Javascript / Browser

Question 1

Context / this

Answer 1

The object to which the executing function belongs:

• In global scope ‘this’ refers to the global object, Window in browser and Global in node; agnostic to strict mode
• In functions ‘this’ refers to the global object unless otherwise bound to another object; method style invocation (dot notation) implicitly binds ‘this’ to the calling object; defaults to undefined in strict mode
• In arrow functions ‘this’ refers to the ‘this’ of its lexical scope

Question 2

EcmaScript

Answer 2

A specification for a general purpose scripting language to which Javascript conforms

Question 3

Event bubbling

Answer 3

The propagation of an event from the target element to all elements up the DOM tree with an event listener of the same type

Question 4

Event delegation

Answer 4

Using event bubbling by assigning an event handler to a parent element rather than all of its children

Question 5

Event loop

Answer 5

The mechanism by which Javascript queues asynchronous functions until after all synchronous functions have executed, and then runs them

Question 6

Hoisting

Answer 6

The placement of variable declarations (not assignments) at the top of their scope before code execution

Question 7

Promise

Answer 7

An object that represents the eventual completion of some asynchronous operation

Question 8

Prototype chain

Answer 8

The linked list of prototypes from any given object’s prototype up to Object.prototype and finally null

Question 9

Prototypal inheritance

Answer 9

The way in which Javascript searches up the prototype chain to find properties or methods invoked but not defined on the calling object

Question 10

Prototype

Answer 10

An object containing properties (including a reference to its own prototype) and methods accessible by all objects with that prototype

Question 11

V8

Answer 11

Google’s Javascript engine; used in Chrome and Node

Question 12

Browser cache

Answer 12

Data from frequently visited website that the browser stores on the client hard drive to improve performance

Question 13

Cookie

Answer 13

A piece of data storing information about a user’s behavior / session; stored on the client and sent back to the server with each subsequent request; used for user analytics, preferences, and session management

Question 14

localStorage

Answer 14

A web API for storing data in browser cache that persists indefinitely or until cleared

Question 15

sessionStorage

Answer 15

A web API for storing data in browser cache that persists for the life of the tab

Question 16

Critical render path

Answer 16

The process by which a browser renders a web page from code:

1. HTML is parsed into the DOM tree; non blocking
2. CSS is parsed into the CSSOM; blocking
3. DOM and CSSOM are combined into the render tree
4. Element layout is determined; re-occurs anytime the render tree is modified
5. Pixels are painted on the screen; finished by Window.onLoad

Question 17

Document object model (DOM)

Answer 17

A browser generated model of an HTML document in nodes that can be manipulated with Javascript

Question 18

CSS object model (CSSOM)

Answer 18

A browser generated model of CSS rules that is mapped onto the DOM

Question 19

Render tree

Answer 19

The composite of the DOM and CSSOM made during the critical render path

Question 20

Service worker

Answer 20

A program separate from the main Javascript file that enables offline functionality

Question 21

Strict mode

Answer 21

A feature of ES5 that enforces a more restricted and safer subset of Javascript

Question 22

Primitive data types

Answer 22

• Number (integers, floats, Infinity, -Infinity, NaN)
• BigInt (integers > (2^53 - 1) or < -(2^53 - 1))
• String (anything enclosed by quotes or backticks)
• Boolean (true, false)
• Null
• Undefined (declared but unassigned)
• Symbol (represents a unique identifier)

All primitives are immutable and assigned by value

Question 23

Number

Answer 23

A primitive data type encompassing integers, floats, Infinity, -Infinity, and NaN

Question 24

BigInt

Answer 24

A primitive data type encompassing integers > (2^53 - 1) or < -(2^53 - 1)

Question 25

String

Answer 25

A primitive data type encompassing anything enclosed by quotes or backticks

Question 26

Boolean

Answer 26

A primitive data that represents either true or false

Question 27

Null

Answer 27

A primitive data type that represents an unknown or empty variable

Question 28

Undefined

Answer 28

A primitive data type that represents an unassigned variable

Question 29

||

Answer 29

The logical OR operator:

• Returns the first truthy operand of an expression
• If there are no truthy operands, it returns the last operand

Question 30

&&

Answer 30

The logical AND operator:

• Returns the first falsy operand of an expression
• If there are no falsy operands, it returns the last operand

Question 31

!

Answer 31

The logical NOT operator:

• Returns the inverse boolean value of its operand

Question 32

=

Answer 32

The assignment operator:

• Assigns a value to a variable

Question 33

==

Answer 33

The equality operator:

• Returns true if both operands evaluate to the same boolean value, and false if not
• If the operands are of different types they are converted to numbers before comparison

Question 34

===

Answer 34

The strict equality operator:

• Returns true if both operands are the same type and value
• Doesn’t attempt type conversion

Question 35

%

Answer 35

The remainder operator:

• Returns the remainder of dividing its first operand by its second

Question 36

**

Answer 36

The exponentiation operator:

• Returns the value of its first operand raised to the power of its second

Question 37

Scope chain

Answer 37

The set of enclosing scopes Javascript will search to find variables used but not declared locally

Question 38

…

Answer 38

The spread / rest operator:

• When prefixed to an iterable, it destructures it into a list of individual items (spread)
• When prefixed to the last parameter of a function, it collects the remaining arguments into a single array (rest)

Question 39

Event capturing

Answer 39

The propagation of an event from the document to all elements down the DOM tree with an event listener of the same type; the inverse of event bubbling; happens before bubbling

Question 40

New

Answer 40

The new operator; creates an instance from a supplied constructor function:

• Creates an empty object
• Adds a __proto__ property to the object that links to its constructor’s prototype
• Binds all references of ‘this’ in the constructor to the object
• Implicitly returns ‘this’

Question 41

Let

Answer 41

Declares a block-scoped variable; can be reassigned but not redeclared; only initialized when parsed (either to undefined or the assigned value)

Question 42

Var

Answer 42

Declares a global or function-scoped variable; can be reassigned and redeclared; initialized to undefined

Question 43

Const

Answer 43

Declares a block-scoped, immutable variable; must be assigned at declaration; only initialized when parsed (either to undefined or the assigned value)

Question 44

Continue

Answer 44

Jumps to the next iteration of the current loop

Question 45

Break

Answer 45

Terminates the current loop

Question 46

Switch…case

Answer 46

Evaluates an expression and executes the code whose case clause strictly matches

Question 47

Try…catch…finally

Answer 47

Try marks a block of code to execute; catch marks a block of code to execute should an exception be thrown in try; finally marks a block of code to execute after catch, regardless of whether an error was thrown or not

Question 48

Throw

Answer 48

Stops execution of the current function, generates an exception, and passes it to the nearest catch block; if there is no catch block the program terminates

Question 49

Exception

Answer 49

Any value after ‘throw’

Question 50

Error

Answer 50

An object representing a runtime error; has name and message properties

Question 51

Assign by value

Answer 51

When a variable stores its assigned value; used with primitives

Question 52

Assign by reference

Answer 52

When a variable stores a reference (pointer) to the memory address of its assigned value; used with objects