. Here's my code
<script>
import { db } from "../main";
export default {
name: "UserDetail",
props: ["data"],
methods: {
verifyUserProfile: () => {
console.log(this.data);
db.collection("users")
.doc(this.data.uid)
.update({
idProofVerified: true
})
}
}
};
</script>
When I use the data prop in template it works just fine but in the function its giving me Cannot read property 'data' of undefined error. What am I doing wrong?
Update: I'm using this component inside View like so:
<div>
<UserDetail :data="users[selected]" />
</div>
Where users is an array of users objects.


You're using () => {} so this points to the global context. Replace it with verifyUserProfile: function () { ... }.


You are most likely calling the method before the props have arrived at the component, typically the props come in only after the lifecycle methods and therefore if you call a function which tries to access them there it will be undefined.
A solution to this would be to place a watcher on the props and then trigger the function once the props have received a value.
Like so:
watch: {
data: {
immediate: true,
handler(val){
if (val){
this.verifyUserProfile()
}
}
}
}

Related

Unexpected mutation of prop in Vue2 [duplicate]

I started https://laracasts.com/series/learning-vue-step-by-step series. I stopped on the lesson Vue, Laravel, and AJAX with this error:
vue.js:2574 [Vue warn]: Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders. Instead, use a data or computed property based on the prop's value. Prop being mutated: "list" (found in component )
I have this code in main.js
Vue.component('task', {
template: '#task-template',
props: ['list'],
created() {
this.list = JSON.parse(this.list);
}
});
new Vue({
el: '.container'
})
I know that the problem is in created() when I overwrite the list prop, but I am a newbie in Vue, so I totally don't know how to fix it. Does anyone know how (and please explain why) to fix it?

This has to do with the fact that mutating a prop locally is considered an anti-pattern in Vue 2
What you should do now, in case you want to mutate a prop locally, is to declare a field in your data that uses the props value as its initial value and then mutate the copy:
Vue.component('task', {
template: '#task-template',
props: ['list'],
data: function () {
return {
mutableList: JSON.parse(this.list);
}
}
});
You can read more about this on Vue.js official guide
Note 1: Please note that you should not use the same name for your prop and data, i.e.:
data: function () { return { list: JSON.parse(this.list) } } // WRONG!!
Note 2: Since I feel there is some confusion regarding props and reactivity, I suggest you to have a look on this thread

The Vue pattern is props down and events up. It sounds simple, but is easy to forget when writing a custom component.
As of Vue 2.2.0 you can use v-model (with computed properties). I have found this combination creates a simple, clean, and consistent interface between components:
Any props passed to your component remains reactive (i.e., it's not cloned nor does it require a watch function to update a local copy when changes are detected).
Changes are automatically emitted to the parent.
Can be used with multiple levels of components.
A computed property permits the setter and getter to be separately defined. This allows the Task component to be rewritten as follows:
Vue.component('Task', {
template: '#task-template',
props: ['list'],
model: {
prop: 'list',
event: 'listchange'
},
computed: {
listLocal: {
get: function() {
return this.list
},
set: function(value) {
this.$emit('listchange', value)
}
}
}
})
The model property defines which prop is associated with v-model, and which event will be emitted on changes. You can then call this component from the parent as follows:
<Task v-model="parentList"></Task>
The listLocal computed property provides a simple getter and setter interface within the component (think of it like being a private variable). Within #task-template you can render listLocal and it will remain reactive (i.e., if parentList changes it will update the Task component). You can also mutate listLocal by calling the setter (e.g., this.listLocal = newList) and it will emit the change to the parent.
What's great about this pattern is that you can pass listLocal to a child component of Task (using v-model), and changes from the child component will propagate to the top level component.
For example, say we have a separate EditTask component for doing some type of modification to the task data. By using the same v-model and computed properties pattern we can pass listLocal to the component (using v-model):
<script type="text/x-template" id="task-template">
<div>
<EditTask v-model="listLocal"></EditTask>
</div>
</script>
If EditTask emits a change it will appropriately call set() on listLocal and thereby propagate the event to the top level. Similarly, the EditTask component could also call other child components (such as form elements) using v-model.

Vue just warns you: you change the prop in the component, but when parent component re-renders, "list" will be overwritten and you lose all your changes. So it is dangerous to do so.
Use computed property instead like this:
Vue.component('task', {
template: '#task-template',
props: ['list'],
computed: {
listJson: function(){
return JSON.parse(this.list);
}
}
});

If you're using Lodash, you can clone the prop before returning it. This pattern is helpful if you modify that prop on both the parent and child.
Let's say we have prop list on component grid.
In Parent Component
<grid :list.sync="list"></grid>
In Child Component
props: ['list'],
methods:{
doSomethingOnClick(entry){
let modifiedList = _.clone(this.list)
modifiedList = _.uniq(modifiedList) // Removes duplicates
this.$emit('update:list', modifiedList)
}
}

Props down, events up. That's Vue's Pattern. The point is that if you try to mutate props passing from a parent. It won't work and it just gets overwritten repeatedly by the parent component. Child component can only emit an event to notify parent component to do sth. If you don't like these restrict, you can use VUEX(actually this pattern will suck in complex components structure, you should use VUEX!)

You should not change the props's value in child component.
If you really need to change it you can use .sync.
Just like this
<your-component :list.sync="list"></your-component>
Vue.component('task', {
template: '#task-template',
props: ['list'],
created() {
this.$emit('update:list', JSON.parse(this.list))
}
});
new Vue({
el: '.container'
})

According to the VueJs 2.0, you should not mutate a prop inside the component. They are only mutated by their parents. Therefore, you should define variables in data with different names and keep them updated by watching actual props.
In case the list prop is changed by a parent, you can parse it and assign it to mutableList. Here is a complete solution.
Vue.component('task', {
template: ´<ul>
<li v-for="item in mutableList">
{{item.name}}
</li>
</ul>´,
props: ['list'],
data: function () {
return {
mutableList = JSON.parse(this.list);
}
},
watch:{
list: function(){
this.mutableList = JSON.parse(this.list);
}
}
});
It uses mutableList to render your template, thus you keep your list prop safe in the component.

The answer is simple, you should break the direct prop mutation by assigning the value to some local component variables(could be data property, computed with getters, setters, or watchers).
Here's a simple solution using the watcher.
<template>
<input
v-model="input"
#input="updateInput"
#change="updateInput"
/>
</template>
<script>
export default {
props: {
value: {
type: String,
default: '',
},
},
data() {
return {
input: '',
};
},
watch: {
value: {
handler(after) {
this.input = after;
},
immediate: true,
},
},
methods: {
updateInput() {
this.$emit('input', this.input);
},
},
};
</script>
It's what I use to create any data input components and it works just fine. Any new data sent(v-model(ed)) from parent will be watched by the value watcher and is assigned to the input variable and once the input is received, we can catch that action and emit input to parent suggesting that data is input from the form element.

do not change the props directly in components.if you need change it set a new property like this:
data() {
return {
listClone: this.list
}
}
And change the value of listClone.

I faced this issue as well. The warning gone after i use $on and $emit.
It's something like use $on and $emit recommended to sent data from child component to parent component.

one-way Data Flow,
according to https://v2.vuejs.org/v2/guide/components.html, the component follow one-Way
Data Flow,
All props form a one-way-down binding between the child property and the parent one, when the parent property updates, it will flow down to the child but not the other way around, this prevents child components from accidentally mutating the parent's, which can make your app's data flow harder to understand.
In addition, every time the parent component is updates all props
in the child components will be refreshed with the latest value. This means you should not attempt to mutate a prop inside a child component. If you do .vue will warn you in the
console.
There are usually two cases where it’s tempting to mutate a prop:
The prop is used to pass in an initial value; the child component wants to use it as a local data property afterwards.
The prop is passed in as a raw value that needs to be transformed.
The proper answer to these use cases are:
Define a local data property that uses the prop’s initial value as its initial value:
props: ['initialCounter'],
data: function () {
return { counter: this.initialCounter }
}
Define a computed property that is computed from the prop’s value:
props: ['size'],
computed: {
normalizedSize: function () {
return this.size.trim().toLowerCase()
}
}

If you want to mutate props - use object.
<component :model="global.price"></component>
component:
props: ['model'],
methods: {
changeValue: function() {
this.model.value = "new value";
}
}

I want to give this answer which helps avoid using a lot of code, watchers and computed properties. In some cases this can be a good solution:
Props are designed to provide one-way communication.
When you have a modal show/hide button with a prop the best solution to me is to emit an event:
<button #click="$emit('close')">Close Modal</button>
Then add listener to modal element:
<modal :show="show" #close="show = false"></modal>
(In this case the prop show is probably unnecessary because you can use an easy v-if="show" directly on the base-modal)

You need to add computed method like this
component.vue
props: ['list'],
computed: {
listJson: function(){
return JSON.parse(this.list);
}
}

Vue.component('task', {
template: '#task-template',
props: ['list'],
computed: {
middleData() {
return this.list
}
},
watch: {
list(newVal, oldVal) {
console.log(newVal)
this.newList = newVal
}
},
data() {
return {
newList: {}
}
}
});
new Vue({
el: '.container'
})
Maybe this will meet your needs.

Vue3 has a really good solution. Spent hours to reach there. But it worked really good.
On parent template
<user-name
v-model:first-name="firstName"
v-model:last-name="lastName"
></user-name>
The child component
app.component('user-name', {
props: {
firstName: String,
lastName: String
},
template: `
<input
type="text"
:value="firstName"
#input="$emit('update:firstName',
$event.target.value)">
<input
type="text"
:value="lastName"
#input="$emit('update:lastName',
$event.target.value)">
`
})
This was the only solution which did two way binding. I like that first two answers were addressing in good way to use SYNC and Emitting update events, and compute property getter setter, but that was heck of a Job to do and I did not like to work so hard.

Vue.js props are not to be mutated as this is considered an Anti-Pattern in Vue.
The approach you will need to take is creating a data property on your component that references the original prop property of list
props: ['list'],
data: () {
return {
parsedList: JSON.parse(this.list)
}
}
Now your list structure that is passed to the component is referenced and mutated via the data property of your component :-)
If you wish to do more than just parse your list property then make use of the Vue component' computed property.
This allow you to make more in depth mutations to your props.
props: ['list'],
computed: {
filteredJSONList: () => {
let parsedList = JSON.parse(this.list)
let filteredList = parsedList.filter(listItem => listItem.active)
console.log(filteredList)
return filteredList
}
}
The example above parses your list prop and filters it down to only active list-tems, logs it out for schnitts and giggles and returns it.
note: both data & computed properties are referenced in the template the same e.g
<pre>{{parsedList}}</pre>
<pre>{{filteredJSONList}}</pre>
It can be easy to think that a computed property (being a method) needs to be called... it doesn't

For when TypeScript is your preferred lang. of development
<template>
<span class="someClassName">
{{feesInLocale}}
</span>
</template>
#Prop({default: 0}) fees: any;
// computed are declared with get before a function
get feesInLocale() {
return this.fees;
}
and not
<template>
<span class="someClassName">
{{feesInLocale}}
</span>
</template>
#Prop() fees: any = 0;
get feesInLocale() {
return this.fees;
}

Assign the props to new variable.
data () {
return {
listClone: this.list
}
}

Adding to the best answer,
Vue.component('task', {
template: '#task-template',
props: ['list'],
data: function () {
return {
mutableList: JSON.parse(this.list);
}
}
});
Setting props by an array is meant for dev/prototyping, in production make sure to set prop types(https://v2.vuejs.org/v2/guide/components-props.html) and set a default value in case the prop has not been populated by the parent, as so.
Vue.component('task', {
template: '#task-template',
props: {
list: {
type: String,
default() {
return '{}'
}
}
},
data: function () {
return {
mutableList: JSON.parse(this.list);
}
}
});
This way you atleast get an empty object in mutableList instead of a JSON.parse error if it is undefined.

YES!, mutating attributes in vue2 is an anti-pattern. BUT...
Just break the rules by using other rules, and go forward!
What you need is to add .sync modifier to your component attribute in the parent scope.
<your-awesome-components :custom-attribute-as-prob.sync="value" />

Below is a snack bar component, when I give the snackbar variable directly into v-model like this if will work but in the console, it will give an error as
Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders. Instead, use a data or computed property based on the prop's value.
<template>
<v-snackbar v-model="snackbar">
{{ text }}
</v-snackbar>
</template>
<script>
export default {
name: "loader",
props: {
snackbar: {type: Boolean, required: true},
text: {type: String, required: false, default: ""},
},
}
</script>
Correct Way to get rid of this mutation error is use watcher
<template>
<v-snackbar v-model="snackbarData">
{{ text }}
</v-snackbar>
</template>
<script>
/* eslint-disable */
export default {
name: "loader",
data: () => ({
snackbarData:false,
}),
props: {
snackbar: {type: Boolean, required: true},
text: {type: String, required: false, default: ""},
},
watch: {
snackbar: function(newVal, oldVal) {
this.snackbarData=!this.snackbarDatanewVal;
}
}
}
</script>
So in the main component where you will load this snack bar you can just do this code
<loader :snackbar="snackbarFlag" :text="snackText"></loader>
This Worked for me

Vue.js considers this an anti-pattern. For example, declaring and setting some props like
this.propsVal = 'new Props Value'
So to solve this issue you have to take in a value from the props to the data or the computed property of a Vue instance, like this:
props: ['propsVal'],
data: function() {
return {
propVal: this.propsVal
};
},
methods: {
...
}
This will definitely work.

In addition to the above, for others having the following issue:
"If the props value is not required and thus not always returned, the passed data would return undefined (instead of empty)". Which could mess <select> default value, I solved it by checking if the value is set in beforeMount() (and set it if not) as follows:
JS:
export default {
name: 'user_register',
data: () => ({
oldDobMonthMutated: this.oldDobMonth,
}),
props: [
'oldDobMonth',
'dobMonths', //Used for the select loop
],
beforeMount() {
if (!this.oldDobMonth) {
this.oldDobMonthMutated = '';
} else {
this.oldDobMonthMutated = this.oldDobMonth
}
}
}
Html:
<select v-model="oldDobMonthMutated" id="dob_months" name="dob_month">
<option selected="selected" disabled="disabled" hidden="hidden" value="">
Select Month
</option>
<option v-for="dobMonth in dobMonths"
:key="dobMonth.dob_month_slug"
:value="dobMonth.dob_month_slug">
{{ dobMonth.dob_month_name }}
</option>
</select>

I personally always suggest if you are in need to mutate the props, first pass them to computed property and return from there, thereafter one can mutate the props easily, even at that you can track the prop mutation , if those are being mutated from another component too or we can you watch also .

Because Vue props is one way data flow, This prevents child components from accidentally mutating the parent’s state.
From the official Vue document, we will find 2 ways to solve this problems
if child component want use props as local data, it is best to define a local data property.
props: ['list'],
data: function() {
return {
localList: JSON.parse(this.list);
}
}
The prop is passed in as a raw value that needs to be transformed. In this case, it’s best to define a computed property using the prop’s value:
props: ['list'],
computed: {
localList: function() {
return JSON.parse(this.list);
},
//eg: if you want to filter this list
validList: function() {
return this.list.filter(product => product.isValid === true)
}
//...whatever to transform the list
}

You should always avoid mutating props in vue, or any other framework. The approach you could take is copy it into another variable.
for example.
// instead of replacing the value of this.list use a different variable
this.new_data_variable = JSON.parse(this.list)

A potential solution to this is using global variables.
import { Vue } from "nuxt-property-decorator";
export const globalStore = new Vue({
data: {
list: [],
},
}
export function setupGlobalsStore() {
Vue.prototype.$globals = globalStore;
}
Then you would use:
$globals.list
Anywhere you need to mutate it or present it.

Passing data from a Child Component to the Parents Pop in VueJs

I have a button component which calls an API, and I want to push the returned response up to the parent, where it will become the 'translatedText' prop, however, I believe I'm using the $emit incorrectly, due to the error: `Uncaught (in promise) TypeError: Cannot read properties of undefined (reading '$emit'). How do I best capture the response data and pass it to my parent prop, and is using $emit the best use in this instance?
TranslationButton.vue
<template>
<b-button type="is-primary" #click="loadTranslations()">Übersetzen</b-button>
</template>
<script>
export default {
name: "TranslationButton",
props: {
translatedText: ''
},
methods: {
loadTranslations() {
fetch('http://localhost:3000/ccenter/cc_apis')
.then(function(response) {
return response.text();
})
.then(function(data) {
console.log(data);
this.$emit('translatedText', this.data);
console.log(data)
})
},
},
};
</script>
Parent Component Props:
props: {
data: Array,
translatedText: '',
showAttachments: {
type: Boolean,
default: false,
}
},
How Child Component is called in Parent Component:
<translation-button #translatedText="loadTranslations()" />

Best practise when passing data from child to parent is emitting events.
this.$root.$emit('translatedText', this.data);
than
this.$root.$on('translatedText', () => { // do stuff })

by emits you pass value to parent component,
#translatedText="loadTranslations()" - its event listner, fireing on your child comp emit
do #translatedText="loadTranslations" instead of #translatedText="loadTranslations()"
and add this loadTranslations as a method to parent comp
BTW
if you dont use arrow funcs, and you use this.data it's pointing to object passed to .then, it will be undefined i guess...

The problem is with the usage of this. It does no longer point to your component inside the promise then() method.
You should create a new variable and initialize it with the value of this and use that variable to emit the event.
E.g.
loadTranslations() {
const _this = this;
fetch().then(response => _this.$emit(response));
}

if you want to pass data from child to parent, you need to use $emit like the below code
child:
<template>
<b-button type="is-primary" #click="loadTranslations">Übersetzen</b-button>
</template>
<script>
export default {
name: "TranslationButton",
props: {
TranslatedText: ''
},
methods: {
loadTranslations() {
const self= this; // change added
fetch('http://localhost:3000/ccenter/cc_apis')
.then(function(response) {
return response.text();
})
.then(function(data) {
console.log(data);
self.$emit('changeTitle', data) // change added
})
}
}
</script>
parent:
<template>
<translation-button #changeTitle="ChangeT" />
</template>
......
methods:{
ChangeT(title)
{
console.log(title)
},
}

Is there any way to check the status of props when passing data from parent to child component

I have been troubled by a question for a long time. Now I am using Vue.js to develop a web project. What I want to do is to pass data from parent to child component. However, the child component's main program would run only after the props data was received, due to the async data transmission mechanism. So I would like to know whether these are some ways to check the status of props data in the child component. Therefore I can make sure the subsequent task would run after the data was passed.
For example, a feasible solution is axios.requset({..}).then(res => {..}).

You can use the watchers in your child component. Consider the following parent component:
Vue.component('parent-comp', {
props: ['myProp'],
template: `
<div>
<child-comp my-prop={someAsyncProp} />
</div>
`,
data() {
return {
// Declare async value
someAsyncProp: null
};
},
mounted() {
// Some async computation
axios
.requset({ url: '/get-data' })
.then(res => {
// Set value asynchronously
this.someAsyncProp = res;
});
}
});
Your child component would use watchers to check if data is available:
Vue.component('child-comp', {
props: ['myProp'],
template: '<div></div>',
watch: {
// Watch the property myProp
myProp(newValue, oldValue) {
if (newValue !== null) {
// Do something with the props that are set asynchronously by parent
}
}
}
})

Vue: Make sure the component cannot modify props (even if it is a reference object)?

I know that the vue model is a unidirectional data flow of props.
However, when prop is a reference object, the component can directly modify its properties. This is wrong, but vue will not check it.
I hope there is a mechanism to ensure that the component cannot modify the props (even if it is a reference object), rather than being checked by the developer.
For example, I have a component
<template>
<input v-model="obj.text" />
</template>
<script>
export default {
props: ['obj']
};
</script>
And a page that uses it
<template>
<my-template :obj="myobj"></my-template>
</template>
<script>
export default {
data() {
myobj: {
text: "hello";
}
}
};
</script>
When data changes in 'input', myobj.text will change together. This violates the unidirectional data flow.
Of course, as shown in the answer, I can use the "get" and "set" methods of the "computed".
But I must be careful not to write 'obj.someProperty' to any 'v-model', but this requires my own attention.
I hope there is a mechanism to give a hint when I make a mistake.

Couldn't find an existing duplicate so here's an answer. If anyone can find one, let me know and I'll make this one a Community wiki.
Use a computed property with getter and setter to represent your v-model value.
The getter gets the value from the prop and the setter emits the new value to the parent.
For example
<input v-model="computedProp">
props: ['referenceObject'],
computed: {
computedProp: {
get () {
return this.referenceObject.someProperty
},
set (val) {
this.$emit('updated', val)
}
}
}
and in the parent
<SomeComponent :reference-object="refObject" #updated="updateRefObject">
data: () => ({ refObject: { someProperty: 'initial value' } }),
methods: {
updateRefObject (newVal) {
this.refObject.someProperty = newVal
}
}

Vuejs emit from grandchild to basic parent

If I have component in nested component 3-4 levels and:
(1) wishes to send data from basic parent component to the final child I must send from each parent to each child props - its nice, but
(2) if I want to receive data in basic parent component from the final child I have to emit to each of the children and get what I gave in the int's parent until it come to the basic component - is not so cool.
That's how it works?
<component1 #change="change">
<component2 :data="data" #change="change">
<component3 :data="data" #change="change">
<component4 :data="data">
</compoent4>
</compoent3>
</compoent2>
</compoent1>
And in each child component I will have:
props: {data: {type: String}},
methods: {
change: function () {
this.$emit('change', this.data)
}
}
And in base parent component I will have:
data() {
data: 1
}
},
methods: {
change: function () {
this.$emit('change', this.data)
}
}
Or it is best to do with vuex?

The proper way to overcome this issue in Vue is to use EventBus.
create a new file, and name it eventBus.js and put the following inside
import Vue from 'vue'
export default new Vue()
Then, on the files you want to communicate from, import the file and use as the following
send event:
import EventBus from '../eventBus'
EventBus.$emit('MESSAGE_NAME', payload)
receive event:
import EventBus from '../eventBus'
EventBus.$on('DATA_PUBLISHED', (payload) => {
this.updateData(payload)
})

Vuex or an event bus may be overkill and it will break your top-down component architecture.
If you really need one way data flow with a trigger and a result passed down:
You could do it react style: pass the change function down as a prop (it has access to parent data via closure), simply call it in the child. You could even use provide/inject.
<component1 :change="change">
<component2 :data="data" :change="change">
<component3 :data="data" :change="change">
<component4 :data="data" :change="change">
</compoent4>
</compoent3>
</compoent2>
</compoent1>
Component 1
props: {data: {type: String}},
methods: {
change: function () {
this.$emit('change', this.data)
}
}
Component 4
props: {data: {type: String}, change: Function},
methods: {
// no methods required! just call: this.change();
}
If you really just want a deep two way binding:
Instead of passing data down as a string prop, you could pass an object containing data down (you could even use provide/inject to skip layers). Then you don't need a change function you can just manipulate the data directly. As always, you can use provide/inject to skip layers.
<component1 :data="data">
<component2 :obj="obj">
<component3 :obj="obj" >
<component4 :obj="obj">
</compoent4>
</compoent3>
</compoent2>
</compoent1>
Component 1
props: {data: {type: String}},
computed: {
obj: function () {
return {data: this.data};
}
}
Component 4
props: {obj: {type: Object}},
methods: {
change: function (newValue) {
this.obj.data = newValue;
}
}