Functions

Question 1

What are functions useful for?

Answer 1

Re-useable code blocks

Efficient computation with built-in functions
e.g. myArray.join(‘ ‘)or Math.random() are part of JS but in another language like C++

Question 2

What’s the difference between parameters and arguments?

Answer 2

Parameters are placeholder for future arguments to be passed to a function.

     function name(parameter1)

Arguments are actual values that are passed to the function as a parameter

     function name('Bob');

Question 3

What are arrow functions? What three things can you apply it to?

How would you express the below as an arrow function?

function name(n) {
return n*2;
}

Answer 3

Brief, concise way to to write function expressions, one-line actions and call backs

let name = n => n*2;

Question 4

What’s the syntax for arrow functions?
(1) 1 argument
(2) no arguments
(3) multi-line steps;

Answer 4

(1) let name = n => n*2;
(2) let name = () => code;
(3) let name = () => {
code
}

Question 5

What are anonymous functions? Give an example

Answer 5

They don’t have an associated variable name, often used where a function expects to get another function as a parameter

For example, the function addEventListener() expects 2 arguments to be passed type and listener, where listener is usually another function.

Without using anon functions, we need to specify a separate function for logkey:

    addEventListener('keydown', logkey);

    function logKey(event) {
              console.log(`You pressed "${event.key}".`);
    }

We can write it like this with anonymous functions:

    textBox.addEventListener('keydown', function(event) {
            console.log(`You pressed "${event.key}".`);
    });

Question 6

What is a function expression? Give an example.

Answer 6

An expression is a function created inside an expression or another syntax contstruct

E.g. assigning a function to a variable name

      let sayHi = function() {
                  alert( "Hello" );
      };

Allow anonymous functions

Question 7

How do you declare a function?

Answer 7

This is a statement

function name(param1, param2 …) {
code;
}

Question 8

What are return values?

Answer 8

Value that is returned after a function executes

Not all functions return a value - can be void or undefined

Question 9

What is function scope?

Answer 9

Each function is discrete (walled off from other functions) - any variables declared in a function cannot be accessed by other functions

Question 10

When is a return value generally used?

Answer 10

For a function that is an intermediate step in a calculation e.g. the result it gets is stored in a variable to be used later on

Question 11

Define ‘invoke’. How would you invoke a function?

Answer 11

Invoke in coding means to instruct to run

To call a function, cite its name:
function();

Question 12

If a function is not a structure, what is it?

Answer 12

A special kind of value

from: https://javascript.info/function-expressions

Question 13

What’s the primary difference between a function expression vs declaration?

Answer 13

A Function Expression is created when the execution reaches it and is usable only from that moment.

A Function Declaration can be called earlier than it is defined.

Question 14

When should we use function expression over a declaration?

Answer 14

We should default to using a declaration at first before considering an expression: flexible organisation - we can call anywhere, easier to locate in a script

Question 15

What is a method?

Answer 15

A function that is a property of an object.

source: https://developer.mozilla.org/en-US/docs/Glossary/Method

Question 16

When should local or global variables be used?

Answer 16

Global variables are storing project level data, otherwise, it’s recommended to use local variables to limit changes and keep code clean

~ source Javascript.info

Question 17

What happens in a local variable if it has the same name as a global variable?

Answer 17

Local variable has precedence i.e. it “shadows” the global variable

Question 18

What are the 3 different ways to specify default parameter values if none are given when a function is called?

Answer 18

(1) assigning something to parameter with =. this can be simple to complex like another function

  function showMessage(from, text = "no text given") {
        alert( from + ": " + text );
  }

(2) compare parameter with undefined

(3) use || operator
function showMessage(text) {
// if text is undefined or otherwise falsy, set it to ‘empty’
text = text || ‘empty’;
…
}

Question 19

The two roles of return when used in a function?

Answer 19

(1) Returns an actual value

(2) Stops code from executing
- value may be returned
- return by itself just triggers an exit

Question 20

What is the output for an empty return or if no return is specified?

(1) function doNothing() { /* empty */ }

(2) function doNothing() {
return;
}

Answer 20

returns undefined

Question 21

What are callbacks or callback functions?

Answer 21

functions we pass as an argument in another function and call/execute later

… like addEventListener??

Question 22

To increase readability and help make our script maintainable, we should write how many actions per line?

Answer 22

One line, one action

Question 23

Do functions have to have arguments?

Answer 23

No