In-template variable definition

[wspl]

Sometimes you will encounter such a situation:

<template>
  <div class="container">
    <p class="row">{{item && item.a && item.a.b && item.a.b.hello}}</p>
    <p class="row">{{item && item.a && item.a.b && item.a.b.world}}</p>
    <p class="row">{{item && item.a && item.a.b && item.a.b.goodbye}}</p>
  </div>
</template>

These code with the same logic need to be written many times.
I hope there is a way to define local variables in the template, then the above code can be abbreviated as:

<template>
  <div class="container">
    <var name="b" :value="item && item.a && item.a.b"></var>
    <p class="row">{{b && b.hello}}</p>
    <p class="row">{{b && b.world}}</p>
    <p class="row">{{b && b.goodbye}}</p>
  </div>
</template>

or

<template>
  <div class="container" v-var:b="item && item.a && item.a.b">
    <p class="row">{{b && b.hello}}</p>
    <p class="row">{{b && b.world}}</p>
    <p class="row">{{b && b.goodbye}}</p>
  </div>
</template>

Q: Why not computed?
A: The item in the above code may come from v-for, so computed is not a feasible solution.

[posva]

It was proposed in the past: vuejs/vue#7325
Currently you can achieve it with https://github.com/posva/vue-local-scope

[skyrpex]

Probably you already know, but if you use JSX you could do this:

export default {
    render() {
        const b = this.item && this.item.a && this.item.a.b;
        return (
            <div class="container">
                <p class="row">{b && b.hello}</p>
                <p class="row">{b && b.world}</p>
                <p class="row">{b && b.goodbye}</p>
            </div>
        )
    }
}

[afontcu]

Q: Why not computed?
A: The item in the above code may come from v-for, so computed is not a feasible solution.

You can also create a method that accepts a parameter, so you can use it in v-for loops.

[phiter]

The issue with methods is that they aren't cached, so a bit less performatic.

[LinusBorg]

Creating a local variable as proposed by OP would also not be cached.

I see, and a local variable could serve as an in-loop cache for method results where computed props can't be used.

[ReinisV]

Q: Why not computed?
A: The item in the above code may come from v-for, so computed is not a feasible solution.

Whenever I hear someone say that, I think "this is a perfect situation for creating a component for that item!". That would give you the ability to use simple computed getters that can be cached.

A component should always have exactly one reason to exist and one mission to execute. This parent components mission is not knowing how to render the child items.

[LinusBorg]

Well, component instantiations come at a price (memory-footprint & render performance), so sometimes it's better to render i list's items in the parent for that reason. There shuold be ways to keep these situations performant, an this RFC aims to address it.

[frehner]

Forgive my ignorance, I'm new to the vue community.

Could the above be done with optional chaining as well? e.g.

<template>
  <div class="container">
    <p class="row">{{ item?.a?.b?.hello }}</p>
    <p class="row">{{ item?.a?.b?.world }}</p>
    <p class="row">{{ item?.a?.b?.goodbye }}</p>
  </div>
</template>

it's stage 3 and has a babel plugin

[LinusBorg]

Not right now, as vue-es2015-template compiler doesn't aupport >es6 features

But that can be solved.

[michaeldrotar]

I was dealing with a similar concern recently in my work so wanted to add 2 cents...

(My day job is in Ember so I'll pseudo-code the concepts)

Generally I want to pass a data model to my components.. like pass a user to a component for showing the user's badge or pass a meeting to a component for showing when and where and who's attending that meeting... it's just simpler to do <my-component meeting=meeting> than to do <my-component name=meeting.name date=meeting.date whatever=meeting.whatever ...>

I've also recently come to appreciate that this allows me to flatten out deeply nested data and to add logic around data that's not directly suited to my use case. So I recently did something like this:

// Component receives `model`
name = model.model_name
date = computed(() => (model.date && date_from_format(model.date, 'YYYY-MM-DD')) || undefined)
instructions = computed(() => (model.location && model.location.meeting_instructions) || undefined)

The same can be done for lists... if I need to display a list of model.users, I originally considered passing them to a nested component as well, but sometimes that's overkill.

users = computed(() => {
  return model.users.map((user) => {
    const spouse = model.spouse || {};
    return {
      name: user.name,
      spouseName: spouse.name
    };
  });
});

Now the template can iterate users without fear of accessing a nested property that might not exist and my component js continues to uphold the contract that it's explicitly handling all the data that the template needs.

[cawa-93]

Will this make the code much less clear? In my opinion, if you have a large component, it will be difficult to understand what the variable is, where it comes from if it is not in the data of component.

Much simpler code that doesn't have any "magic" variables:

<template>
  <div class="container" v-if="item && item.a && item.a.b">
    <p class="row">{{item.a.b.hello}}</p>
    <p class="row">{{item.a.b.world}}</p>
    <p class="row">{{item.a.b.goodbye}}</p>
  </div>
</template>

[woothu]

Computed can also solve v-for performance problem, as items could be mapped to have additional keys for display purposes + you can use object destructuring.

Btw. most of times proper Item structurizing can solve this issue.

[chriswoodle]

I think I found some kind of way to accomplish in-template variables in vue 3. Here is an example where I create the "in-template variable" inside of a v-for. Basically declare an inline array and iterate over the single item.

<div v-for='item in items'>
    <div v-for="record of [cache.find(item.otherId).value]">
        <span v-if="record">{{ record.name }}</span>
    </div>
</div>

In this case cache.find(item.otherId) returns back a ref<{ name: string } | null>

I feel like it's its kinda hacky, and I'm not really sure the performance impact.

[julie777]

The issue with methods is that they aren't cached, so a bit less performatic.

Which is why there are computed functions that do cache results. :-)

[liho00]

<div v-data="{x:'my temp variable'}">{{x}}</div>

[komonjlep]

I think I found some kind of way to accomplish in-template variables in vue 3. Here is an example where I create the "in-template variable" inside of a v-for. Basically declare an inline array and iterate over the single item.

<div v-for='item in items'>
    <div v-for="record of [cache.find(item.otherId).value]">
        <span v-if="record">{{ record.name }}</span>
    </div>
</div>

In this case cache.find(item.otherId) returns back a ref<{ name: string } | null>

I feel like it's its kinda hacky, and I'm not really sure the performance impact.

It's great how this approach can be used to cache cross-referenced (calculated) item when I have two v-for for matching correct item from big list. This way I don't need to re-evaluate fetching of correct item for each action separately, but can simply fetch item only once and refer to it.

It looks like this works correctly, even with events. I would hope to see this implement in more readable manner and without using v-for in hackish-way. Please consider implementing this as built-in-feature.

[dima-hx]

There is also a way to set multiple variables in :set directive in such way:

                        <div :set="br = {variable1: someCalculation(), variable2: anotherCalculations()}">
                            <pre>{{br}}</pre>
                        </div>

https://stackoverflow.com/a/56249209/2114398

https://dev.to/pbastowski/comment/7fc9

[myWsq]

Svelte implements this feature and has good Typescript support.

https://svelte.dev/docs#template-syntax-const

[mon-jai]

<div v-data="{x:'my temp variable'}">{{x}}</div>

I can't find any documentation on v-data.

[darrylnoakes]

I can't find any documentation on v-data.

It was just a suggestion for the syntax, I believe.

[DevilTea]

I've a solution that is using a component to solve this issue.

Only tested in vue >= 3.3.0

https://www.npmjs.com/package/vue-temp-var

Example

<script setup lang="ts">
import TempVar from 'vue-temp-var'

function getRandomNumber() {
  return Math.random()
}
</script>

<template>
  <div v-for="i in 10" :key="`random-${i}`">
    <!-- cause the return value would be random, we should store it somewhere -->
    <!-- bind a prop 'define' to 'TempVar'. Because it is a prop, it would still be reactive -->    
    <!-- destruct from slot: defined, the type would be kept -->
    <TempVar 
      :define="{ randomNum: getRandomNumber() }"
      #defined="{ randomNum }"
    >
      <span>"{{ randomNum }}"</span>
      <span v-if="randomNum > 0.5"> is larger than 0.5</span>
      <span v-else> is smaller than 0.5</span>
    </TempVar>
  </div>
</template>

[iamandrewluca]

LocalScope.vue

<template>
	<slot v-bind="$attrs" />
</template>

<template>
	<LocalScope v-slot="{ classes }" classes="py-3 font-semibold">
		<p :class="classes">some text</p>
		<p :class="classes">some text</p>
	</LocalScope>
</template>

[DevilTea]

This looks more neat! It's not necessary to use another template to consume bounded slot props!

But unfortunately, this method would not keep the type of the local scope variables.🤔

[iamandrewluca]

I suppose this now can be achieved using generics component 🤔

https://vuejs.org/api/sfc-script-setup.html#generics

[DevilTea]

Yes, my solution is a component that is achieved by using generics component. I just wrapped it and distributed it to npm 😂.

Here is the component's source code

<script setup lang="ts" generic="T extends { [idx: string]: { [idx: string]: object | U; } | U; }, U extends string | number | bigint | boolean | symbol | null | undefined">
defineProps<{
    define: T
}>()

defineSlots<{
  defined(props: T): any
}>()
</script>

<template>
    <slot name="defined" v-bind="define"></slot>
</template>

[iamandrewluca]

Strange why defineProps<T>() cannot be used 🤔 it does not work

Error: [@vue/compiler-sfc] Unresolvable type reference or unsupported built-in utility type

src/Comp.vue
4  |  
5  |  <script setup lang="ts" generic="T">
6  |  defineProps<T>()
   |              ^
7  |  </script>

[dfidalg0]

I guess the most simple solution looks like this

<!-- local-scope.vue -->
<script setup lang="ts" generic="T extends Record<string, unknown>">
defineProps<{ set: T }>();
</script>

<template>
  <slot v-bind="set" />
</template>

And you can use it like this

<!-- my-component.vue -->
<script setup lang="ts">
import LocalScope from './local-scope.vue';
</script>

<template>
  <local-scope
    :set="{ someVar: someValue }"
    v-slot="{ someVar }"
  >
    {{ someVar }}
  </local-scope>
</template>

The thing is - it's overhead having a component lifecycle just to define a variable while in JSX you could easilly define a variable inside a function.

Maybe we could have something like v-def="{ someVar: someValue }" that transpiles to const someVar = someValue inside the render function.

Or, to make it look more like standard JS, we could have

v-def="someVar = someValue, anotherVar = anotherValue"

transpiled to

const someVar = someValue, anotherVar = anotherValue

And, with Typescript,

v-def="
  someVar: number = someValue,
  anotherVar = anotherValue as string
"

would just work.

[fardolieri]

I use a functional component and force (sort of roughly) a type onto it. That way, consumers can choose a fitting name without having to rename the variable. It even allows multiple assignments. What do you think?

See the demo. (Note: In the playground typing for v-slot does not seem to work but it works in my editor.)

<script setup>
import TempVar from './temp-var';
</script>

<template>
  <TempVar :foo="Math.random()" v-slot="{ foo }">
    <div>{{ foo }}</div>
  </TempVar>
</template>

// temp-var.ts
const TempVar = defineComponent(
  (props, ctx) => () => ctx.slots.default?.(ctx.attrs),
  { inheritAttrs: false },
) as unknown as new <T>(props: T) => {
  $props: T;
  $slots: { default?: (context: T) => unknown };
};

export default TempVar;