JavaScript-middle-level

(https://hh.ru/applicant/skills/674/verification_methods?rank=2&kind=theory)

1. How can you find out exactly which element was clicked as part of event delegation on the parent?
   -> using event.target
   (other options: Through window.event.srcElement; Only by manually selecting a selector; via this inside the handler; via event.currentTarget;)

2. You need to implement an interface localization system. Each interface string should have several translations into different languages. Required functionality:

• Storing translations in Russian, English, and other languages
• Fast lookup of a translation by key and language
• Ability to update and add new translations
  -> Create an object where the keys are languages, and the value is an object with string keys and translation values
  (other options: Use an array where each element is a “key-string, translation” pair; Store everything in one huge JSON string and parse it regularly; Make a global variable for each language and write translations like: en_key: translation; Build a single array of all translations and search it by index;)

3. Which inheritance concept in JavaScript does the code below demonstrate?

function A() {}
A.prototype.shared = 'value'
function B() {}
B.prototype = Object.create(A.prototype)
B.prototype.unique = 'another'
const b = new B()

-> Manual prototype chain
(other options: Shallow inheritance; Object-level encapsulation; Property inheritance; Formal cloning;)

4. How can you change an object’s prototype in JavaScript?
   -> Using Object.setPrototypeOf(obj, prototype)
   (other options: The prototype is replaced automatically when the constructor is called; only with the help of classes; You need to call prototype.override(); The prototype cannot be changed after creation;)

5. What will happen when the code executes?

function createCounter() {
  let count = 0;
  return {
    increment() {
      count++;
      return count;
    }
  };
}  
const counter = createCounter();
console.log(counter.increment());

-> The increment method uses a closure to access count and increments it;
(other options: Each call to increment returns 1, because count is re-created and incremented by 1 each time; A global variable count is created along with an increment method in counter that increments count; The createCounter function returns nothing; count becomes inaccessible after the first iteration;)

6. Why can’t an arrow function be used as a constructor (with the new operator)?
   -> It has no `[[Construct]]` and does not create its own this object;
   (other options: It doesn’t support the new keyword; It inherits the parent’s prototype; It always returns undefined; It doesn’t accept arguments;)

7. Which of the following expressions will evaluate to false?
   -> 4 = '4'
   (other options: ‘a’ < ‘z’; 5+2>3; true ! false; 10>2)

8. When is it appropriate to use setTimeout?
   ->When you need to artificially shift a function's execution in time within the main queue;
   (other options: When the called function forms a result based on an external flag; When you need to respond to a user event and save data; When the logic is built around iterative array transformations; When the structure implies multiple loops with predictable execution time;)

9. What will the expression output?

let count = 5;
count = ++count + 6;
console.log(count)

->12

10. What happens to the original object if it is passed into a function and one of its properties is changed inside that function?
    ->The original object also changes, because the object is passed by reference;

11. A function is declared inside a for loop body via let fn = () => …; What happens to the variable fn after the loop finishes?
    ->If the function is declared inside the loop body using let, then the variable fn has block scope;

12. Which of the statements is incorrect?

• A string can be mutated
• The type of null is “object”
• undefined is used when a value is not set
  ->A string can be mutated

index JavaScript-easy-level hh