Changes to the scroll behavior API for routing

Changes to the Scroll Behavior API in Vue Router

The scroll behavior API in Vue Router has undergone multiple adjustments across version iterations, evolving from basic configurations to more refined control methods. These changes directly impact how developers handle scroll position restoration during page navigation, particularly in SPAs where maintaining scroll behavior consistent with traditional multi-page applications is crucial.

Basic Scroll Control in Early Versions

In Vue Router 2.x, a simple scrollBehavior function was sufficient to define basic scroll behavior:

const router = new VueRouter({
  scrollBehavior(to, from, savedPosition) {
    if (savedPosition) {
      return savedPosition
    } else {
      return { x: 0, y: 0 }
    }
  }
})

This implementation had clear limitations:

• Only supported synchronous position object returns
• Could not handle scroll positioning for asynchronously loaded content
• Lacked support for container scrolling (limited to window)

Promise Support in Vue Router 3.x

Version 3.x introduced asynchronous solutions, allowing the return of Promise objects:

scrollBehavior(to, from, savedPosition) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve({ x: 0, y: 200 })
    }, 500)
  })
}

Typical use cases included:

1. Waiting for dynamic content to load
2. Coordinating with page transition timing
3. Calculating positions of dynamic elements

// Wait for specific DOM elements to load  
scrollBehavior(to) {
  if (to.hash) {
    return new Promise((resolve) => {
      setTimeout(() => {
        resolve({
          selector: to.hash,
          offset: { x: 0, y: 10 }
        })
      }, 800)
    })
  }
}

Major Changes in Vue Router 4.x

Version 4.0 introduced a complete API overhaul, with key changes including:

1. Parameter signature updates:

// New parameter structure  
scrollBehavior(to, from, savedPosition) {
  // Return type: ScrollPosition | ScrollPositionNormalized  
}

2. Standardized position objects:

interface ScrollPositionNormalized {
  left: number
  top: number
  behavior?: ScrollOptions['behavior']
}

3. Container scrolling support:

const router = createRouter({
  scrollBehavior(to, from, savedPosition) {
    // Specify scroll container  
    return {
      el: '#app-container',
      top: 100
    }
  }
})

Combining Scroll Behavior with Route Meta Fields

Modern practices often integrate meta fields for fine-grained control:

routes: [
  {
    path: '/detail/:id',
    component: DetailPage,
    meta: {
      scrollPos: {
        selector: '.comments-section',
        offset: { y: -20 }
      }
    }
  }
]

Corresponding scroll behavior handling:

scrollBehavior(to) {
  if (to.meta.scrollPos) {
    return {
      ...to.meta.scrollPos,
      behavior: 'smooth'
    }
  }
  return { top: 0 }
}

Integration with Modern Browser APIs

Leveraging ScrollToOptions for advanced effects:

scrollBehavior() {
  return {
    top: 500,
    behavior: 'smooth',
    // New browser features  
    inline: 'center',
    block: 'nearest'
  }
}

Complete example handling edge cases:

scrollBehavior(to, from, savedPosition) {
  // Restore position on browser forward/back navigation  
  if (savedPosition) {
    return savedPosition
  }
  
  // Hash navigation positioning  
  if (to.hash) {
    return {
      el: decodeURIComponent(to.hash),
      behavior: 'smooth',
      top: 20
    }
  }
  
  // Special handling for specific routes  
  if (to.matched.some(record => record.meta.keepScroll)) {
    return false // Disable scrolling  
  }
  
  // Default behavior  
  return new Promise(resolve => {
    requestAnimationFrame(() => {
      resolve({ top: 0, left: 0 })
    })
  })
}

Collaboration with the Composition API

Managing scroll state in setup environments:

import { useRouter } from 'vue-router'

export default {
  setup() {
    const router = useRouter()
    
    router.afterEach((to) => {
      if (to.meta.scrollTop !== false) {
        window.scrollTo({
          top: 0,
          behavior: 'auto'
        })
      }
    })
  }
}

Practical techniques for dynamically modifying scroll behavior:

// Dynamically update scroll target within a component  
onMounted(() => {
  const route = useRoute()
  if (route.query.scrollTo) {
    const target = document.querySelector(route.query.scrollTo)
    target?.scrollIntoView({
      behavior: 'smooth'
    })
  }
})

Handling Race Conditions in Scroll Restoration

Addressing common issues with dynamic content loading:

scrollBehavior(to) {
  return new Promise((resolve) => {
    const checkContentLoaded = () => {
      const targetEl = document.querySelector(to.hash)
      if (targetEl) {
        resolve({
          el: targetEl,
          behavior: 'smooth'
        })
      } else {
        requestAnimationFrame(checkContentLoaded)
      }
    }
    checkContentLoaded()
  })
}

Implementation with timeout fallback:

scrollBehavior(to) {
  return Promise.race([
    new Promise(resolve => {
      const timer = setTimeout(() => {
        resolve({ top: 0 }) // Fallback on timeout  
      }, 1000)
      
      const target = document.querySelector(to.hash)
      if (target) {
        clearTimeout(timer)
        resolve({
          el: target,
          offset: { y: 20 }
        })
      }
    }),
    new Promise(resolve => {
      window.addEventListener('DOMContentLoaded', () => {
        const target = document.querySelector(to.hash)
        resolve(target ? { el: target } : { top: 0 })
      }, { once: true })
    })
  ])
}

Debugging Scroll Behavior

Development tools for debugging:

router.afterEach((to, from) => {
  if (process.env.NODE_ENV === 'development') {
    console.log('[Router] Scroll position:', {
      from: from.path,
      to: to.path,
      savedPosition: router.options.scrollBehavior(to, from, null)
    })
  }
})

Performance monitoring implementation:

const scrollBehavior = router.options.scrollBehavior
router.options.scrollBehavior = function(...args) {
  const start = performance.now()
  const result = scrollBehavior.apply(this, args)
  
  if (result instanceof Promise) {
    return result.finally(() => {
      console.log(`Scroll took ${performance.now() - start}ms`)
    })
  } else {
    console.log(`Scroll took ${performance.now() - start}ms`)
    return result
  }
}