Inertia adapter(s) affected

• React
• Vue 3
• Svelte
• Not Applicable

JS package version

2.3.18

Backend stack (optional)

(Not applicable, the bug is in @inertiajs/core)

Describe the problem

In getPendingVisit

However, options parameters is typed as VisitOptions (which is Partial<Visit & VisitCallbacks>), which means TypeScript allows properties to be explicitly set to undefined. E.g. when an options object like { only: undefined } is spread into mergedOptions, the explicit undefined value overwrites the default [], producing an object whose runtime value contradicts its declared type (Visit), without exactOptionalPropertyTypes TypeScript does not flag this.

Later in the visit method, visit.only.length, visit.except.length, and visit.reset.length are accessed directly (introduced by PR #2843) without any null/undefined guard (which is fine because the type tell that these properties should be defined). This causes a runtime crash:

Uncaught TypeError: Cannot read properties of undefined (reading 'length')

It happens in practice when options are built dynamically (e.g. conditionally spreading partial option objects, or when a wrapper function passes through options that may be undefined). This bit me in production while migrating from 1.x to 2.x, which is how I ended up here 😅

Expected behavior: router.visit('/url', { only: undefined }) should behave the same as router.visit('/url', {}). The default value ([]) should not be overwritten by an explicit undefined. Otherwise, types should not allow this.

Possibles solutions:

• Strip undefined values from options and configuredOptions before merging. This is the most pragmatic and simple fix.
• Enable exactOptionalPropertyTypes in tsconfig.json to make TypeScript reject { only: undefined } at the type level. This is the "correct" TS solution but has a much larger blast radius (tried it, a lot of errors comes up, unrelated to this, although this does indeed involve other potential risks).

Steps to reproduce

import { router } from '@inertiajs/react' // or vue, or svelte

router.visit('/some-url', { only: undefined })
// => Uncaught TypeError: Cannot read properties of undefined (reading 'length')

The issue is reproducible on the latest main branch (v3).

I'm willing to open a PR if solution 1 or 2 is fine.