The constructor property

The `constructor` property of a JavaScript object points to the constructor function that created the instance and plays a crucial role in the prototype chain. It exists by default on the prototype object and can be used for type checking and dynamic object creation. It is accessed through the instance's prototype chain. When overriding the prototype, the `constructor` property must be manually restored. In inheritance systems, this property needs to be handled correctly. Primitive types undergo implicit boxing when accessing this property, while `null` and `undefined` do not have it. ES6 class syntax automatically maintains the `constructor` relationship. Performance-wise, it is slightly faster than `instanceof`. It has specific interactions with features like `Symbol.species` and `new.target`.

Prototype and prototype chain

In JavaScript, prototypes are the foundation of object inheritance. Every object has a prototype, and the prototype itself is also an object. Constructors point to the prototype via the `prototype` property, while instances access the prototype through `__proto__`. The prototype chain is the mechanism for implementing inheritance. When accessing a property, if the object does not have it, the search moves up the prototype chain until the property is found or the end is reached. JavaScript provides prototype-related methods like `Object.getPrototypeOf`. The relationship between constructors, prototypes, and instances is close. Prototypal inheritance has advantages like high memory efficiency but also drawbacks like shared reference-type properties. ES6 class syntax is syntactic sugar for prototypal inheritance. Prototype pollution can pose security risks, which can be mitigated with methods like `Object.create(null)`. Overly long prototype chains can impact performance, so deep inheritance chains should be avoided, and composition is often a better approach.

Object enumeration and iteration

In JavaScript, enumerating and iterating over object properties are key operations for handling object data. Enumeration refers to listing all accessible properties of an object, while iteration involves sequentially accessing these properties or their values. The enumerability of object properties is an important characteristic that determines whether a property appears in certain enumeration operations. Common enumeration methods include the `for...in` loop, `Object.keys()`, and `Object.getOwnPropertyNames()`. Modern iteration methods include `Object.values()` and `Object.entries()`, as well as their inverse operation, `Object.fromEntries()`. Properties of the `Symbol` type require the use of `Object.getOwnPropertySymbols()` for access. In terms of performance, `for...in` is generally slower, while methods like `Object.keys()` are faster. By implementing the `Symbol.iterator` method, an object can be made iterable. Enumeration methods are widely applied in practical scenarios such as handling immutable objects, deep copying, property filtering, and transformation. The ES6 specification defines the enumeration order of an object's own properties: numeric keys in ascending order, string keys in creation order, and `Symbol` keys in creation order.

Function declaration and function expression

In JavaScript, function definitions mainly include function declarations and function expressions. Function declarations start with the `function` keyword followed by the function name, are hoisted to the top of the scope, and can be called before declaration. Function expressions assign a function to a variable, which can be anonymous or named, and are not hoisted. The internal name of a named function expression is only visible within the function. Arrow functions are concise function expressions without their own `this` binding, making them suitable for callbacks. The IIFE pattern creates a private scope. Function declarations are ideal for scenarios requiring hoisting, while function expressions are better for controlling creation timing. Both have distinct applications in higher-order function passing, constructor usage, recursive implementation, and module patterns. Modern engines show no significant optimization differences between the two, though function declarations may parse slightly faster. Named functions provide clearer call stacks during debugging, while function expressions support dynamic function creation at runtime.

Strict mode translates this sentence into English.

Strict mode is a special execution mode introduced in ECMAScript 5 that enhances code safety and optimization by restricting syntax features and throwing more errors. It is enabled using the `"use strict"` directive. In strict mode, variables must be explicitly declared, deleting undeletable properties is prohibited, function parameters cannot have duplicate names, `this` in global functions is `undefined`, variables created by `eval` do not leak to outer scopes, `arguments` is not bound to parameters, octal literals are forbidden, extending non-extensible objects is disallowed, the `with` statement is disabled, and accessing `caller` and `callee` is restricted. Modern module systems and classes automatically enable strict mode. Strict mode offers performance benefits by eliminating dynamic features, allowing engines to optimize better. Compatibility with older environments must be considered, and the best practice is to migrate gradually while leveraging tools. Many new features require strict mode.

Error handling mechanism

JavaScript error handling mechanisms are crucial for ensuring code robustness. The `try-catch` statement captures and handles exceptions, while the `finally` block is used for resource cleanup. JavaScript includes built-in error types like `TypeError` and `ReferenceError`, and custom errors can be created by inheriting from the `Error` class. The `throw` statement actively throws errors. Promises handle errors via `catch` or `async/await`. Global error handling involves events like `window.onerror` and `unhandledrejection`. React uses error boundaries to catch component errors. In production environments, error logs should be recorded. Defensive programming practices include parameter validation and type checking. Asynchronous iterators and Node.js have specific error handling patterns. Proper error handling ensures stable program execution.

Array Basics

Arrays are a fundamental data structure in JavaScript for storing ordered collections of elements, where each element has a numeric index starting from 0. Arrays can contain values of any type. There are three ways to create an array: literal notation, constructor, and the `Array.of` method. Basic operations include accessing and modifying elements via their index, as well as retrieving the array length. Common methods include `push`, `pop`, `unshift`, and `shift` for adding or removing elements, `concat` for merging arrays, `indexOf` and `includes` for searching elements, and iteration methods like `for` loops, `for...of` loops, and `forEach`. Transformation methods include `map`, `filter`, and `reduce`. Arrays can be multidimensional. ES6 introduced destructuring assignment and the spread operator for simplified operations, as well as typed arrays for handling binary data. Performance-wise, inserting elements at the beginning is less efficient, while direct index assignment is faster. Arrays and strings can be converted to each other. ES6 added static methods like `Array.isArray`, `Array.from`, and `Array.of`, as well as iterator methods (`keys`, `values`, `entries`), search methods (`find` and `findIndex`), filling methods (`fill`), inclusion methods (`includes`), flattening methods (`flat`), and sorting methods (`sort`). Arrays are special objects with an automatically maintained `length` property.

Object basics

JavaScript objects are composite data types that store key-value pairs, consisting of properties and methods. Properties are key-value pairs, while methods are function properties. Objects can be created using object literals, support dynamic addition and deletion of properties, and constructors can be used to create similar objects. Prototype chains enable inheritance, and `Object.create` can set prototypes. ES6 introduced destructuring assignment to simplify property extraction. Property descriptors control property behavior, and object freezing/sealing restricts modifications. The `class` syntax provides a clearer object-oriented writing style. Object iteration methods include `keys`, `values`, and `entries`. Shallow and deep copying handle reference issues. `Map` is suitable for specific scenarios. Optional chaining simplifies deep access, and the nullish coalescing operator helps set default values.

Property descriptors

In JavaScript, property descriptors are the core mechanism of object properties, divided into two types: data descriptors and accessor descriptors. Data descriptors include characteristics such as `value`, `writable`, `enumerable`, and `configurable`, while accessor descriptors control access through `get` and `set` functions. The article details how to retrieve and define property descriptors, as well as the specific roles of `writable`, `enumerable`, and `configurable`. It also explores practical applications of property descriptors in development, such as creating immutable properties and implementing private property patterns. Additionally, it discusses the behavior of property descriptors in prototype chains, their integration with class syntax, and their collaboration with `Proxy`. Finally, it outlines various constraints and limitations of property descriptors.

Object creation methods

JavaScript provides multiple ways to create objects. Object literals are the simplest and most straightforward method, suitable for creating simple objects. Constructors are ideal for creating multiple similar object instances, allowing methods to be shared via prototypes. The `Object.create` method enables precise control over the prototype chain. ES6 class syntax is syntactic sugar for constructors, offering a clearer approach to object-oriented programming. Factory functions encapsulate object creation logic and can achieve private members. The singleton pattern ensures only one instance of an object exists. Destructuring and spread operators provide new ways to create and merge objects. Dynamic property names allow expressions to be used as property keys. Property descriptors enable precise control over property behavior. Prototypal inheritance is the core mechanism for implementing inheritance in JavaScript, with various methods available to set an object's prototype.