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Frequent questions in front-end JS interviews cover many aspects, including basic syntax, data structure and algorithm, DOM operation, asynchronous programming, modularization, framework/library usage, etc. The following are some common frequent questions in front-end JS interviews and brief analysis:

1. Basic Grammar

• Data type and storage differences: What data types are there in JavaScript? How are they stored in memory? (e.g. the difference between primitive data types and reference data types)
• Variable Declaration：var、let、constWhat is the difference between ? What is their scope and life cycle?
• Operators：i++and++iWhat is the difference?breakandcontinueHow is it different when used in a loop?

2. Data Structure and Algorithm

• Array Operations: What are the commonly used methods of arrays? (such aspush、pop、shift、unshift、splice、slice、join、sortwait)
• Deep copy and shallow copy: What are deep and shallow copies? How to implement them? (e.g. usingJSON.parse(JSON.stringify(obj)), spread operator, recursion, etc.)
• Sorting Algorithm: Understand and implement a sorting algorithm (such as bubble sort, quick sort, etc.)

3. DOM Manipulation

• Common DOM Operations: How to select, add, delete, modify and query DOM elements?
• innerHTML vs innerText: What are the differences between the two and what are their applicable scenarios?
• Event Handling: What is the event model in JavaScript? How to implement event delegation?

4. Asynchronous Programming

• Promise: What is Promise? What are its three states? How to use itPromise.allandPromise.race？
• Async/Await: How to use async/await to handle asynchronous operations? What is its relationship with Promise?
• Callback functions and event loops: Understand JavaScript's event loop mechanism and how callback functions work?

5. Modularity

• ES6 Modules: Understand the modular syntax in ES6 (import/export), and what are its advantages?
• CommonJS and AMD/CMD：Understand and compare the differences and applicable scenarios of CommonJS, AMD and CMD module specifications.

6. Framework/library usage

• React/Vue/Angular：Understand and compare the features, advantages and applicable scenarios of these front-end frameworks/libraries.
• life cycle: What is the life cycle of components in frameworks such as React and Vue?
• State Management: How to use state management tools such as Redux or Vuex in React?

7. Other high-frequency issues

• Prototypes and prototype chains: Understand the prototype and prototype chain mechanisms in JavaScript and their functions.
• this keyword: Understanding JavaScriptthisPointing rules and performance in different scenarios.
• Closure: Understand the concept, function and usage scenarios of closures.
• Garbage Collection Mechanism: Understand the garbage collection mechanism in JavaScript (such as mark-and-sweep method, reference counting method, etc.).

Example questions and solutions

Sample Questions: Please explain the prototype chain mechanism in JavaScript.

Analysis：

• The prototype chain is a mechanism for implementing inheritance in JavaScript.
• Each object has a__proto__Attributes (recommended in ES6)Object.getPrototypeOf()Method to obtain), this property points to its constructorprototypeAttributes.
• When accessing a property or method of an object, if the object itself does not have the property or method, it will search up along its prototype chain until it is found or reaches the top of the prototype chain (Object.prototype）。
• The top of the prototype chain isnull, indicating that there are no more prototype objects to look up.
• Through the prototype chain, property sharing and method inheritance between objects can be achieved.

1. What are the data types in JavaScript? What are the differences between them?

answer：
There are 8 data types in JavaScript, including primitive types and reference types.

• Basic Data Types：
  • Undefined: Undefined, the value when the variable is declared but not assigned a value.
  • Null: Null value, indicating an empty object reference.
  • Boolean: Boolean type, with only two values: true and false.
  • Number: Numeric type, including integers and floating-point numbers.
  • String: String type, used to represent text data.
  • Symbol: Unique value type, used to create unique identifiers.
  • BigInt: Large integer type, used to represent integers of arbitrary precision.
• Reference Data Types：
  • Object: Object type, which is the base class of all complex data types in JavaScript, including arrays (Array), functions (Function), dates (Date), etc.

The main difference between primitive data types and reference data types is how they are stored and assigned. The values ​​of primitive data types are stored in stack memory, and the values ​​are directly copied when assigned; while the values ​​of reference data types are stored in heap memory, and the stack memory stores references (i.e. addresses) to the values ​​in heap memory, and the references are copied when assigned.

2. Talk about scope and closure in JavaScript?

answer：

• Scope: refers to the effective scope of identifiers such as variables and functions in a code block. JavaScript has two main scopes: global scope and local scope (including function scope, block scope, etc.). Variables in the global scope are visible throughout the script, while variables in the local scope can only be accessed within the code block in which it is defined.

• Closure: It means that a function remembers and has access to its lexical scope, even if the function is executed outside its lexical scope. The main use of closures is to encapsulate private variables, create modules, etc. Closures allow functions to access and operate variables outside the function, and these variables are not easily polluted or changed even outside the function.

3. Explain asynchronous programming and Promise in JavaScript?

answer：

• Asynchronous Programming: It means that the execution order of the code is not executed in the order in which it is written, but is determined by the completion of certain conditions (such as network requests, file reading and writing, etc.). JavaScript is single-threaded, but it implements asynchronous programming through event loops and callback functions.

• Promise: It is a new object introduced in ES6 for handling asynchronous operations. The Promise object represents an operation and its result value that may not be completed yet, but will be completed (or failed) in the future. Promise has three states: pending, fulfilled, and rejected. With Promise, we can write asynchronous code in a synchronous way, making the code more concise, easier to understand and maintain.

4. Talk about event bubbling and event capturing in JavaScript?

answer：

• Event Bubbling: It means that the event starts from the target element and then propagates upward to the top level of the DOM tree (i.e. the document object). During the event bubbling process, any level of DOM element can capture the event and process it.

• Event Capture: In contrast to event bubbling, event capture starts from the top level of the DOM tree and then propagates down to the target element level by level. During the event capture process, any level of DOM element can capture the event and process it.

In JavaScript, you can set the event handling method through the third parameter of the addEventListener method, that is, whether to use event bubbling or event capture. If the third parameter is true, it means using event capture; if it is false or omitted, it means using event bubbling.

5. What new features are introduced in ES6+?

answer：
ES6 (ECMAScript 2015) and subsequent versions introduced many new features, including but not limited to:

• let and const: Used to declare local variables and constants in block scope.
• Arrow Functions: Provides a more concise way to write functions, and does not bind its own this, arguments, etc.
• Template Strings: Allows embedded expressions and supports multi-line strings.
• Destructuring assignment: Allows you to extract data from an array or object and assign it to a declared variable.
• Default parameter values, rest parameters, and the spread operator: Enhanced the function's parameter processing capabilities.
• Promises and async/await: Used to handle asynchronous operations, making asynchronous code more concise and easier to understand.
• Class Syntax: Provides syntactic sugar for classes that are closer to traditional object-oriented programming.
• Modules: Supports ES6 module syntax and implements modular programming of JavaScript.