The import syntax in ECMAScript 6 revolutionized JavaScript modularization by providing both static and dynamic import support. Static imports determine dependencies at compile time, including named imports, default imports, and namespace imports. Named exports allow exposing multiple values, while default exports are used for a module's primary functionality. Dynamic imports, implemented via `import()`, enable on-demand loading. ES modules feature live bindings, differing from CommonJS. Browsers support module loading via the `<script type="module">` tag, with module resolution following specific path rules. Static analysis enables advantages like tree-shaking, and the toolchain fully supports ES modules. Future developments include new features like Import Maps.
Read more
The `export default` syntax introduced in ECMAScript 6 allows a module to specify a default export value, which can be imported with a custom name. Each module can have only one default export, typically used for exporting the main functionality or value. Unlike named exports, default exports do not require the same name when importing and offer greater flexibility, allowing direct exports of anonymous functions, classes, or object literals. A module can contain both a default export and named exports. Common use cases include exporting React components, configuration objects, or service classes. The `export from` syntax can be used to re-export the default export of another module. During dynamic imports, the default export appears as the `default` property. In TypeScript, type annotations can be added to default exports. Key considerations include: each module can have only one default export, naming flexibility during import, and differences from CommonJS's `module.exports`.
Read more
The `export` keyword in the ECMAScript 6 module system is used to export functions, objects, or primitive values for use by other modules. It primarily includes two forms: named exports and default exports. Named exports allow multiple values to be exported, each with a name, and imports must use the same names. They support direct export at declaration, unified export after declaration, and renaming with the `as` keyword. Default exports allow only one per module, and the import name can be customized. A module can contain both named and default exports and also supports re-exporting content from other modules. Exports are dynamic bindings to values rather than copies—when the internal value of a module changes, the imported value updates accordingly. Export statements are affected by hoisting, but it is recommended to explicitly list all exports at the top of the module. In practice, common export patterns include utility libraries, configuration objects, classes combined with helper functions, etc. Exported identifiers must follow variable naming rules, and in strict mode, exported variables are read-only. ES6 modules support circular dependencies, and exported bindings are live—modifications to exported values inside the module update all import locations. Exports must be at the top level of the module and cannot be inside conditional statements or functions. Compared to CommonJS, ES6 exports are static, export bindings, and support both named and default exports. Good export practices aid in Tree Shaking optimization, and proper use of exports enables module encapsulation and information hiding. In TypeScript, type annotations can be added.
Read more
The issue of `this` binding in ECMAScript 6 classes is a key challenge in JavaScript object-oriented programming. While class methods default to binding `this` to the instance, passing them as callbacks can lead to `this` loss. This article provides a detailed analysis of four solutions, including using arrow functions, constructor binding, call-time wrapping, and class field syntax. It also explores `this` behavior and solutions in various scenarios such as inheritance, static methods, DOM event handlers, higher-order functions, asynchronous contexts, class decorators, proxy objects, factory functions, object property parameter passing, array methods, and module exports. This comprehensive guide helps developers fully understand and correctly handle `this` binding issues in classes.
Read more
ECMAScript 6 classes are essentially syntactic sugar over prototype-based inheritance. JavaScript has used a prototype inheritance mechanism since its inception, where each object has an internal property pointing to its prototype object. The ES6 class syntax encapsulates prototype inheritance: class declarations create special functions, class methods are added to the constructor's prototype, the `extends` keyword implements prototype inheritance chains, and static methods exist as constructor properties. Class fields are ultimately converted to prototype-based implementations. Class methods are non-enumerable by default. The `super` keyword relies on the prototype chain. Classes and prototype inheritance have identical performance. The `instanceof` operator checks the prototype chain. Classes are constrained by the prototype model and cannot support multiple inheritance, though factory functions can serve as an alternative. ES2022 introduced private fields, which do not appear in the prototype chain. Understanding the relationship between classes and prototypes aids debugging. Classes can be transpiled to ES5 code. Prototype inheritance is more flexible, allowing dynamic prototype modifications. Class methods are shared, saving memory.
Read more
After ECMAScript 6 introduced the class concept, default members were public. To address encapsulation issues, the TC39 committee proposed the private fields proposal, using a prefix to identify private members. This proposal was eventually incorporated into the ECMAScript 2022 standard, becoming the official solution for achieving true privacy in JavaScript class definitions. Private fields are declared by adding a prefix to the field name and must be explicitly declared at the top level of the class, ensuring strict syntactic enforcement of privacy. They feature compile-time checks, hard privacy, uniqueness, and mandatory prior declaration. Unlike TypeScript's `private`, which is only a compile-time constraint, ECMAScript private fields are truly private at runtime. Private fields follow strict encapsulation rules—subclasses cannot access their parent class's private fields. They can be combined with private methods to achieve fully private implementation details. Static private fields are implemented by adding the prefix after the `static` keyword. Private fields come with restrictions: they must be pre-declared in the class, cannot be dynamically created, and cannot use computed property names. They are suitable for encapsulating internal state, hiding implementation details, and preventing naming conflicts. Compared to the WeakMap approach, private fields offer cleaner syntax, better performance, and improved debugging. Modern browsers already support private fields. TC39 chose the prefix design for its explicitness, consistency, and extensibility. TypeScript 4.3 supports ECMAScript private field syntax and integrates it with the type system. Modern browser developer tools also support private field inspection while maintaining their privacy.
Read more
ECMAScript 6 introduced class expression syntax, allowing classes to be defined within expressions. Class expressions can be named or anonymous. Unlike class declarations, class expressions are not hoisted. The name of a named class expression is only visible inside the class. Class expressions can be immediately invoked to create singleton objects, passed as arguments, or used for inheritance, making them particularly suitable for dynamically generating classes. They support private fields, static blocks, generator methods, and asynchronous methods, though attention must be paid to `this` binding issues. Class expressions can be combined with `Proxy` to enable metaprogramming or used in the module pattern for implementation. They provide flexible and powerful object-oriented programming capabilities.
Read more
ECMAScript 6 introduced concise getter and setter syntax for object property access control. Getters execute when reading a property, while setters execute when setting a property. In ES6 classes, these methods are defined using the `get` and `set` keywords. Computed property names can be combined with getters/setters to achieve dynamic property names. The `Object.defineProperty` method provides finer-grained property control. Getters/setters exhibit dynamic computation characteristics in prototype inheritance. Proxy objects can be combined with getters/setters to implement advanced access control. In practical applications, getters/setters are used for scenarios like data validation, computed properties, and logging. Performance-sensitive situations require attention to the overhead of getters/setters. During JSON serialization, only data properties are included. Custom `toJSON` methods can control serialization behavior.
Read more
In ECMAScript 6, the `super` keyword is used in subclasses to call the constructor or methods of the parent class, with two usage modes: as a function call and as an object. When used as a function call, it can only be used in the subclass constructor and must be called before `this`. When used as an object, it can be called in any method of the subclass. In ordinary methods, `super` refers to the parent class's prototype object, while in static methods, it points to the parent class itself. The binding of `super` is static, determined at method definition and unaffected by the calling object. Arrow functions inherit the `super` binding from their surrounding context. When using `super` in object literals, shorthand method definitions are required. `super` can also be used in async methods and Proxy-wrapped classes. JavaScript does not support multiple inheritance, but mixin patterns can simulate `super` behavior.
Read more
ECMAScript 6 introduced the `class` syntactic sugar, which enables class inheritance via the `extends` keyword. Subclasses can inherit properties and methods from parent classes, support method overriding, and use `super` calls. In constructors, `super` must be called before using `this`. In methods, `super` refers to the parent class prototype. Static methods are also inherited. Built-in types like `Array` can be extended, with the underlying mechanism still relying on prototype chains. Multi-level inheritance forms a chain structure. Abstract base classes can be created using `new.target`. The mixin pattern simulates multiple inheritance. The `instanceof` operator checks the prototype chain. Public and private fields exhibit different inheritance behaviors. Built-in types require overriding the `Symbol.species` method. Overriding methods must account for `this` binding. Constructor return values affect instance creation. Asynchronous constructors can be combined with inheritance.
Read more
