title: Template Expressions Must Be Single Expressions impact: MEDIUM impactDescription: Using statements instead of expressions in templates causes compilation errors type: capability tags: [vue3, template, expressions, interpolation, syntax]
Template Expressions Must Be Single Expressions
Impact: MEDIUM - Vue templates only support single JavaScript expressions, not statements. Using variable declarations, if statements, or multiple statements will cause template compilation errors.
Template interpolation {{ }} and directive bindings evaluate JavaScript expressions that produce a value. Statements like if, for, variable declarations, or multi-line code blocks are not allowed.
Task Checklist
- Use only single expressions in
{{ }}interpolation - Use ternary operators instead of if/else statements
- Move complex logic to computed properties or methods
- Avoid variable declarations in templates
- Use
v-if/v-elsedirectives for conditional rendering
Incorrect:
<template>
<!-- ERROR: Variable declaration is a statement, not expression -->
<p>{{ var greeting = 'Hello' }}</p>
<p>{{ let x = 1 }}</p>
<p>{{ const name = 'Vue' }}</p>
<!-- ERROR: if statement not allowed -->
<p>{{ if (ok) { return message } }}</p>
<p>{{ if (user) return user.name }}</p>
<!-- ERROR: Multiple statements not allowed -->
<p>{{ count++; return count }}</p>
<p>{{ items.push(newItem); items.length }}</p>
<!-- ERROR: for/while loops not allowed -->
<p>{{ for (let i = 0; i < 5; i++) { } }}</p>
</template>
Correct:
<template>
<!-- OK: Simple expressions -->
<p>{{ message }}</p>
<p>{{ count + 1 }}</p>
<p>{{ items.length }}</p>
<!-- OK: Ternary operators for conditionals -->
<p>{{ ok ? 'YES' : 'NO' }}</p>
<p>{{ user ? user.name : 'Guest' }}</p>
<p>{{ score >= 60 ? 'Pass' : 'Fail' }}</p>
<!-- OK: Method/function calls -->
<p>{{ formatDate(date) }}</p>
<p>{{ items.filter(i => i.active).length }}</p>
<!-- OK: Chained expressions -->
<p>{{ message.split('').reverse().join('') }}</p>
<!-- OK: Template literals -->
<p>{{ `Hello, ${name}!` }}</p>
<!-- OK: Object/array expressions -->
<p>{{ { name: 'Vue', version: 3 } }}</p>
</template>
<script setup>
import { ref, computed } from 'vue'
const ok = ref(true)
const message = ref('Hello')
const user = ref({ name: 'Alice' })
const score = ref(85)
// Move complex logic to computed properties
const greeting = computed(() => {
if (user.value) {
return `Welcome back, ${user.value.name}!`
}
return 'Hello, Guest!'
})
// Or use methods for reusable logic
function formatDate(date) {
return new Date(date).toLocaleDateString()
}
</script>
Use Directives for Control Flow
<template>
<!-- Instead of if/else in expressions, use v-if/v-else -->
<p v-if="user">Welcome, {{ user.name }}!</p>
<p v-else>Please log in</p>
<!-- Instead of loops in expressions, use v-for -->
<ul>
<li v-for="item in items" :key="item.id">{{ item.name }}</li>
</ul>
<!-- Conditional display without removing from DOM -->
<p v-show="isVisible">This toggles visibility</p>
</template>