TypeScript Type Manipulation : Template Literal Types

Welcome to our tutorial on TypeScript type manipulation with template literal types. In this tutorial, we will explore the power of template literal types in TypeScript and how they can be used to manipulate and define complex types.

TypeScript is a superset of JavaScript that adds static types to the language. This allows developers to catch errors and bugs at compile-time rather than at runtime, resulting in more robust and maintainable code. TypeScript provides various features for type manipulation, one of which is template literal types.

Template literal types allow us to manipulate string literals and create more sophisticated and precise types. They enable us to create type-safe string manipulation functions, generate types based on computed values, and create recursive types. Understanding and mastering template literal types can greatly enhance our ability to write scalable and safe TypeScript code.

In this tutorial, we will start with the basics of template literal types and gradually delve into more advanced concepts and use cases. We will cover topics such as string concatenation, union types, conditional types, and mapped types with template literal types. By the end of this tutorial, you will have a solid understanding of template literal types and how to leverage them to create powerful and expressive TypeScript types.

TypeScript Type Manipulation: Template Literal Types Tutorial

Introduction

Template Literal Types in TypeScript allow for powerful type manipulation by combining literal types and string interpolation. This tutorial explores how to use template literal types to dynamically create and manipulate types.

Creating Template Literal Types

To create a template literal type, enclose the string literal within backticks (\`), preceded by the `typeof` keyword. For example:

type Greeting = \`Hello, \${string}\`;

In this example, `Greeting` is a template literal type that represents a string starting with “Hello, ” followed by any string value.

Concatenating Template Literal Types

Template literal types can be concatenated using the `|` (union) operator. This allows for the creation of more complex types. For example:

type Greeting = \`Hello, \${'world' | 'friend'}!\`;

In this example, `Greeting` represents a string that can either be “Hello, world!” or “Hello, friend!”.

Manipulating Template Literal Types

Template literal types can be manipulated using conditional types and mapped types.

Conditional Types

Conditional types allow for the conditional selection of types based on a condition. They can be used to create more dynamic template literal types.

type Greeting = T extends 'world' ? \`Hello, \${T}!\` : 'Invalid greeting';

In this example, the `Greeting` type uses a conditional type to check if the generic type `T` is equal to `’world’`. If it is, it returns a template literal type representing a valid greeting. Otherwise, it returns the string `’Invalid greeting’`.

Mapped Types

Mapped types allow for the transformation of each property in a type. They can be used to manipulate template literal types by transforming specific parts of the string.

type CapitalizeFirstLetter = T extends `${infer First}${infer Rest}` ? `${Uppercase}${Rest}` : T;

In this example, the `CapitalizeFirstLetter` type uses mapped types to capitalize the first letter of a string literal. It achieves this by splitting the string into the first letter (`First`) and the rest of the string (`Rest`), then capitalizing the first letter using the `Uppercase` utility type.

Conclusion

Template Literal Types in TypeScript provide a powerful way to dynamically create and manipulate types. By combining literal types, string interpolation, conditional types, and mapped types, developers can create complex and flexible type systems for their applications.

Understanding String Template Literal Types

String template literal types are a powerful feature in TypeScript that allow you to create more expressive and flexible string types. They are a type system enhancement that takes advantage of TypeScript’s support for literal types and template literals.

What are String Template Literal Types?

String template literal types in TypeScript are a way to define string types that can include specific patterns or formats. They allow you to express complex string constraints that are enforced by the TypeScript compiler.

How do String Template Literal Types work?

String template literal types work by using a combination of literal types and template literals. Template literals are enclosed in backticks (\` \`) and can contain placeholders (expressed using `${}`) which are replaced with values when the template literal is evaluated.

When defining a string template literal type, you can use template literal syntax along with literal types to create a string type that matches specific patterns. For example, you can define a string type that only allows strings starting with “user-” followed by a number:

type Username = \`user-\${number}\`;

In this example, the type `Username` is defined as a string type that must start with the literal “user-” followed by a number. The TypeScript compiler will enforce this constraint and show an error if you try to assign a value that doesn’t match this pattern.

Benefits of String Template Literal Types

String template literal types provide several benefits:

• Enhanced readability: String template literal types allow you to express complex string patterns in a more readable and concise way, improving the clarity of your code.
• Improved type safety: By defining specific string patterns, you can catch errors at compile-time and prevent mismatched or invalid string values from being assigned to variables.
• Code completion and IntelliSense: IDEs that support TypeScript can provide code completion and IntelliSense for string template literal types, making it easier to write code that conforms to the specified pattern.

Overall, string template literal types are a powerful tool in TypeScript for enforcing string patterns and improving type safety. With their expressive syntax and enhanced readability, they provide a flexible and robust way to define string types in your code.

Working with Number Template Literal Types

Number template literal types allow you to create complex types by manipulating literal types that represent numeric values. With number template literal types, you can perform mathematical operations, define ranges, and create type-safe numeric patterns.

Creating Range Types

Number template literal types can be used to define ranges of numbers. This can be done by specifying a minimum and maximum value using the `…` syntax. For example, to create a range type from 1 to 10, you can use:

type Range = 1...10;

This will create a type `Range` that can only accept values between 1 and 10.

Performing Mathematical Operations

Number template literal types also allow you to perform basic mathematical operations. For example, you can add, subtract, multiply, and divide literal types to create new types. Here are some examples:

type Sum = 5 + 3; // Result: 8

type Difference = 10 - 6; // Result: 4
type Product = 2 * 4; // Result: 8
type Quotient = 20 / 5; // Result: 4

These operations can be nested and combined to create more complex types. For example:

type Calculation = ((2 + 3) * (4 - 1)) / 2; // Result: 7

Type-Safe Numeric Patterns

Number template literal types can be used to create type-safe numeric patterns. This can be useful when defining value ranges, enumerations, or patterns that follow a specific numeric structure. Here’s an example:

type EvenNumber = number & { toString: `${number}`; } & (number extends number ? number : never) & (number % 2 extends 0 ? number : never);

This creates a new type `EvenNumber` that can only accept even numbers. It combines different constraints on the numeric type to ensure that only even numbers are valid.

In Conclusion

Number template literal types provide powerful tools for working with numeric values in TypeScript. They allow you to define ranges, perform mathematical operations, and create type-safe numeric patterns. By leveraging number template literal types, you can create more accurate and expressive type definitions in your TypeScript code.

Manipulating Boolean Template Literal Types

Template literal types in TypeScript allow you to manipulate and constrain string values in TypeScript. In addition to manipulating string values, you can also manipulate boolean values using template literal types.

Defining Boolean Template Literal Types

To define a boolean template literal type, you can use the `true` and `false` keywords as literal values in a union type. For example:

“`typescript

type YesOrNo = true | false;

“`

This `YesOrNo` type represents a boolean template literal type that can only be either `true` or `false`.

Using Boolean Template Literal Types

You can use boolean template literal types to create more precise type constraint situations. For example, consider the following function:

“`typescript

function validateYes(value: YesOrNo): boolean {

return value === true;

}

“`

This function takes a boolean template literal type `YesOrNo` as an argument and returns `true` only if the value is `true`. In all other cases, it returns `false`.

Combining Boolean Template Literal Types with Conditional Types

Boolean template literal types can be combined with conditional types to create more advanced type constraints. For example, consider the following type:

“`typescript

type CheckYesOrNo = T extends true ? “Yes” : “No”;

“`

This `CheckYesOrNo` type is a conditional type that takes a boolean template literal type `T` and returns the string literal type `”Yes”` if `T` is `true`, and `”No”` if `T` is `false`.

You can use this type to create more specific type constraints based on boolean template literal types.

Conclusion

Boolean template literal types in TypeScript provide a way to manipulate and constrain boolean values. They can be used to create more accurate and precise type constraints in your TypeScript code.

Combining Template Literal Types with Union Types

Template literal types and union types can be combined to create more flexible and powerful types in TypeScript. With the ability to define a pattern of strings using template literal types, and the ability to represent multiple possible types using union types, we can create complex type definitions that suit our needs.

Defining Union Types with Template Literal Types

One way to combine template literal types with union types is by defining a union type where each member is a template literal type. This allows us to define a set of strings that our type can be.

“`typescript

type Color = ‘red’ | ‘blue’ | ‘green’;

“`

In the example above, we define a type called `Color` which can only be one of the three specified values: `’red’`, `’blue’`, or `’green’`.

Combining Template Literal Types with Union Types

We can also combine template literal types with union types in more complex ways. For example, suppose we want to define a type for a CSS class name, which can start with either `’btn-‘` or `’text-‘`, followed by a specific class name. We can achieve this by combining template literal types with union types.

“`typescript

type ClassName =

| `btn-${‘primary’ | ‘secondary’}`

| `text-${‘small’ | ‘medium’ | ‘large’}`;

“`

In the example above, we define a type called `ClassName` which can be either a button class name (e.g., `’btn-primary’` or `’btn-secondary’`) or a text class name (e.g., `’text-small’`, `’text-medium’`, or `’text-large’`).

Using Combined Types

Once we have defined our combined types, we can use them in various contexts, such as function parameters, return types, or object properties.

“`typescript

function applyClass(className: ClassName): void {

// Apply the class name to the element

}

const button: Button = {

text: ‘Click me’,

class: ‘btn-primary’,

};

applyClass(button.class);

“`

In the example above, we define a function called `applyClass` that takes a `ClassName` as a parameter. We then define a button object with a `class` property as a `ClassName`. Finally, we pass the `class` property to the `applyClass` function.

Conclusion

By combining template literal types and union types in TypeScript, we can create powerful and flexible type definitions that accurately represent the data we are working with. This allows us to catch potential errors at compile-time and write more robust and reliable code.

Applying Template Literal Types in Function Signatures

If we want to apply template literal types in function signatures, we can use them as generic type parameters.

Let’s take a look at an example:

“`typescript

function capitalizeFirstCharacter(

str: T

): `Captialized ${T}` {

return `Captialized ${str.charAt(0).toUpperCase()}${str.slice(1)}`;

}

“`

In this example, we define a function called `capitalizeFirstCharacter` which takes a generic type parameter `T` that extends `string`. The function takes a string `str` as an argument and returns a template literal type that starts with the word “Capitalized” followed by the capitalized version of the input string.

Here’s how we can use the `capitalizeFirstCharacter` function:

“`typescript

const result = capitalizeFirstCharacter(“hello”);

// result: “Capitalized Hello”

“`

The return type of the `capitalizeFirstCharacter` function is automatically inferred to be the template literal type `”Capitalized Hello”`. This is because the generic type parameter `T` is inferred to be the string literal type `”hello”`, and the return type is then constructed based on this type.

By using template literal types in function signatures, we can create type-safe functions that generate more specific return types based on the input types.

Conclusion

Template literal types in TypeScript allow us to define and manipulate string literal types in a more expressive way. By using them in function signatures, we can create functions with more specific return types based on the input types.

Hopefully, this tutorial has given you a good understanding of how to use template literal types in TypeScript. Happy coding!

Defining Objects with Template Literal Types

Template Literal Types in TypeScript provide a powerful way to define and manipulate object types at compile-time. They allow us to create object types using string templates, enabling dynamic and flexible typing.

Basic Syntax

The basic syntax for defining objects with template literal types is as follows:

type MyObject = { [Key in keyof SomeType]: ValueType };

Here, SomeType is the type of an existing object, and ValueType is the desired type for each property of the new object. The resulting MyObject type will have the same keys as SomeType, but with all values replaced by ValueType.

Example

Let’s say we have an existing object type called Person with the following structure:

type Person = {

name: string;
age: number;
email: string;
};

We can use template literal types to define a new object type called PartialPerson which has the same keys as Person, but with all values marked as optional:

type PartialPerson = { [Key in keyof Person]?: Person[Key] };

The resulting PartialPerson type will have the following structure:

{

name?: string;
age?: number;
email?: string;
}

This allows us to create objects with partial property values, as shown in the following example:

const person1: PartialPerson = {

name: "John",
age: 30,
};
const person2: PartialPerson = {
name: "Jane",
email: "[email protected]",
};

Conclusion

Template Literal Types in TypeScript provide a powerful way to define object types using string templates. They enable us to create dynamic and flexible typing for object properties, allowing us to manipulate and transform types at compile-time.

Using Template Literal Types to Generate Custom Types

Template literal types in TypeScript allow you to generate custom types by combining string literals and type expressions using the `${}` syntax. This powerful feature enables you to create reusable and type-safe types.

Dynamic Property Names

Template literal types can be used to generate types with dynamic property names. By using template literal types, you can create types where property names are based on other types or values.

For example, consider the following code:

“`typescript

type User = {

id: number;

name: string;

email: string;

};

type KeyofUser = keyof User; // “id” | “name” | “email”

type DynamicPropertyNames = {

[K in keyof T as `get${Capitalize}ById`]: (id: number) => T[K];

};

type UserAPI = DynamicPropertyNames;

const userApi: UserAPI = {

getNameById: (id) => “John Doe”,

getEmailById: (id) => “[email protected]”,

};

“`

In this example, the `DynamicPropertyNames` type is a mapped type that generates new property names with the prefix “get” and the capitalized property name followed by “ById”. The resulting `UserAPI` type will have properties like `getNameById` and `getEmailById` that accept an `id` parameter and return the corresponding property value from the `User` type.

Conditional Types

Template literal types can also be used in conditional types to create custom types based on conditionals. By combining template literal types with conditional types, you can generate types that are dependent on certain conditions.

Here’s an example:

“`typescript

type IsEqual = A extends B ? (B extends A ? true : false) : false;

type IsString = IsEqual;

type Foo = T extends string ? `${T} is a string` : `${T} is not a string`;

type Bar = Foo; // “string is a string”

type Baz = Foo; // “number is not a string”

“`

In this example, the `IsString` type is a conditional type that checks if a given type `T` is equal to `string`. The `Foo` type uses template literal types to create a custom type based on the result of the condition. If `T` is a `string`, the resulting type will be `T is a string`. Otherwise, it will be `T is not a string`.

Parameterize Types

Template literal types can also be parameterized to generate more flexible and reusable types. By using type parameters within template literal types, you can create types that adapt to different input values.

Consider the following example:

“`typescript

type FormatString = T extends “JSON” ? JSON.stringify : T extends “CSV” ? “some csv formatting” : never;

type TypeFormat = { format: T extends string ? FormatString : never };

const jsonType: TypeFormat<"json"> = { format: JSON.stringify };

const csvType: TypeFormat<"csv"> = { format: “some csv formatting” };

const invalidType: TypeFormat<"xml"> = { format: “invalid format” }; // Error: Type ‘”XML”‘ is not assignable to type ‘”JSON” | “CSV”‘

“`

In this example, the `FormatString` type is parameterized with the input value and returns a corresponding formatting function or string. The `TypeFormat` type takes a type parameter and generates a custom type where the `format` property is assigned the formatting function or string based on the input value.

Conclusion

Template literal types in TypeScript provide a powerful way to generate custom types by combining string literals and type expressions. They allow you to create dynamic property names, conditionally generate types, and parameterize types to create flexible and reusable types. By leveraging the capabilities of template literal types, you can enhance the type safety and expressiveness of your TypeScript code.

Creating Conditional Types with Template Literal Types

Template literal types in TypeScript allow us to create powerful conditional types that make our code more flexible and reusable. By using conditional types, we can define types that depend on some condition or input value.

Conditional Types

Conditional types are a feature introduced in TypeScript 2.8 that allow us to define generic types based on a condition. They are similar to conditional statements in other programming languages.

One of the main use cases for conditional types is to create type guards. Type guards are expressions that define a condition as a type predicate. They help the TypeScript compiler narrow down the type of a variable based on some condition.

Defining Conditional Types with Template Literal Types

Template literal types can be combined with conditional types to create powerful type definitions. By using template literal types, we can define custom types that depend on the specific values of variables.

Here’s an example:

“`typescript

type CheckTruthy = T extends true ? “Truthy” : “Falsy”;

type Result = CheckTruthy; // Result: “Truthy”

“`

In this example, we define a conditional type `CheckTruthy` that takes a generic parameter `T`. If `T` extends `true`, the type is set to “Truthy”. Otherwise, the type is set to “Falsy”.

Using Template Literal Types with Conditional Types

We can also use template literal types inside a conditional type to create more complex type definitions. For example, we can define a conditional type that depends on the length of an array:

“`typescript

type ArrayLength = T[“length”] extends 0 ? “Empty” : “Non-empty”;

type EmptyArray = ArrayLength<[]>; // Result: “Empty”

type NonEmptyArray = ArrayLength<[1, 2,="" 3]="">; // Result: “Non-empty”

“`

In this example, we define a conditional type `ArrayLength` that takes a generic parameter `T` which extends `any[]` (an array). If the length of the array is 0, the type is set to “Empty”. Otherwise, the type is set to “Non-empty”.

Conclusion

Template literal types combined with conditional types allow us to create flexible and reusable type definitions in TypeScript. By using conditional types, we can define types that depend on some condition or input value, making our code more robust and maintainable.

Implementing Template Literal Types in Generic Types

Template literal types in TypeScript allow us to create generic types that are based on the specific string literal value that is passed as a type parameter. This powerful feature can be used to create more precise and expressive types.

Using Template Literal Types in Generic Types

To implement template literal types in generic types, we can use the syntax `${string}` where the string inside the curly braces represents the allowed string literal values. Let’s consider an example where we want to create a generic type that represents an object with specific keys and values:

“`typescript

type ObjectWithKeys = {

[key in K]: V;

};

“`

In the code above, we define a generic type ObjectWithKeys with two type parameters: K represents the allowed keys as a string literal type, and V represents the values for each key.

We use the template literal type syntax `${string}` to define the keys in the resulting object type. This ensures that only the specified keys are allowed when creating objects using this generic type.

Example Usage

Now, let’s see how we can use the ObjectWithKeys generic type:

“`typescript

const person: ObjectWithKeys<'name' |="" 'age',="" string="" |="" number=""> = {

name: ‘John’,

age: 30,

};

“`

In the example above, we create an object person using the ObjectWithKeys type. We specify the allowed keys as the string literal type 'name' | 'age', and the allowed values as string | number. This ensures that the resulting object must have the keys 'name' and 'age', and their corresponding values can only be of type string or number.

Benefits of Template Literal Types in Generic Types

• Increased type safety: By using template literal types in generic types, we can enforce specific keys and values in our code, reducing the chance of errors.
• Improved code readability and expressiveness: Template literal types allow us to create more self-descriptive types by specifying the allowed keys and values.
• Code reusability: We can create reusable generic types that can be used across different parts of our codebase, ensuring consistency and reducing duplication.

Overall, implementing template literal types in generic types in TypeScript can significantly enhance the type system and improve the development experience by providing more precise and expressive types.

Applying Template Literal Types in Mapped Types

In TypeScript, we can use template literal types to create powerful mapped types that allow us to transform and manipulate types dynamically based on string patterns. By combining template literal types with mapped types, we can create type transformations that are both concise and expressive.

What are Mapped Types?

Mapped types in TypeScript allow us to create new types based on an existing type by applying a mapping operation to each property in the original type. The mapping operation is defined through a mapped type parameter that specifies how each property should be transformed.

For example, consider the following mapped type that transforms all properties in an object type to be optional:

type Optional<T> = {[K in keyof T]?: T[K]};

Here, the mapped type parameter K iterates over each key in the original type T and transforms it into an equivalent optional property.

Using Template Literal Types in Mapped Types

We can apply template literal types within mapped types to create even more powerful transformations. Template literal types allow us to create types that depend on string patterns, enabling us to dynamically generate type names or perform operations based on specific string values.

For example, we can use template literal types to create a mapped type that adds a prefix to each property in an object type:

type AddPrefix<T> = { [K in keyof T as `prefix_${K}`]: T[K] };

In this example, the mapped type parameter K is transformed using a template literal type `prefix_${K}`, which adds the prefix prefix_ to each key in the original type. The resulting mapped type AddPrefix<T> will have the same property names as the original type, but with the prefix added.

Other Use Cases

Template literal types in mapped types can be used in various scenarios, such as:

• Removing specific properties from an object type based on a string pattern
• Renaming specific properties in an object type based on a string pattern
• Transforming the values of specific properties in an object type based on a string pattern

By combining the flexibility of template literal types with the power of mapped types, we can create complex type transformations that align with our specific needs.

Conclusion

In this article, we have explored the application of template literal types in mapped types in TypeScript. By leveraging template literal types, we can create dynamic type transformations by applying string patterns to existing types. This allows us to create concise and expressive type manipulations that suit our specific requirements.

FAQ:

What are template literal types in TypeScript?

Template literal types in TypeScript allow you to create new types by manipulating string literals. They use backticks (\`) and the ${} syntax to interpolate values into strings.

How do you create a template literal type in TypeScript?

To create a template literal type, you use the `as` keyword followed by the template literal expression. For example: `let myVar: `Hello ${‘World’}`;`

Can you use template literal types to manipulate other types?

Yes, you can use template literal types to manipulate other types. For example, you can concatenate two types using template literal types: `type Concat = `${T}${U}`;`.

What are some common use cases for template literal types?

Some common use cases for template literal types include creating new types based on existing types with slight modifications, generating complex key names for object types, and creating typesafe URL routing systems.

How do you infer values with template literal types?

You can infer values with template literal types using conditional types and type inference. For example, you can create a utility type that infers the keys of an object type: `type Keys = keyof T;`

What are some potential drawbacks of using template literal types?

Some potential drawbacks of using template literal types include increased complexity and reduced readability of code, as well as potential limitations or edge cases in type inference and manipulation.

Are template literal types supported in all versions of TypeScript?

Template literal types were introduced in TypeScript 4.1, so they are not supported in earlier versions of TypeScript.