Error in Index Signature Assignment

🔎 Search Terms

"index signature error", "numeric index incompatible with interface", "index signature assignment error", "object literal known properties error", "interface index signature bug", "interface property compatibility error"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about index signatures

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.4.5#code/PQKgBAxghgzgpmAjGEwBQBLAdgFzgJwDMoIEBJAFQHsAHCgTxoQG80wwcBpOegLjBg582AOYBuNAF80mXAWKkwZAMoBXAEYMmYOAA88WACYwl1OoxZsB3PgKGiJ02XiIlyFOILJyFl9gG0aAEF+QWEsEQBdfkpaLTgJAJoAIX4sVQBbdQJopTVNC0cZCCosQQ5PHAB5dQArGI8vHzcwAF4wVnYofmYOG34AIhwANSgAGwGwSQAaK0Qevp5BkfGB6eslsAGYUYmptGBgdmOT07OAPTAACwI4AHITHBudfHwqfCkJA6OAWj+wP4-AH-GSgSCwBAAJhQ6GwLl8pjiFgA+tDOot6KjQvYIkVnPIWioNPFUTp9HAjCZYuYmKT0TAbFi7OFxFIZHCCYpKJVvHhfHSrIEgkywqJctSSZDEmBAskmQADGAaZEAEmYooiknl4vykrxJTKOAqghqtSZ3KafLcpPa6O6HQxTKGu1JkxmViV6mR6gWXB4TpWY1d6wZ-shgx241RbqshzO8YT8cuN3w90ezwIbw+kjEQA

💻 Code

/* case 1 */
interface ITopType {
  tKey: string;
}

interface ISubType extends ITopType {
  sKey: string;
}

interface ITestInteface {
  [pA: string]: ITopType;
  [pB: number]: ISubType;
}

const testObj: ITestInteface = {
  a: { tKey: "tVal" },
  1: { tKey: "tVal", sKey: "sVal" }
//                    ^ here's the error
};

// --- --- --- 

/* case 2 */
interface ITopType_2 {
  tKey_2: string;
}

interface ISubType_2 extends ITopType_2 {
  sKey_2: string;
}

interface ITestInteface_2 {
  [pA_2: string]: ITopType_2;
  [pB_2: `sub_${string}`]: ISubType_2;
}

const testObj_2: ITestInteface_2 = {
  a: { tKey_2: "tVal_2 " },
  sub_b: { tKey_2: "tVal_2 ", sKey_2: "sVal_2" }
  //                            ^ here's the error
};

🙁 Actual behavior

The TypeScript handbook states that it is possible to support both types (string and number) of indexers, but the type returned from a numeric indexer must be a subtype of the type returned from the string indexer. However, when attempting to implement this, I encountered an error.

/* case 1 */
interface ITopType {
  tKey: string;
}

interface ISubType extends ITopType {
  sKey: string;
}

interface ITestInteface {
  [pA: string]: ITopType;
  [pB: number]: ISubType;
}

const testObj: ITestInteface = {
  a: { tKey: "tVal" },
  1: { tKey: "tVal", sKey: "sVal" }
//                    ^ here's the error
};

The error:

Type '{ a: { tKey: string; }; 1: { tKey: string; sKey: string; }; }' is not assignable to type 'ITestInteface'.
  Property '1' is incompatible with index signature.
    Object literal may only specify known properties, and 'sKey' does not exist in type 'ITopType'.(2322)

There are several approaches to rectify this error (type assertion etc.), but they involve rewriting code and modifying constructs.

The same issue can occur here even without the use of a number index signature.

/* case 2 */
interface ITopType_2 {
  tKey_2: string;
}

interface ISubType_2 extends ITopType_2 {
  sKey_2: string;
}

interface ITestInteface_2 {
  [pA_2: string]: ITopType_2;
  [pB_2: `sub_${string}`]: ISubType_2;
}

const testObj_2: ITestInteface_2 = {
  a: { tKey_2: "tVal_2 " },
  sub_b: { tKey_2: "tVal_2 ", sKey_2: "sVal_2" }
  //                            ^ here's the error
};

The second error:

Type '{ a: { tKey_2: string; }; sub_b: { tKey_2: string; sKey_2: string; }; }' is not assignable to type 'ITestInteface_2'.
  Property 'sub_b' is incompatible with index signature.
    Object literal may only specify known properties, but 'sKey_2' does not exist in type 'ITopType_2'. Did you mean to write 'tKey_2'?(2322)

🙂 Expected behavior

The TypeScript compiler should allow assignment to the number index signature and the template string pattern index signature.

Additional information about the issue

No response

Start with the linked TypeScript Playground examples and reproduce both assignment errors. Then trace the compiler's handling of object-literal excess-property checks for numeric and template-string index signatures. Done means the intended assignments are accepted without weakening unrelated index-signature checks, with regression coverage added for both cases.