JS Fundamentals

Question 1

How do JS engines work?

Answer 1

1. Parse the script
2. Compile script to bytecode
3. Executes the bytecode

• used to be a mere interpreter
• now uses a lot of optimizations, e.g. JIT compilation

Question 2

How do we add JS code to HTML?

Answer 2

• external file:

- inline: console.log(“hello”)

Question 3

What are statements?

Answer 3

• syntax constructs and commands that perform actions

Question 4

What roles do semicolons play in JS code?

Answer 4

• separate statements
• maybe omitted in most cases when a line break exists
• > implicit semicolon

Question 5

What does “use strict” do in JS code?

Answer 5

• directive to enable modern JS (ES5+)

- advanced language structures enable “use strict” automatically, e.g. modules, classes

Question 6

What is a variable?

Answer 6

• labeled storage for data

Question 7

What is the difference between let and const?

Answer 7

• let enables to reassign a variable

- const does not allow reassigning a variable

Question 8

Different data types in JS?

Answer 8

• primitive data types: number, BigInt, string, Boolean, null, undefined, Symbols
• non-primitive data types: Object (array, function)

Question 9

Outputs of:
Number(undefined)
Number(null)
Number(true)
Number(false)
Number("")

Answer 9

Number(undefined) // NaN
Number(null) // 0
Number(true) // 1
Number(false) // 0
Number("") // 0

Question 10

Outputs of:
2 + 2 + “1”
“1” + 2 + 2

Answer 10

2 + 2 + “1” // “41”

“1” + 2 + 2 // “122”

Question 11

What does the unary + do?

Answer 11

• nothing if applied to numbers
• if applied to non-numbers, convert to number
• same behavior as Number(…)

Question 12

What does the operator = do?

Answer 12

• assigns a value to a variable
• low precedence
• actually returns a value

Question 13

Difference between postfix and prefix increment/decrement?
counter++
++counter

Answer 13

• postfix: increments and returns old value

- prefix: increments and return new value

Question 14

How are strings compared in JS?

Answer 14

• letter by letter in lexicographical order

Question 15

Difference between == and ===?

Answer 15

• ==: loose equality -> compares values with type conversion to numbers
• ===: strict equality -> compares values without type conversion

Question 16

What does the nullish coalescing operator ?? do

Answer 16

• return first argument if it is not null/undefined, otherwise second one

result = a ?? b
result = (a !== null && a !== undefined) ? a : b

Question 17

Differences between Function Expression and Function Declaration?

Answer 17

Function Expression:

• function created inside an expression or inside another syntax construct
• not hoisted (are created when execution flow reaches them)

Function Declaration:

• function declared as a separate statement in the main code flow
• hoisted (processed before the code block is executed)

Question 18

What are transpilers?

Answer 18

• software that can parse modern code and rewrite it using older syntax
• enables dev to use modern language syntax features without worrying about backward compatibility
• Babel is one of the most modern

Question 19

What are polyfills?

Answer 19

• new language features can also contain built-in functions, e.g. Array.flatMap
• polyfills functions that fill the gap of missing functions