1---
2title: Template Expressions Must Be Single Expressions
3impact: MEDIUM
4impactDescription: Using statements instead of expressions in templates causes compilation errors
5type: capability
6tags: [vue3, template, expressions, interpolation, syntax]
7---
8 
9# Template Expressions Must Be Single Expressions
10 
11**Impact: MEDIUM** - Vue templates only support single JavaScript expressions, not statements. Using variable declarations, if statements, or multiple statements will cause template compilation errors.
12 
13Template 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.
14 
15## Task Checklist
16 
17- [ ] Use only single expressions in `{{ }}` interpolation
18- [ ] Use ternary operators instead of if/else statements
19- [ ] Move complex logic to computed properties or methods
20- [ ] Avoid variable declarations in templates
21- [ ] Use `v-if`/`v-else` directives for conditional rendering
22 
23**Incorrect:**
24```vue
25<template>
26  <!-- ERROR: Variable declaration is a statement, not expression -->
27  <p>{{ var greeting = 'Hello' }}</p>
28  <p>{{ let x = 1 }}</p>
29  <p>{{ const name = 'Vue' }}</p>
30 
31  <!-- ERROR: if statement not allowed -->
32  <p>{{ if (ok) { return message } }}</p>
33  <p>{{ if (user) return user.name }}</p>
34 
35  <!-- ERROR: Multiple statements not allowed -->
36  <p>{{ count++; return count }}</p>
37  <p>{{ items.push(newItem); items.length }}</p>
38 
39  <!-- ERROR: for/while loops not allowed -->
40  <p>{{ for (let i = 0; i < 5; i++) { } }}</p>
41</template>
42```
43 
44**Correct:**
45```vue
46<template>
47  <!-- OK: Simple expressions -->
48  <p>{{ message }}</p>
49  <p>{{ count + 1 }}</p>
50  <p>{{ items.length }}</p>
51 
52  <!-- OK: Ternary operators for conditionals -->
53  <p>{{ ok ? 'YES' : 'NO' }}</p>
54  <p>{{ user ? user.name : 'Guest' }}</p>
55  <p>{{ score >= 60 ? 'Pass' : 'Fail' }}</p>
56 
57  <!-- OK: Method/function calls -->
58  <p>{{ formatDate(date) }}</p>
59  <p>{{ items.filter(i => i.active).length }}</p>
60 
61  <!-- OK: Chained expressions -->
62  <p>{{ message.split('').reverse().join('') }}</p>
63 
64  <!-- OK: Template literals -->
65  <p>{{ `Hello, ${name}!` }}</p>
66 
67  <!-- OK: Object/array expressions -->
68  <p>{{ { name: 'Vue', version: 3 } }}</p>
69</template>
70 
71<script setup>
72import { ref, computed } from 'vue'
73 
74const ok = ref(true)
75const message = ref('Hello')
76const user = ref({ name: 'Alice' })
77const score = ref(85)
78 
79// Move complex logic to computed properties
80const greeting = computed(() => {
81  if (user.value) {
82    return `Welcome back, ${user.value.name}!`
83  }
84  return 'Hello, Guest!'
85})
86 
87// Or use methods for reusable logic
88function formatDate(date) {
89  return new Date(date).toLocaleDateString()
90}
91</script>
92```
93 
94## Use Directives for Control Flow
95 
96```vue
97<template>
98  <!-- Instead of if/else in expressions, use v-if/v-else -->
99  <p v-if="user">Welcome, {{ user.name }}!</p>
100  <p v-else>Please log in</p>
101 
102  <!-- Instead of loops in expressions, use v-for -->
103  <ul>
104    <li v-for="item in items" :key="item.id">{{ item.name }}</li>
105  </ul>
106 
107  <!-- Conditional display without removing from DOM -->
108  <p v-show="isVisible">This toggles visibility</p>
109</template>
110```
111 
112## Reference
113- [Vue.js Template Syntax - Using JavaScript Expressions](https://vuejs.org/guide/essentials/template-syntax.html#using-javascript-expressions)
114- [Vue.js Conditional Rendering](https://vuejs.org/guide/essentials/conditional.html)
115