Functions

Question 1

What is a general definition of a function?

Answer 1

A function is a block of code that executes tasks in a specific order.

Question 2

How do you declare a function in JavaScript

Answer 2

function [functionName] (parameters) {

[workingCode]

}

Question 3

What are parameters of a function?

Answer 3

Parameters are optional, comma separated variables that you wish to declare for the function.

These parameters can be assigned values when you use the function.

Question 4

How can you use a function?

Answer 4

You can use a function by calling (evoking, executing) it with its name and the brackets that surround any parameters (even if there are no parameters).

functionName(parameters)

Question 5

When do parameters become arguments in functions execution.

Answer 5

Parameters become arguments when we give actual values during a function execution.

Question 6

What is the difference between parameters and arguments when using functions?

Answer 6

Parameters - variables used when declaring a function.

Arguments - values used in place of parameters when invoking a function.

Question 7

What is the result of defining a parameter in a function but not passing in an argument during execution

Answer 7

The result is that the parameter value will be ‘undefined’.

Question 8

What is returned from a function if a return statement is not included in the code block?

Answer 8

Undefined is returned

Question 9

What types of values can JavaScript pass around?

Answer 9

JavaScript can pass Primitives and Objects.

Expressions need to be evaluated to either of these two values before being passed.

Question 10

What happens if you pass a function with parameters as an argument to another function?

Answer 10

The function argument needs to be evaluated before being used in the calling function (in this case it is an expression)

Question 11

What is the term for a function (without parameters) that is passed as an argument to another function.

Answer 11

In this case the function is a callback (because you call-it-back within the code-block of the calling function).

Question 12

What is function hoisting in JavaScript.

Answer 12

Function hoisting is when function declarations are moved to the top of the scope regardless of their place in the code.

Question 13

Should we rely on function hoisting?

Answer 13

No, we should always declare our functions before invoking them.

Question 14

What is a function expression?

Answer 14

A function expression is when a variable is defined using an anonymous function.

Question 15

Are function expressions hoisted?

Answer 15

No, variables defined with a function are not hoisted in the scope.

Question 16

What is the syntax of an arrow function?

Answer 16

(param1, param2) => { code-block }

Question 17

When do we need to use parenthesis around parameters in an arrow function?

Answer 17

When there are zero, or more than one parameter.

Question 18

What is the syntax for an arrow function with one parameter?

Answer 18

param1 = > {code-block}

Question 19

If your arrow function has zero parameters, what can you replace the parenthesis with?

Answer 19

An underscore _

Question 20

How do we use implicit returns in arrow functions

Answer 20

Implicit returns are used when the function occupies one line of code and includes no curly braces { }

Question 21

What data type are functions in JavaScript?

Answer 21

Functions are objects in JavaScript.

Question 22

How is it useful that functions are objects in JavaScript?

Answer 22

Because functions are objects, they can be a value to an object property.

Question 23

What is another name for a value of an object property (or key) that is also a function?

Answer 23

These functions are called methods.

Question 24

What is the syntax for calling a method?

Answer 24

objectName.propertyName()

or

objectName‘propertyName’

The parenthesis are needed to invoke the function

Question 25

Can methods include parameters?

Answer 25

Yes, the definition of a function as a method allows for parameters?

Question 26

Can you pass methods arguments?

Answer 26

Yes, depending on the method, they will accept arguments.

Question 27

Can you use functions as a variable?

Answer 27

Yes, functions can defined as a variable - that is a function expression.

Question 28

What is a callback function?

Answer 28

A callback function, or callback, is a function passed to another function as an argument, to be executed later.

Question 29

Give an example of a commonly used callback function.

Answer 29

The event handler of an event listener is a callback function, it is passed to the eventlistener to be called once an event has occured

Question 30

How could we rewrite an event listener to explicitly show that a callback is being used?

Answer 30

function callback(){do code}

button.addEventListener(‘click’, callback)

Question 31

What are two situations where we might want to use callback functions?

Answer 31

1. When we want to allow code to be easily swapped
2. To prevent blocking situations in asynchronous code

Question 32

What are two benefits of callback functions?

Answer 32

They allow code to be easily swapped by changing which callback is used.

Callback functions prevent blocking operations in asynchronous code

Question 33

How can we easily swap code using a callback function?

Answer 33

If we create a function expression we can then pass the expression into the function as a callback. By changing which expression we use we swap which code is run during the callback

Question 34

Give an example of commonly seen asynchronous code in JavaScript.

Answer 34

An event listener is an example of asynchronous code because it does not execute immediately when found in the code.

Question 35

What is synchronous code in JavaScript?

Answer 35

Synchronous code runs in a top-to-bottom, left-to-right manner in the executable file.

Question 36

What is the idea of JavaScript being single threaded?

Answer 36

The idea is that JavaScript can only be performing one block of code at a time.

Question 37

What is blocking in JavaScript?

Answer 37

Blocking is the behavior of only running one part of a code base at a time. If something has not completed, nothing else can be undertaken.

Question 38

What is the event loop in JavaScript?

How many parts does the event loop have?

Answer 38

The event loop has three parts:

1. The call-stack: basically a to-do list of code that needs to run
2. The web-api: a waiting-list of code that is awaiting an event
3. The event queue: when a waiting-list result happens the continuation step gets added to the event queue.

The event loop is how JavaScript decides what it need to execute at any given time.

Question 39

How can we set a default value for a parameter in a function?

Answer 39

We can set the parameter = to some value:

function sayName(name = ‘Stranger’){ }

Question 40

What is a helper function?

Answer 40

A helper function is a function called within another function’s code body

Question 41

How do we pass arguments to a helper function

Answer 41

We can pass arguments to a helper function by defining them as parameters in the ‘base’ function. The parameters must have the same name as in the helper function.