TS1170: A computed property name in a type literal must refer to an expression whose type is a literal type or a 'unique symbol' type

TypeScript is a statically typed superset of JavaScript that compiles down to plain JavaScript. It enhances the language by introducing a type system, allowing developers to specify types for variables, function parameters, return values, and more. This adds a layer of safety, making it easier to catch errors during development rather than at runtime.

Types in TypeScript define the shape and behavior of data. They can be simple types like string, number, and boolean, or complex types like interfaces (which define the structure of an object), enums (which define a set of named constants), and more. The ability to explicitly specify types helps improve code readability and maintainability.

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Understanding TS1170

The TypeScript error TS1170 states: A computed property name in a type literal must refer to an expression whose type is a literal type or a 'unique symbol' type. This error typically occurs when you try to use a computed property name in a type definition, but the expression you use is not of a suitable type.

What is a Computed Property Name?

A computed property name allows you to define property names dynamically within an object or type declaration using square brackets []. Here's an example:

const propName = 'dynamicProp';
const obj = {
    [propName]: 'someValue' // This is a computed property name
};

Example that Causes TS1170

Here’s an example that could lead to the TS1170 error:

const dynamicKey = 'firstName';

type User = {
    [dynamicKey]: string; // Causes TS1170
}

Explanation of the Error

In the above code, dynamicKey is a variable and its type is inferred as string. However, TS1170 prompts that computed property names in type literals must either be a literal type (like a string literal "firstName") or a 'unique symbol' type. Since dynamicKey is a variable, it doesn’t meet the requirement.

How to Fix TS1170

To fix this error, you can either use a string literal or create a unique symbol:

Using String Literal:

type User = {
    firstName: string; // Correct: Using string literal directly instead of a variable
}

Using Unique Symbol:

const dynamicKey = Symbol('firstName');

type User = {
    [dynamicKey]: string; // Correct: unique symbol can be used
}

Important to Know!

1. Literal Types: These are types that represent specific values (like 'firstName' or 42).
2. Unique Symbols: Introduced in TypeScript, unique symbols provide a way to create unique identifiers which can be used for computed property names.

Common Misunderstandings

• Why can't I use a variable? Computed property names in types need to ensure consistent shapes for type checking, hence they require literal types or unique symbols.
• What happens if I ignore TS1170? Ignoring this error could lead to unpredictable behavior in your TypeScript code, as object shapes may not be enforced properly.

More Examples that Cause TS1170

Consider another example that can raise TS1170:

const userField = 'age';

type Person = {
    [userField]: number; // Causes TS1170
};

To resolve it:

type Person = {
    age: number; // Correct use with string literal
};

Important to Know!

• Computed Properties are Powerful: They allow for dynamic property names, which can be useful in many scenarios, including working with APIs or dynamic forms.
• Type Safety: TypeScript strives for compile-time safety, ensuring any dynamic property easily fits within the expected type.

FAQ's

Q: What is a unique symbol in TypeScript?

A: A unique symbol is a special type that ensures that every instance created is distinct, which can be useful in preventing naming collisions in large applications.

Q: Can I mix types within a computed property?

A: No, when defining a computed property name in type literals, you must strictly use either a literal type or a unique symbol type.

Conclusion

In conclusion, understanding the error TS1170: A computed property name in a type literal must refer to an expression whose type is a literal type or a 'unique symbol' type is essential for effective TypeScript programming. By ensuring your computed property names adhere to these rules, you can write more robust and type-safe code. If you are eager to learn more about TypeScript and its powerful type system, consider following our blog or utilizing resources like gpteach.