Fix: Invalid Entry Inside getStaticPaths Return Value in Astro

What Causes This Error?

This error occurs when the array returned by getStaticPaths() contains entries that are not objects. Each entry must be an object with at least a params property.

The Problem

---
// ❌ Array contains strings instead of objects
export function getStaticPaths() {
  return ['post-1', 'post-2', 'post-3'];
}

// ❌ Array contains null
export function getStaticPaths() {
  return [null, { params: { slug: 'valid' } }];
}

// ❌ Array contains numbers
export function getStaticPaths() {
  return [1, 2, 3];
}
---

The Fix

Use Object Entries

---
// ✅ Each entry is an object with params
export function getStaticPaths() {
  return [
    { params: { slug: 'post-1' } },
    { params: { slug: 'post-2' } },
    { params: { slug: 'post-3' } },
  ];
}
---

Transform Data Properly

---
const slugs = ['post-1', 'post-2', 'post-3'];

// ❌ Wrong
export function getStaticPaths() {
  return slugs;
}

// ✅ Correct - map to objects
export function getStaticPaths() {
  return slugs.map((slug) => ({
    params: { slug },
  }));
}
---

Common Scenarios

Filtering Returns Invalid Entries

---
const posts = [
  { slug: 'post-1', published: true },
  { slug: 'post-2', published: false },
  { slug: 'post-3', published: true },
];

// ❌ Wrong order - filter then map leaves non-objects
export function getStaticPaths() {
  return posts
    .map((post) => post.published ? { params: { slug: post.slug } } : null)
    .filter(Boolean); // Works but confusing
}

// ✅ Better - filter first, then map
export function getStaticPaths() {
  return posts
    .filter((post) => post.published)
    .map((post) => ({
      params: { slug: post.slug },
    }));
}
---

Async Data with Nulls

---
export async function getStaticPaths() {
  const data = await fetchData();

  // ❌ Some items might be null
  return data.map((item) => item && {
    params: { id: item.id },
  });
}

// ✅ Filter out nulls
export async function getStaticPaths() {
  const data = await fetchData();

  return data
    .filter((item) => item != null)
    .map((item) => ({
      params: { id: item.id },
    }));
}
---

Flattening Nested Arrays

---
const categories = [
  { name: 'tech', posts: ['post-1', 'post-2'] },
  { name: 'life', posts: ['post-3'] },
];

// ❌ Results in nested arrays
export function getStaticPaths() {
  return categories.map((cat) =>
    cat.posts.map((slug) => ({ params: { slug } }))
  );
}

// ✅ Use flatMap
export function getStaticPaths() {
  return categories.flatMap((cat) =>
    cat.posts.map((slug) => ({
      params: { category: cat.name, slug },
    }))
  );
}
---

Spread Operator Issues

---
// ❌ Spreading strings creates character entries
const slug = 'post';
export function getStaticPaths() {
  return [...slug].map((char) => ({ params: { slug: char } }));
  // Results in 'p', 'o', 's', 't' as separate paths
}

// ✅ Handle as single item
export function getStaticPaths() {
  return [{ params: { slug: 'post' } }];
}
---

Quick Checklist

• Every array entry must be an object
• Each object needs { params: { ... } }
• Filter out null/undefined before mapping
• Use .filter() before .map() for cleaner code
• Use .flatMap() for nested data structures