TypeScript errorTypeScript: “Type 'X' is not assignable to type 'Y'” (TS2322)

Why the types don’t match, and the fix

1. 01

   The value can be undefined or null

   With strict mode on, string | undefined does not fit into string. Values from optional props, find(), Map.get(), process.env and form fields are often like this. Check for the missing case, give a default, or let the target accept undefined.

   const user = users.find((u) => u.id === id);
   if (!user) throw new Error(`No user ${id}`);
   setName(user.name); // user is User here
   
   const port: string = process.env.PORT ?? '3000';

2. 02

   An object is missing a property, or has the wrong shape

   “The basic rule for TypeScript’s structural type system is that x is compatible with y if y has at least the same members as x.” TypeScript, Type compatibility

   TypeScript compares objects by their properties, not by their names. An object fits a type when it has at least the properties that type requires, with compatible types. The nested lines of the error point to the property that differs.

   interface Props { title: string; count: number }
   
   // error: property count is missing
   // <Card title="Inbox" />
   <Card title="Inbox" count={unread.length} />

3. 03

   A string where a union of literals is expected

   let size = 'small' is typed as string, which can hold any text, so it does not fit a prop typed 'small' | 'large'. Type the variable with the union, or use const or as const so TypeScript keeps the literal value.

   type Size = 'small' | 'large';
   let size: Size = 'small';
   
   const options = { size: 'small' } as const;
   <Button size={options.size} />

4. 04

   Using as to force it

   “TypeScript only allows type assertions which convert to a more specific or less specific version of a type.” TypeScript, Everyday types: type assertions

   An as cast tells TypeScript to trust you, and it only allows casts between related types. If you need as unknown as Something to make the error go away, the types really do not match: fix the data or the type instead.

   const canvas = document.getElementById('chart') as HTMLCanvasElement; // fine when you know the element
   
   // a warning sign:
   const user = response as unknown as User;

Checked against the vendor’s docs on November 8, 2026: TypeScript Handbook: Type compatibility; TypeScript Handbook: Everyday types.

Rather hand it over?

Fixed scope, fixed price, paid upfront. The work happens in writing; no call needed.

• from $450App Fix TicketOne reproducible bug in an existing mobile or web app, diagnosed and fixed.See scope and start
• from $450AI-Built App RescueA Lovable, Bolt or Cursor app checked end to end in 3 days, then its worst problems fixed at a fixed price.No issues worth fixing, no charge. If the Launch Check finds nothing rated medium or higher, Simon refunds the full $450. You keep the report either way. This applies to the first 10 Launch Checks bought after October 30, 2026.See scope and start

Related: TypeScript “Property does not exist on type” · TypeScript “Cannot find name” · “Cannot read properties of undefined”