Reading Time Estimation in Svelte: Implementation & Practical Examples

This readingTime.ts utility helps calculate estimated content reading time. Let’s break down how it works and see practical Svelte implementations.

The Utility Functions Explained

1. calculateReadingTime(text: string | null | undefined): number

Purpose: Calculate estimated reading time in minutes
Parameters:

  • text: Content string (can contain HTML)

Process:

  1. Sanitize HTML tags (if present)
  2. Count words
  3. Convert to minutes using 230 words/minute baseline

Full Code:

// src/lib/utils/readingTime.ts

const WORDS_PER_MINUTE = 230; // Average reading speed

export function calculateReadingTime(text: string | null | undefined): number {
    if (!text || text.trim() === '') {
        return 0;
    }

    // 1. Remove HTML tags
    const plainText = text.replace(/<[^>]*>/g, ' ');

    // 2. Count words
    const words = plainText.trim().split(/s+/).filter(Boolean);
    const wordCount = words.length;

    if (wordCount === 0) {
        return 0;
    }

    // 3. Calculate minutes
    const minutes = Math.ceil(wordCount / WORDS_PER_MINUTE);
    
    return Math.max(1, minutes);
}

2. formatReadingTime(text: string | null | undefined): string

Purpose: Format raw minutes into user-friendly string
Parameters:

  • text: Content string

Process:

  1. Get minutes from calculateReadingTime
  2. Return formatted string or empty string

Full Code:

export function formatReadingTime(text: string | null | undefined): string {
    const minutes = calculateReadingTime(text);
    return minutes > 0 ? `${minutes} min read` : '';
}

Svelte Implementation Examples

1. Basic Article Component

<!-- Article.svelte -->
<script lang="ts">
  import { formatReadingTime } from '$lib/utils/readingTime';
  
  export let content = '';
</script>

<article>
  <header>
    {@html content}
    <div class="meta">
      <span>📖 {formatReadingTime(content)}</span>
    </div>
  </header>
</article>

2. Blog Post with Dynamic Content

<!-- BlogPost.svelte -->
<script lang="ts">
  import { formatReadingTime } from '$lib/utils/readingTime';
  import { page } from '$app/stores';
  
  // Get content from CMS/markdown
  $: readingTime = formatReadingTime($page.data?.content);
</script>

<div class="post-header">
  <h1>{$page.data.title}</h1>
  {#if readingTime}
    <small class="reading-time">{readingTime}</small>
  {/if}
</div>

Handling Edge Cases

1. HTML Content

Automatically sanitized:

{formatReadingTime('<p>Hello <strong>World</strong></p>')} 
<!-- Output: "1 min read" -->

2. Empty Content

Returns empty string:

{formatReadingTime('')} <!-- Output: "" -->

Customization Options

1. Adjust Reading Speed

Modify the WORDS_PER_MINUTE constant:

// For faster readers
const WORDS_PER_MINUTE = 300;

2. Custom Formatting

Modify formatReadingTime:

// Localized version
return `${minutes} min de lectura`; // Spanish

// With emoji
return `⏱️ ${minutes}m`;

Best Practices

  1. Use Cases:

    • Blog posts
    • Documentation pages
    • News articles

  2. Performance Tips:

    • Precompute on server with SvelteKit
    • Cache results for static content
    • Use Web Workers for long-form content (>10k words)

  3. SEO Integration:

<svelte:head>
  <meta name="twitter:label1" value="Reading time">
  <meta name="twitter:data1" value={formatReadingTime(content)}>
</svelte:head>

Live Demo Implementation

<script>
  import { formatReadingTime } from '$lib/utils/readingTime';
  let content = '';
</script>

<textarea bind:value={content} placeholder="Type content here..." />
<div class="result">
  {formatReadingTime(content) || 'Start typing...'}
</div>

<style>
  textarea {
    width: 100%;
    height: 200px;
    padding: 1rem;
  }
  
  .result {
    margin-top: 1rem;
    font-size: 1.2em;
    color: #4CAF50;
  }
</style>

FAQ

Q: Why 230 words per minute?
A: Based on average adult reading speed. Adjust for your audience.

Q: Is this XSS-safe?
A: Yes - HTML tags are replaced with spaces before processing.

Q: How accurate is this?
A: ±10% variance typical. Not suitable for scientific use.

Q: Can I use this with Markdown?
A: Absolutely! Works with any text input.

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