When working with libraries, sometimes we do not know the data type of a previously declared index signature, so it is easy to encounter the Property is incompatible with the index signature in TypeScript error. This article will show you how to fix it, such as converting the index signature to a union type or using any type.
Why does Property is incompatible with index signature in TypeScript error occur
When you assign an object with a property that is not same data type as the index signature of the previously declared data type, the Property is incompatible with the index signature error will appear. Let’s take a look at an example of an error
Error Example:
type Student = {
[key: string]: string;
};
const student = {
age: 20,
name: "John",
};
// Assign an object with a property that is not same data type as the index signature
const newStudent: Student = student;Error Output:
Property 'age' is incompatible with index signature.
Type 'number' is not assignable to type 'string'.In the above example, we create a Student data type with an index signature that we want to receive a string data type, then we create a new student object and assign this student to the newStudent variable of data type is Student, the error occurs because the age attribute is a numeric data type, while the Student data type expects to receive a string.
Solution for Property is incompatible with index signature in TypeScript error
To solve this error, you can change the data type of the index signature to union type, or you can also change the data type to any and disable the data type checking.
Use the union type
You can set the union data type for the index signature to be able to receive different data types.
Example:
type Student = {
// Set the union data type for the index signature to be able to receive different data types
[key: string]: string | number;
};
const student = {
age: 20,
name: "John",
};
const newStudent: Student = student;
console.log(newStudent);Output:
{
"age": 20,
"name": "John"
}In the above example, we add a numeric data type to the index signature, and the error is resolved.
Use any data type
You can use the method of converting the data type of the index signature to any, but that will cause TypeScript to disable error checking.
Example:
type Student = {
// Set the data type of the index signature to any
[key: string]: any;
};
const student = {
age: 20,
name: "John",
};
const newStudent: Student = student;
console.log(newStudent);Output:
{
"age": 20,
"name": "John"
}Summary
Through this article, we have solved the error Property is incompatible with index signature in TypeScript by setting the data type as the union or any. The most common way is to convert it to the union because it will not disable error checking. Hope the article is helpful to you.
Carolyn Hise has three years of software development expertise. Strong familiarity with the following languages is required: Python, Typescript/Nodejs, .Net, Java, C++, and a strong foundation in Object-oriented programming (OOP).