There are four main types of high-frequency JavaScript problems in front-end interviews: 1. Scope and closures, you need to understand the application scenarios and memory leak causes, and you can implement such as counters; 2. The direction of this, you need to master the call context and arrow functions; 3. Prototype and inheritance, understand the implementation process of constructors, prototypes, proto relationships and new; 4. Asynchronous programming and event loops, distinguish between macro tasks and micro tasks, and master the principles and execution order of Promise and async/await. These issues are examined in-depth understanding and practical application capabilities, and need to master the core mechanism in combination with practice.

If you are preparing for an interview for a front-end development position, especially the JavaScript part, you may be wondering: What exactly are the questions that are most often asked? Are there any core knowledge points that must be mastered?

The answer is yes. Below we have compiled several JS knowledge points and related issues that appear frequently in front-end interviews and are easily overlooked to help you check for omissions.

Scope and closure: Not just a conceptual question

This is what almost every front-end interview involves. Many students know that "closures are functions accessing external variables", but not many people can really explain the application scenarios and side effects.

• Examples of FAQs:
  • What is a closure?
  • Why is closures likely to cause memory leaks?
  • Implement a counter using closure?

In practical applications, closures are often used in modular code and encapsulating private variables. for example:

 function createCounter() {
  let count = 0;
  return function () {
    count ;
    console.log(count);
  };
}
const counter = createCounter();
counter(); // 1
counter(); // 2

At this time, the interviewer may ask: "How else can it be achieved without closures?" or "When will this count variable be released?" This kind of question is actually examining your understanding of the garbage collection mechanism.

This point of view: Stop memorizing it by rote

This is one of the most error-prone keywords in JS. Many people rely on memorizing different binding rules to judge this direction, but once they encounter variant questions during the interview, they are likely to get stuck.

• The key point is to understand the call context:
  • Who called the function?
  • Has the function changed this by bind, call, apply?
  • Is it an arrow function? (The arrow function does not have this of its own)

For example:

 const obj = {
  name: 'Alice',
  greet: function () {
    setTimeout(function () {
      console.log(this.name); // undefined
    }, 100);
  },
};
obj.greet();

At this time this points to window or undefined (strict mode), not obj. The workaround is usually to use arrow functions or cache this in advance:

 setTimeout(() => {
  console.log(this.name); // Alice
}, 100);

Prototype and inheritance: basic but easy to ignore details

Although ES6 now introduces class syntax, the underlying layer is still based on the prototype chain. Many developers only stay at the "class extends" level and ignore the essence of the prototype chain.

• Frequently asked questions include:
  • What is the relationship between constructor, prototype, and proto ?
  • What did the new keyword do?
  • How to implement an inheritance?

The process of new can be summarized into the following steps:

• Create an empty object {}
• Set the proto of this object to point to the prototype of the constructor
• Point this in the constructor to the object and execute
• Return this object (unless the constructor returns another object)

Understanding these will help you avoid errors when writing complex class structures.

Asynchronous Programming and Event Loop: Things that must be explained clearly

Asynchronous processing is a major feature of JS and is also a must-have for interviews. From callback to promise to async/await, progressing layer by layer.

• Frequently asked questions include:
  • What is the difference between macro tasks vs micro tasks?
  • Promise status flow?
  • How is the underlying async/await implemented?

To give a classic example:

 console.log('Start');

setTimeout(() => {
  console.log('Timeout');
}, 0);

Promise.resolve().then(() => {
  console.log('Promise');
});

console.log('End');

The output order is:

 Start
End
Promise
Timeout

This is because Promise.then belongs to micro-tasks and takes precedence over macro-task execution. This kind of question is very common, but many students just remember the order and do not understand the event loop mechanism behind it.

Basically that's it. These questions cannot be fully grasped by cramming temporarily. It is recommended to think more about the principles when writing code and understand them in combination with practice, so that you can deal with them freely during the interview.

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