Index signature for type ‘#’ is missing in type ‘##’ in TypeScript – How to fix?

The error “Index signature for type ‘#’ is missing in type ‘##'” in TypeScript is a common error when using the index signature. Let’s find out the cause of this error and how we can fix it.

Why do we get this error?

The reason why you got the “Index signature for type is missing in type” error is that TypeScript doesn’t consider a specific type and a type that uses an index signature to be compatible.

The example below is a case where the error occurs:

interface PersonalProfile {
  name: string;
  id: string;
}

const myProfile: PersonalProfile = {
  name: "Bellingham",
  id: "0123456789",
};

const getProfile = (myProfile: { [key: string]: string }) => {
  console.log("My name is: " + myProfile["name"]);
  console.log("My identifier: " + myProfile["id"]);
};

getProfile(myProfile);

Error:

Index signature for type 'string' is missing in type 'PersonalProfile'.

Solution for the error “Index signature for type ‘#’ is missing in type ‘##'” in TypeScript

Using the spread syntax

The spread syntax allows an iteration over the elements of an array or object. 

Spread syntax creates an object’s shallow copy and makes it possible for TypeScript to recognize it as a compatible type.

The spread syntax is shown as a sign …

You can see the example below for a better understanding.

interface PersonalProfile {
  name: string;
  id: string;
}

const myProfile: PersonalProfile = {
  name: "Bellingham",
  id: "0123456789",
};

const getProfile = (myProfile: { [key: string]: string }) => {
  console.log("My name is: " + myProfile["name"]);
  console.log("My identifier: " + myProfile["id"]);
};

getProfile({ ...myProfile });

Output:

"My name is: Bellingham"
"My identifier: 0123456789"

Using the Type Aliases

The type keyword in TypeScript provides type aliases for our variables, objects, and functions. The type aliases describe what our data should look like. To describe what our data type looks like, we use basic types (string, number,…) or by creating our own custom types.

Now, let’s apply to our problem:

type PersonalProfile = {
  name: string;
  id: string;
};

const myProfile: PersonalProfile = {
  name: "Bellingham",
  id: "0123456789",
};

const getProfile = (myProfile: { [key: string]: string }) => {
  console.log("My name is: " + myProfile["name"]);
  console.log("My identifier: " + myProfile["id"]);
};

getProfile(myProfile);

Output:

"My name is: Bellingham"
"My identifier: 0123456789"

As you can see, now you don’t need to use the spread operator.

Using the Type Alias ​​can be considered a safer way to infer an implicit index signature for a type alias rather than an interface because it cannot be augmented.

Summary

In summary, there are two ways for you to resolve the error “Index signature for type ‘#’ is missing in type ‘##'” in TypeScript. We hope you can successfully apply one of the methods mentioned above. That’s the end of this article. Thanks for reading.

Maybe you are interested:

• “Types have separate declarations of a private property” in TypeScript
• No index signature with a parameter of type ‘string’ was found on type
• Type ‘#’ is missing the following properties from type ‘##’

Christopher

My name’s Christopher Gonzalez. I graduated from HUST two years ago, and my major is IT. So I’m here to assist you in learning programming languages. If you have any questions about Python, JavaScript, TypeScript, Node.js, React.js, let’s contact me. I will back you up.

Name of the university: HUST
Major: IT
Programming Languages: Python, JavaScript, TypeScript, Node.js, React.js