I have the following Vue component:
Vue.component('result', {
props: ['stuff'],
data: () => ({}),
template: "<img :src='tag' class='result'></img>",
computed: {
tag: function tag() {
return `pages/search/img/${this.stuff.type.toLowerCase()}_tag.png`;
}
}
});
When the component is created, an error is thrown:
TypeError: Cannot read property 'toLowerCase' of undefined
at VueComponent.tag
However, when I remove the call to toLowerCase(), the method executes properly, generating a string with the expected type. I could work around this by changing my filenames to have capital letters, but I would rather understand why Vue is behaving this way. Why would a property be undefined only when methods are called on it?
Update: after some troubleshooting, I found that this.stuff.type is undefined the first time tag() is computed. Calling toLowerCase() just forces an error on an otherwise silent bug. Is there a reason the props aren't defined when computed functions are called for the first time? Should I be writing my component differently?
The stuff prop is undefined when the result component is created.
There are two options to fix this problem:
Either use the v-if directive in the parent component's template to make sure stuff has a value when the result component is created:
<template>
<result v-if="stuff" :stuff="stuff" />
</template>
Or handle the stuff prop being undefined in the result component.
Vue.component('result', {
props: {
// Object with a default value
stuff: {
type: Object,
// Object or array defaults must be returned from
// a factory function
default: () => ({ type: 'someType'})
},
},
data: () => ({}),
template: "<img :src='tag' class='result' >",
computed: {
tag: function tag() {
return `pages/search/img/${this.stuff.type.toLowerCase()}_tag.png`;
}
}
})
Note: The img element is a void element, it does not require an end tag.
Props are by default null but you can give them a default value to overcome this problem.
Example :
Vue.component('result', {
props: {
stuff: {
type: Object,
default: {
type: ''
}
}
},
data: () => ({}),
template: "<img :src='tag' class='result'></img>",
computed: {
tag: function tag() {
return `pages/search/img/${this.stuff.type.toLowerCase()}_tag.png`;
}
}
});
Related
I have this computed prop:
methods: {
url_refresh: function (id) {
return `${this.url_base}?start=${Date.now()}`
}
}
And when i try to console log on mount:
mounted() {
console.log(this.url_refresh);
},
It logs the function instead of the value:
How do I get the value instead of the function?
Its supposed to return http://localhost/admin/agenda/refresh?agenda_id=2&start=2020-11-29T00:00:00-03:00
You are probably use methods instead of computed, look here
Should Work for you (tested):
<template>
<div>
Your Template
</div>
</template>
<script>
export default {
mounted() {
console.log(this.url_refresh);
},
data() {
return { url_base: "http://localhost/admin/agenda/refresh" };
},
computed: {
url_refresh() {
return `${this.url_base}?start=${Date.now()}`;
}
},
};
</script>
Will log: http://localhost/admin/agenda/refresh?start=1607431794589
Computed Caching vs Methods
Main point of using computed is caching behavior. Hence it doesn't make sense to pass any argument into computed (the result should depend only on other reactive data). If you need argument, use methods instead...
Is there any way to set default function for prop with function type?
props: {
clickFunction: {
type: 'Function'
default: ????
}
}
Yes:
Vue.component('foo', {
template: '<div>{{ num }}</div>',
props: {
func: {
type: Function,
default: () => 1,
},
},
data() {
return {
num: this.func()
}
}
})
new Vue({
el: '#app',
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<div id="app">
<foo></foo>
</div>
I'd say, always provide a default value that matched the type you specified: makes things much more easier to reason about. Here, what you are looking for would be a noop function (a.k.a. a function that does nothing):
props: {
clickFunction: {
type: Function
default: () => {}
}
}
You can then use this.clickFunction() in your code without checking it defensively first: it is always a function. Be sure you don't get mixed up with the default value for an Object prop, that would be an empty object:
props: {
someObject: {
type: Object
default: () => ({})
}
}
In Vue, Object and Array props must have their default value returned by a function to ensure they are distinct for every instance of the component (which does not seem to be required for Function props, but docs indeed aren't crystal clear about this).
This way should work with Vue 2.0
props: {
clickFunction: {
type: 'Function'
default(){
return () => true
}
}
}
I think this is not the right approach. VUE do not work that way. Why do want to pass reference to function as props? Component can emit its own event that parent can catch.
Here you have available props types:
https://v2.vuejs.org/v2/guide/components-props.html
And here you can read about cutom events:
https://v2.vuejs.org/v2/guide/components-custom-events.html#Event-Names
Edit
Yeah, my answer is not correct and does not answer the question.
Straight answer: yes, there are some any ways to set default function for prop with function type (look other answers).
Patterns and use-cases are off-topic so I end here (however, someone mentioned an interesting thing in response to my answer).
can you tell me what I do wrong? I need access to props in my data object in component.
I have defined component like this:
export default {
components: {...},
computed: {...},
props: {
userCode: {
type: String,
default: null
}
},
data: () => ({
options: {
callback: function() {
console.log(this.userCode) // prints undefined
return ...;
}
}
}),
methods: {...},
...,
}
prop value I define in router like this:
{
path: '/user/bbb',
name: 'users',
component: userView,
meta: {
requiresLoggedIn: true,
},
props: {userCode: 'XXX'}
}
When I tried in same component render this prop in html like this {{this.userCode}} so it's worked and display my passed code. How to access to prop in options data object? Thanks.
Vue.js best practices aside the reason this.userCode is undefined is because in that case the callback function defines its own this and the global this is not being used. To use the global this either use
callback: () => {}
or
callback: function() {
console.log(this.userCode) // prints undefined
return ...;
}.bind(this)
you can read more about the arrow function here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions
I have the code (vuejs2) -
Vue.component('competetion-list', {
template: `<div>{{totalCompetetions}}</div>`,
props: ['values'],
data: function () {
return { totalCompetetions: this.values.length}
}
})
Nothing is printed on the page but if I change the template value to
template: `<div>{{this.values.length}}</div>`
it prints 15. What am I doing wrong and how can I pass the props to the data?
Any help is much appreciated.
I was unable to assign the prop values to data totalCompetetions in the following way -
data: function () {
return { totalCompetetions: this.values.length}
}
But I was able to do it using the watch, computed, and methods properties.
With watch property -
watch: {
values: function(){
this.totalCompetitions = this.values;
}
}
With computed property -
computed:{
competition:{
get: function(){
return this.values.length;
}
}
With methods property -
methods:{
competitionn: function(){
return this.values.length;
}
}
But for computed and methods properties, I needed to set totalCompetetions in the following way -
For computed -
template: `<div><p>{{totalCompetitions = competition}}</p></div>` //without parenthesis
For methods -
template: `<div><p>{{totalCompetitions = competition()}}</p></div>` //with parenthesis
You code does work.
I guess the problem is your parent component. Did you pass the values correctly? for example:
<competetion-list :values="[1, 2, 3]"></competetion-list>
Besides, for your case I'd say computed properties is a better solution.
computed: {
totalCompetetions () {
return this.values.length
}
}
From the data() method, you should be able to reference the component's properties using this.
Try following:
Vue.component('competetion-list', {
template: `<div>{{totalCompetetions}}</div>`,
props: ['values'],
data: function () {
var data = { totalCompetetions: this.values.length}
return data
}
})
As validly mentioned in the comment, if values array is changing later, you may have to put a watcher on the prop and inside watcher, set totalCompetetions as this.values.length.
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.