JavaScript

Question 1

Can you name two programming paradigms important for JavaScript developers?

Answer 1

• Imperative programming, with procedural and object-oriented programming paradigms supported
• Functional programming, an example of which being closures

Question 2

What is the difference between classical inheritance and prototypal inheritance?

Answer 2

• Inheriting from a class means an object will have the attributes and methods that the super class defines
• Prototype inheritance means that an object will have a reference to attributes and methods that the prototype has, and uses these as a fallback

Question 3

What does map() do? (2)

Answer 3

• An array method, the map() method creates a new array with the results of calling a function for every array element.
• The map() method calls the provided function once for each element in an array, in order.

Question 4

What does reduce() do? (3)

Answer 4

• An array method, the reduce() method reduces the array to a single value.
• The reduce() method executes a provided function for each value of the array (from left-to-right).
• The return value of the function is stored in an accumulator (result/total).

Question 5

What does filter() do? (2)

Answer 5

• An array method, the filter() method creates an array filled with all array elements that pass a test (provided as a function).
• The filter() method executes a provided test for each value of the array (from left-to-right).

Question 6

What is a stack? How would you implement a stack in JS? (3)

Answer 6

• A stack is a FILO data structure, can be an array in JS
• Use the pop() method to remove a value from the top of the stack
• Use the push() method to add value(s) to the top of the stack

Question 7

What is a queue? How would you implement a queue in JS? (3)

Answer 7

• A queue is a FIFO data structure, can be an array in JS
• Use the shift() method to remove a value from the front of the queue
• Use the unshift method to add value(s) to the front of the stack

Question 8

What does call() do in JS? (2)

Answer 8

• It can be used to invoke (call) a method with an owner object as an argument (parameter).
• What this means is if a method includes this, then this will refer to the argument object
• It takes arguments separately, i.e if there are n values then there are n arguments

Question 9

What does apply() do in JS? (2)

Answer 9

• It can be used to invoke (call) a method with an owner object as an argument (parameter).
• What this means is if a method includes this, then this will refer to the argument object
• It takes arguments as an array, i.e if there are n values then the argument is an array of n elements

Question 10

What is lexical scope?

Answer 10

Scope created within a function, defined with var

Question 11

What is block scope?

Answer 11

The keywords let and const define block scoped variables in pairs of curly brackets, otherwise JS doesn’t support block scope

Question 12

What is scope chaining?

Answer 12

If you think of nested scoping as a tree structure, where a function b() nested in a() is a child node to a(), a scope chain is the path from a node to the root node

Question 13

What sits at the top of every scope chain?

Answer 13

A global scope object, which is window in the browser and global in node

Question 14

What is scope shadowing?

Answer 14

In a nested function, declaring a lexically scoped variable with the same name as a lexically scoped variable in the parent function is valid, and the nested function only has access to it’s locally scoped variable (normally it would have access to the parents variable too)

Question 15

What is a closure?

Answer 15

If an inner function accesses a value in an outer function, it is said to close over that value, hence is a closure. This inner function can then be returned from the outer function, and still access the outer variable.

In other words, closure is when a function is able to remember and access its lexical scope even when that function is executing outside its lexical scope.

Question 16

What is garbage collection?

Answer 16

When the JS runtime decides when/if to release any allocated memory.

Question 17

How does JS runtime garbage collect a closure? (2)

Answer 17

• For a closure to correctly resolve its reference to the closed over variable, not only does the memory for the variable need to be kept but the scope chain must also be kept
• Once the closure is no longer required, it is marked for garbage collection, followed by the closed over variable and finally the scope chain

Question 18

What does splice() do? (2)

Answer 18

• An array method, to add/remove items to/from an array, and returns the removed item(s).
• Pass starting index and number to remove, or negative index from end of array

Question 19

What is hoisting? (2)

Answer 19

• Hoisting is the JavaScript interpreter’s action of moving all variable and function declarations to the top of the current scope.
• However, only declarations are hoisted, and it’s legal to refer to a variable before its defined, so it’s possible to refer to a variable that is undefined

Question 20

How does this work in JS?

Answer 20

The context of a function, what is referred with the this keyword, in JavaScript depends on how a function is invoked, not how it’s defined.

Question 21

What is a set in JS?

Answer 21

A set is a collection of items which are unique i.e no element can be repeated.

Question 22

What does spread (…) do in JS?

Answer 22

• Spread syntax allows an iterable such as an array expression or string to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected
• It can also be used for an object expression to be expanded in places where zero or more key-value pairs (for object literals) are expected.

Question 23

Why would you use a Set in JS? (2)

Answer 23

• When there are no repetitions

* When order doesn’t matter

Question 24

What are the advantages of declarative programming? (3)

Answer 24

• Generally terser and easier to read, which aids readability and maintainability in large codebases
• Often avoid creating and mutating state, so less need for unnecessary variable and storage
• You only need to describe what you want to achieve, and there’s no need to list instructions on how to achieve it

Question 25

What is transpilation?

Answer 25

Compiling source code into new source code, rather than into an executable

Question 26

What is hoisting?

Answer 26

All declarations, both variables and functions, are moved to the “top” of a scope and processed first before any part of your code is executed. Functions are hoisted first before variables.

Question 27

How does JS handle a variable declaration initialisation (e.g var a = 2;)

Answer 27

JS sees this as two statements: var a and then a = 2. Since (variable) declarations are hoisted, the declaration is processed during compilation and the rest is left in place for execution

Question 28

What is the difference between a function declaration and a function expression?

Answer 28

• Simply, if the function keyword is the first keyword in a statement then it is a declaration, otherwise (e.g for an IIFE where ( precedes it, or when you do var a = function …) it is an expression.
• Function declarations are hoisted, whereas function expressions are not

Question 29

What is meant by functions are “first class”?

Answer 29

Functions are a sub type of object, and can be handled like any other object (i.e can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable)

Question 30

What is a module?

Answer 30

A function (generally captialised) that returns an object, which contains references to inner functions, where the object acts as an API to the hidden and private inner functions.

Question 31

What are the requirements for a module? (2)

Answer 31

• There must be an outer enclosing function, and it must be invoked at least once (each time creates a new module instance)
• The enclosing function must return back at least one inner function, so that this inner function has closure over the private scope, and can access and/or modify that private state

Question 32

How are arrow function different from normal function declarations?

Answer 32

Arrow functions discard all the normal rules for this binding, and instead take on the this value of their immediate lexical enclosing scope.