Union types and intersection types

In TypeScript, union types and intersection types are two important ways of combining types in the type system. Union types are used to indicate that a value can be one of several types, while intersection types are used to merge multiple types into a single type. Union types are commonly used for variables that may be of multiple types and require type guards to narrow down the scope, whereas intersection types are primarily used to combine multiple interfaces or type aliases to create a new type that includes all properties. The key difference is that unions represent an "or" relationship, while intersections represent an "and" relationship. This article delves into their application scenarios, including their combination with literal types, conditional types, and mapped types, as well as their practical use in React higher-order components. It also introduces advanced techniques such as type guards, type inference, and function overloads, demonstrating how to leverage these features to build a more precise and flexible type system.

Function type definition

TypeScript's function type definitions provide rich functionality for describing function behavior, including parameter and return value constraints. The basic syntax uses type annotations and interfaces, with arrow syntax to define parameter and return types. It supports optional parameters, default parameters, and rest parameters for handling variable numbers of arguments. Function overloading allows multiple type definitions for the same function. Generic functions handle multiple types while preserving type information. Higher-order functions process functions as parameters or return values. Constructor types are defined using the `new` keyword. Asynchronous functions handle `Promise` return values. Interfaces fully describe function types, supporting type inference and contextual typing. Complex examples combine multiple features. Function type compatibility adopts a structural type system. Advanced patterns use conditional types and mapped types to create flexible function types.

Object types and interfaces

In TypeScript, object types and interfaces are essential tools for defining data structures. Object types describe the shape of values, while interfaces are more powerful. Both can constrain the structure of variables, function parameters, and return values. Interfaces can include optional and read-only properties, providing flexibility, and can also describe function types and indexable types. Interfaces support inheritance and mixed types, allowing them to extend other interfaces to form complex definitions. Type aliases and interfaces share similarities, but type aliases can represent primitive types, union types, and tuples, whereas interfaces are better suited for defining object shapes. Interfaces have a close relationship with classes—classes can implement interfaces to ensure all members are provided. Interfaces can also define function overloads to handle different parameter combinations. When combined with type assertions and type guards, interfaces enable safer code. Generic constraints allow limiting the scope of generic parameters. While decorators are primarily used with classes, they can be combined with interfaces through techniques to enhance code maintainability.

Array and tuple types

In TypeScript, arrays and tuples are two fundamental compound types. Arrays are used to store sequences of elements of the same type with variable length, supporting two declaration syntaxes and enforcing consistent element types. Empty arrays require explicit type annotations; otherwise, they are inferred as `any[]`. Readonly arrays can be created using `ReadonlyArray` or the `readonly` modifier. Tuples represent collections of elements with a known quantity and type, where each element can have a different type. They enforce strict checks on position and type, support optional elements, and allow rest syntax. Arrays are variable in length, while tuples have a fixed length—though the `push` method can bypass this restriction. Arrays handle homogeneous data, whereas tuples are suited for precise structures, such as React's `useState` or CSV parsing. Union-type arrays allow mixed elements. Const assertions can create readonly tuples, and mapped types can transform tuple structures. Common issues include tuple length overflow and mutating tuple members, which can be resolved through type assertions or precise type definitions. A defensive approach involves using the `readonly` modifier.

Primitive types and literal types

In TypeScript, primitive types include number, string, boolean, null, undefined, symbol, and bigint. These types represent the most basic, indivisible units of data and are immutable. Literal types are unique to TypeScript and include string literals, numeric literals, and boolean literals. They are often combined with union types to create precise type constraints. Template literal types allow for the creation of more complex types based on strings. Enums and literal union types each have their own advantages and disadvantages. Literal types are erased during compilation and do not affect runtime performance. Type guards are particularly useful when working with literal types. Literal types are well-suited for defining the types of configuration objects and can be combined with generics to create flexible APIs.

Common compilation errors and solutions

TypeScript, as a superset of JavaScript, provides type checking during compilation, but developers often encounter type mismatch errors, such as inconsistencies in variable or function types, requiring type consistency. Undefined variable errors necessitate declaring variables first. Optional and default parameters are easily confused and must be correctly distinguished. Module import/export errors require checking paths and export methods. Type assertions may lead to runtime errors; it is recommended to use type guards. Generic constraint issues require adding appropriate constraints. Decorator applications must be placed correctly. Enum types should avoid duplicate values. Interface implementations must be complete. Read-only properties cannot be modified. Function overloads must match actual implementations. Type aliases should avoid circular references. Index signature conflicts require loosening types. Strict null checks need to handle potential null values. Type extensions may be insufficient, requiring interface improvements. Async functions must correctly handle Promise return types. Type inference may not meet expectations. Union type operations have limitations; type safety must be considered.

The TypeScript ecosystem

The core advantage of TypeScript lies in its powerful type system, which enables static type checking to improve code quality. Basic types include `number`, `string`, `boolean`, etc. Union types and intersection types provide flexible ways to combine types. Generics are widely used in collection operations, while built-in utility types simplify type manipulation. Conditional types and the `infer` keyword enable complex type inference. It supports both ES modules and CommonJS module systems and provides type support for JavaScript libraries through declaration files. The development toolchain is robust, including `tsc`, `ts-node`, and ESLint. Major frontend frameworks like React and Vue offer excellent support, and it integrates well with testing frameworks like Jest. Build tools such as Webpack have built-in support, and it excels in Node.js backend development—TypeORM exemplifies database operations. It is well-suited for handling configuration objects and improving error-handling patterns. The compiler offers multiple optimization options, and the community provides abundant resources, including official documentation and open-source projects. Future developments will continue to introduce new features, such as standardized decorators and more powerful type inference.

Type checking in strict mode

TypeScript's strict mode significantly enhances type safety through seven core compiler options: - `noImplicitAny` prohibits implicit `any` types. - `strictNullChecks` enforces handling of `null` and `undefined`. - `strictFunctionTypes` strengthens contravariant function parameter checks. - `strictPropertyInitialization` requires explicit class property initialization. - `strictBindCallApply` ensures correct binding of method parameters. - `noImplicitThis` forbids implicit `any`-typed `this`. - `alwaysStrict` enforces strict mode parsing. The article provides a detailed breakdown of each option's mechanism and practical use cases, offering step-by-step migration strategies and advanced type techniques. It also discusses interactions between strict mode and third-party libraries, along with performance optimization recommendations. Finally, it notes that test code may relax certain restrictions. Together, these measures help developers write more robust and type-safe code.

Core knowledge points of TypeScript

TypeScript, as a superset of JavaScript, enhances code maintainability and development efficiency through its static type system. Its core concepts include type annotations, type inference, and strict mode checking. The type system covers primitive types, object types, function types, and advanced features such as union types, intersection types, conditional types, and template literal types. Object-oriented programming is supported with features like classes, inheritance, access modifiers, abstract classes, and decorators. Advanced type utilities like mapped types, the `keyof` operator, and built-in utility types improve type programming capabilities. Modules and namespaces help manage code organization, while generics provide flexible type abstraction. Engineering practices involve project configuration, type declarations, and build tool integration. TypeScript integrates deeply with mainstream frameworks like React, Vue, and Node.js, as well as testing tools. Its comprehensive technical system makes it a vital choice for modern web development.

Introduction to basic type systems

TypeScript's type system is a core feature that enhances code quality and maintainability through static type checking. Basic types include number, string, boolean, null, undefined, symbol, and bigint. Arrays and tuples are used for ordered data collections. Union types allow variables to hold multiple types. Literal types restrict values to specific constants. Type aliases and interfaces define complex structures and support extension. Enums provide named constants. Type inference automatically determines types, while type assertions manually specify them. Function types clarify parameters and return values. Generics enable type reuse. Advanced types like mapped, conditional, and indexed types handle complex scenarios.