Basics

Question 1

How to define variable in global and local scopes

Answer 1

if (true) {
  var x = 5; // global scope
  let y = 5; // local scope
}

Question 2

Ternary operator example

Answer 2

let status = age >= 18 ? "adult" : "minor";

Question 3

What is variable hoisting

Answer 3

• Variables (var) in JavaScript are, in a sense, “hoisted” (or “lifted”) to the top of the function or statement.
• However, variables that are hoisted return a value of undefined (cuz why not xd)
  
  • “Temporal Dead Zone” (TDZ) - when variable is observable but unusable variables.
• Because of hoisting, all var statements in a function should be placed as near to the top of the function as possible.
• let and const are hoisted but not initialized. Referencing the variable in the block before the variable declaration results in a ReferenceError.

Question 4

Convert variable to Boolean

Two ways

Answer 4

Boolean(-1);
!!value

Question 5

Convert variable (non string) to Number

Answer 5

Number(" -12.34 ");

Question 6

Convert variable to String

Answer 6

String(12.34);
${x}
toString() if supported on object

Question 7

Number literal for hex, binary, BigNum numbers

Answer 7

0xff, 0b1101, 123n

Question 8

Convert string to Float

Answer 8

parseFloat("12.5");

Question 9

Convert string to Number

Answer 9

parseInt("101", 2); // binary to number

Question 10

Conditionals: if

Answer 10

if (condition_1) {
  statement_1;
} else if (condition_2) {
  statement_2;
} else {
  statement_last;
}

Question 11

Conditionals: switch

Answer 11

switch (expression) {
  case label_1:
    statements_1;
    break;
  default:
    statements_def;
    break;
}

Question 12

Conditionals: for loop (C like)

Answer 12

for (let i = 0; i < selectObject.options.length; i++) {
  // index for - use by default
}

for (var i = 0, j = 9; i <= j; i++, j--) {
  // Two variables at once
}

Question 13

Conditionals: While loop(s)

Answer 13

while (n < 3) {
  n++;
  x += n;
}

do {
  i += 1;
  console.log(i);
} while (i < 5);

Question 14

Conditionals: For each loop(s)

Answer 14

Creates a loop Iterating over iterable objects (including Array, Map, Set, arguments object and so on), invoking a custom iteration hook:

for (let i of arr) {
  console.log(i);
}

Will iterate all properties (numeric indexes + custom properties) or methods you define on Array object (or other object):

for (let i in obj) {
  console.log(i); 
}

Question 15

Exception handling structure

Answer 15

try {
  throw new Error("The message");
} catch (e) {
  throw e; // re-throw
} finally {
  console.log("Resource cleanup...");
}

Question 16

JS Exceptions:
* How to throw
* Exception object properties
* How to create custom exception

Answer 16

• How to throw:
  
  • throw new Error("The message");
• Exception object properties:
  
  • console.error(e.name);
  • console.error(e.message);
• How to create custom exception:
  

    class CustomError extends Error {
      constructor(message) {
        super(message); // Pass message to the Error class
        this.name = "CustomError"; // Set a custom error name
      }
    }
   

Question 17

What happens when you return value in finally block?

Answer 17

It overwrites previous return (including one in catch block)

Question 18

What is a closure. Use cases.

Answer 18

Closure is when a function remembers and continues to access variables from outside its scope, even when the function is executed in a different scope.

A closure is the combination of a function and the lexical environment within which that function was declared.
The Lexical Environment is a structure that holds identifier-variable mapping

Use cases:
* Data Encapsulation (Private Variables)
* Function Factories
* Memoization (Performance Optimization)
* Event Handlers (Retaining State)
* Iterators
* Partial Application / Currying

Question 19

Variadic function parameters

Answer 19

function multiply(multiplier, ...theArgs) {
  // theArgs is an actual array
  return theArgs.map((x) => multiplier * x);
}

Question 20

Renamed export

Answer 20

export {function1 as newFunctionName};

Question 21

Export from another module

Answer 21

export { name } from 'x.js'

Question 22

Renamed import

Answer 22

import {function1 as newFunctionName} from "x"

Question 23

• Parse JSON string to object.
• Convert object to JSON string
• Convert object to JSON string, use 2 spaces for indentation (what are all parameters)

Answer 23

JSON.stringify(obj)
JSON.parse(jsonString)
JSON.stringify(value, undefined, 2)
Second arg is replacer - a function that alters the behavior of the stringification process

Question 24

How to handle cases where dog in adventurer.dog.name can be an undefined object? Name the techniques.

Answer 24

• Use ?? (nullish coalescing - scalania) and ?. (optional chaining)
  
  • adventurer.dog?.name ?? "backup value"
• a.b?.c will return undefined insatead of Error
• ?? "backup value" will return "backup value" only if the calculated value is undefined.

Question 25

Check typeof variable x

Answer 25

typeof x