67 lines
2.2 KiB
Markdown
67 lines
2.2 KiB
Markdown
---
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title: v-for Range Iteration Starts at 1, Not 0
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impact: LOW
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impactDescription: v-for with a number range starts at 1, unlike JavaScript arrays which start at 0
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type: gotcha
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tags: [vue3, v-for, list-rendering, range]
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---
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# v-for Range Iteration Starts at 1, Not 0
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**Impact: LOW** - When using `v-for` with a number (range iteration), the iteration starts at `1`, not `0`. This differs from typical JavaScript behavior where arrays are 0-indexed.
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This gotcha commonly causes off-by-one errors when the generated numbers are used for calculations or array indexing.
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## Task Checklist
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- [ ] Remember `v-for="n in 10"` produces 1 through 10, not 0 through 9
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- [ ] When using range values for array indexing, subtract 1: `items[n - 1]`
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- [ ] Consider creating a computed array if you need 0-based indices
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**Incorrect Assumption:**
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```html
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<!-- Developer expects 0-9, but gets 1-10 -->
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<span v-for="n in 10">{{ n }}</span>
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<!-- Output: 1 2 3 4 5 6 7 8 9 10 -->
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<!-- WRONG: Off-by-one error when used for array access -->
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<li v-for="n in items.length" :key="n">
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{{ items[n].name }} <!-- Error! items[10] is undefined when length is 10 -->
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</li>
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```
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**Correct:**
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```html
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<!-- Correct understanding: 1-based range -->
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<span v-for="n in 10" :key="n">{{ n }}</span>
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<!-- Output: 1 2 3 4 5 6 7 8 9 10 -->
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<!-- CORRECT: Adjust index for array access -->
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<li v-for="n in items.length" :key="n">
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{{ items[n - 1].name }} <!-- n-1 converts to 0-based -->
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</li>
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<!-- BETTER: Just iterate the array directly -->
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<li v-for="(item, index) in items" :key="item.id">
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{{ index + 1 }}. {{ item.name }} <!-- index is 0-based, add 1 for display -->
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</li>
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```
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```html
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<!-- Range for repeating elements (no array involved) -->
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<div v-for="n in 3" :key="n" class="skeleton-row">
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Loading placeholder {{ n }} of 3...
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</div>
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<!-- Output: Loading placeholder 1 of 3, 2 of 3, 3 of 3 -->
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```
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## When Range Iteration Is Useful
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- Rendering a fixed number of placeholder/skeleton elements
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- Creating pagination buttons: `v-for="page in totalPages"`
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- Generating star ratings: `v-for="star in 5"`
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- Repeating template structures a set number of times
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## Reference
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- [Vue.js List Rendering - v-for with a Range](https://vuejs.org/guide/essentials/list.html#v-for-with-a-range)
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