No index signature with a parameter of type 'string' was found on type

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Borislav Hadzhiev

Tue Mar 08 20223 min read

No index signature with a parameter of type 'string' was found on type #

The error "No index signature with a parameter of type 'string' was found on type" occurs when we use a value of type string to index an object with specific keys. To solve the error, type the string as one of the object's keys using keyof typeof obj.

Here is an example of how the error occurs.

index.ts
const key = 'country' as string; const obj = { name: 'Tom', country: 'Germany', }; // ⛔️ Error: No index signature with a parameter of type // 'string' was found on type '{ name: string; country: string; }'.ts(7053) console.log(obj[key]);

The key variable has a type of string and this could be any string.

We get the error when we try to access an object that has a name and country properties.

TypeScript is telling us that the type string is too broad and not all strings are keys in the object, so we have to make sure the specific string is one of the object's keys.

The first way to solve the error is to use a type assertion.

index.ts
const key = 'country' as string; const obj = { name: 'Tom', country: 'Germany', }; // 👇️ "Germany" console.log(obj[key as keyof typeof obj]); // 👇️ type OnlyKeys = 'name' | 'country' type OnlyKeys = keyof typeof obj;

We used a type assertion to indicate to TypeScript that the key variable is not of type string, but rather it is a union type containing only the keys of the object.

Now TypeScript lets us access the specific property without throwing the error.

We used keyof typeof to get a union type of the object's keys.

If you are working with a type directly, you would just use keyof MyType to get a union of the object's keys.

index.ts
interface Employee { name: string; country: string; } const obj: Employee = { name: 'Tom', country: 'Germany', }; const key = 'country' as string; // 👇️ "Germany" console.log(obj[key as keyof Employee]); // 👇️ type OnlyKeys = 'name' | 'country' type OnlyKeys = keyof Employee;

Notice that we used keyof Employee and not keyof typeof Employee, because Employee is a type, and not an object.

An even better way to solve this issue is to type the key variable as keyof Employee to indicate to TypeScript that the string will only ever be one of the object's keys.

index.ts
interface Employee { name: string; country: string; } const obj: Employee = { name: 'Tom', country: 'Germany', }; // 👇️ key can only be one of the object's keys const key: keyof Employee = 'country'; // 👇️ "Germany" console.log(obj[key as keyof Employee]);

Now we don't have to use any type assertions, which is much better.

Type assertions are used when we have information about the type of a value that TypeScript can't know about.

When using them, we effectively tell TypeScript that value X will be of type Y and not to worry about it. This could cause runtime errors if we are wrong.

Here is another example of how you could type a value to only be one of the object's keys.

index.ts
interface Employee { name: string; country: string; } const obj1: Employee = { name: 'Tom', country: 'Germany', }; interface AccessEmployee { keyName: keyof Employee; // 👈️ one of Employee's keys } const obj2: AccessEmployee = { keyName: 'country', }; // 👇️ "Germany" console.log(obj1[obj2.keyName]);

We are using the obj2.keyName property to access a property in obj1.

To be able to do this, we had to type the keyName property in obj2 to be of type keyof Employee.

The keyName property in obj2 can only ever have a value of name or country, so TypeScript allows us to safely access the specific properties in obj1.

This is necessary, because not all string values in the world are keys in the specific object. For TypeScript to let us access the object's properties, we have to convince it that the string is one of the object's keys.

This is the most type safe solution to the error, because if we ever try to change the value of obj2.keyName to an incompatible type, we'd get an error.

index.ts
interface Employee { name: string; country: string; } interface AccessEmployee { keyName: keyof Employee; // 👈️ one of Employee's keys } const obj2: AccessEmployee = { keyName: 'country', }; // ⛔️ Type '"something else"' is not assignable // to type 'keyof Employee'.ts(2322) obj2.keyName = 'something else';

Trying to set obj2.keyName to any other value than name or country causes the type checker to issue an error.

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