TypeScript errorTypeScript: “Argument of type 'X' is not assignable to parameter of type 'Y'” (TS2345)

Why the types do not match, and the fix

1. 01

   undefined or null in the argument

   Optional props, find(), environment variables and form values can all be undefined. Handle that case before the call, so the type inside the if is the plain type.

   const apiUrl = process.env.API_URL;
   if (!apiUrl) throw new Error('API_URL is not set');
   connect(apiUrl); // string, not string | undefined

2. 02

   A string where specific strings are expected

   let method = 'GET' widens to string, which a parameter typed 'GET' | 'POST' rejects. Use const, as const, or annotate the variable with the union.

   type Method = 'GET' | 'POST';
   const method: Method = 'GET';
   
   const options = { method: 'GET' } as const;

3. 03

   An object that 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 members. The lines under the error show the first property that does not fit. Add it, fix its type, or change the parameter type if the function really accepts less.

4. 04

   React event and ref types

   Passing a handler typed for one element to another (ChangeEvent<HTMLInputElement> on a select) or a ref of the wrong element type gives the same kind of mismatch. In JSX it is reported as TS2322 on the prop; TS2345 appears when you call the handler yourself. Type the handler for the element it is attached to.

   const onChange = (e: React.ChangeEvent<HTMLSelectElement>) => setPlan(e.target.value);

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

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Related: TypeScript “Type is not assignable to type” · TypeScript “Object is possibly null” · TypeScript “Property does not exist on type”