Question

What is an arrow function?

Answer 1

A function defined as follows:

let myFunc = () => {
console.log("Hello World")
}

Answer 2

A parameter are the variables defined in the function declaration.

function addThree(parameter1, parameter2, parameter3) {
return parameter1 + parameter2 + parameter3
}

Answer 3

An argument passes variables or values to a function in the function call

function addThree(parameter1, parameter2, parameter3) {
return parameter1 + parameter2 + parameter3;
}

let argument1 = 3;
let argument2 = 5;
let argument3 = 9;

addThree(argument1, argument2, argument3);  //returns 17

Answer 4

Returns NaN - because 1 + undefined is not a number.

Answer 5

initial expression: for (let i = 0; ... ) {}
condition: for (let i = 0; i < array.length; ... ) {}
loopEnd expression: for (let i = 0; i < array.length; i++)

Answer 6

Comments are used in code to further define what the code is doing. They are NOT to be executed.

Answer 7

1 and 3. #1 is for multiple lines of comments. The last '*/' will close the comment area. // may also be a comment, but you must use it at the beginning of each comment line.

Answer 8

No! This will return a syntax error because it is trying to evaluate "it". '//' must be used before each line on a comment. To comment out multiple lines of code use the pair: '/* */'

Answer 9

Use console.log() to print out variable values.

Answer 10

Use string concatenation: console.log("myVar Value: " + myVar)
Use interpolation: console.log(`myVar Value: ${myVar}`)
Also string concatenation, using "," console.log("myVar Value1: ", myVar, " and adding 1 is: ", myVar + 1)

Answer 11

Yes, but it may not do what you intended. If, for instance, you have two variables a=1 and b=2, and you run the code above it will not add a and b. Instead, it will concatenate the two characters 'a' and 'b' into another string of 'ab'.

Answer 12

Yes, but again, it may not give you what you intended. Here JavaScript sees a letter and a number. It will attempt to convert the number to a letter, then concatenate. The result will be '1b'

Answer 13

A float or an integer. It can be negative, or positive.

Answer 14

The result will be 1020. Because you took a string, used the concatenate operator and added x and y. To get the result of 30, you would use:

let x = 10;
let y = 20;
console.log("The result is: ", x + y)  ... or
console.log(`The result is: ${x + y}`);

Answer 15

REPLS: Read, Evaluate, Print Loops. Invoking node at the command line will provide a REPL.

Answer 16

Variables cannot begin with Numbers, and "-" is never allowed, nor is "#" ever allowed.

Variables can be any alpha/numeric value, $, or _; and cannot begin with a number

let 1Number = 0;
let -Number = 0;
let #Number = 0;

Answer 17

False

Variables may only begin with underscore (_), dollar sign($), or alphabetic characters

Answer 18

my#Variable: # is neither alphabetic, numeric, nor $ nor _

1Variable: Although numeric, the variable cannot begin with a number

Answer 19

"bootcamp".length ==> 8

Answer 20

'bootcamp'[3] ==> 't'

Answer 21

'bootcamp'[0] = 'b'

Answer 22

Index is a zero based number. Length counts starting at 1.

Answer 23

Overrunning the length of the array or string. 'bootcamp'.length = 8, but 'bootcamp'[8] is undefined

Answer 24

Use the indexOf method: 'bootcamp'.indexOf('t') => 3;

Answer 25

Use the '+' operator: 'Hello' + 'World' = 'HelloWorld'

Answer 26

+, -, *, /, %

Answer 27

! = not
&& = and
|| = or

Answer 28

multiplication, division : left to right
addition, subtraction : left to right

Answer 29

NOT equal to

Answer 30

The expression on the left must be true AND the expression on the right must be true; otherwise the answer is false.

Answer 31

The expression on the left must be true, and if it is not, the expression on the right is evaluated to see if it is true. If neither are true, the expression is evaluated to false. Otherwise the expression is evaluated to true.

Answer 32

Only the expression that is required to determine the outcome is evaluated, starting from left to right. For instance, if true && false, both the left 'true' and the right 'false' must be evaluated. However, if 'false' && 'false' then only the left needs to be evaluated, because only one expression must be false for an '&&' operation to evaluate to false.

In the case of ||, if the left expression is true, the expression evaluates to true. So the right expression is never evaluated. If the left expression evaluates to false, then the right must be evaluated to determine if the statement evaluates to false.

Answer 33

DeMorgan's law refers to the associativity of && and || expressions. Given a grouped expression, how would you break it apart into individual expressions.

!(A || B) === !A && !B
!(A && B) === !A || !B

My memory aid is: "NOT of AND is OR, and NOT of OR is AND"

Answer 34


A	B	A&&B
false	false	false
false	true	false
true	false	false
true	true	true

Answer 35

'a' < 'b' = true

Answer 36

"Apple" < "apple" = true

Answer 37

'A' > 'a'

Answer 38

1 > 2 = false

Answer 39

PEMDAS: Parentheses, Exponents, Multiplication and Division (from left to right), Addition and Subtraction (from left to right)

Answer 40

==: Used for comparing two variables, but it ignores the datatype of the variable

===: Used for comparing two variables, but this operator also checks the datatype when comparing.

Answer 41

5 == "5" => true Because == does not compare the datatype.

Answer 42

5 === "5" //false Because === evaluates the datatype as well. The number 5 is not equal to the string '5'.

Answer 43

    while (condition) {
        ... code to be executed here
    }

Answer 44

    for (let i = 0; i < 5; i++) {
        ... do something
    }

Answer 45

An iteration is each pass through the loop, or each time the loop executes.

Answer 46

    function forLoopDoubler (array) {
        let i = 0;
        while (i < array.length) {
            array[i] = array[i] * 2;
            i++;
        }
        return array;
    }

Answer 47

Use the .length method: myArray.length

Answer 48

Arrays are always defined with square brackets: let myArray = ['a', 'b', 'c']

Answer 49

Either use indexOf, or because 9 is the last element in the array you can use array.length-1;

myArray.indexOf(9)

myArray[myArray.length - 1]

Answer 50

numbersAndLetters.indexOf(17) ==> 2

Notice that because 17 is a number, it is stored in the array index of 3. It is NOT stored as index of 3 AND 4. Similarly with 'cat', stored at index 3. This element is a string. Arrays can store different data types, and each data type gets its own index.

Answer 51

Arrays are 0-based -- meaning the first element is ALWAYS 0.

Answer 52

Using length as an index. Because arrays are zero based, the last index of the array would be length-1, NOT length! myArray.length

Answer 53

1, 2, 34, 5, 6 : Javascript tries to convert your array into a string. :-(

Answer 54

console.log([1, 2, 3].concat([4, 5, 6])); // => [1, 2, 3, 4, 5, 6]

Answer 55

The number 4 is not included in the array. So the indexOf method would return -1

Answer 56

Use the .concat method: arrayOne.concat(arrayTwo)

Answer 57

myArray.pop() will return the last element of the array. Further, it will remove the last element from the array.

Answer 58

myArray.shift(): This will both remove the first element from the array, and return that element to the caller.

Answer 59

If you want to add the element to the end of the array, use .push(<element>): myArray.push('newElement')

Answer 60

Use the unshift method: myArray.unshift('newFirstElement')

Answer 61

Array.map()

    let myArray = [ 1, 2, 3, 4 ]

    myArray.map( (elt) => {
        return elt * 2
    });

    //returns:  [2, 4, 6, 8 ]
    //does NOT mutate original array.

Answer 62

Array.filter

    let myArray = [ 1, 4, 7, 6, 12, 9]

    myArray.filter( (elt) => {
        if (elt % 2 === 0) return elt;
    });

    //returns [4, 6, 12]

Answer 63

accumulator.

Answer 64

If accumulator does not have an initial value set, then accumulator is set to the first element of the array

Answer 65

    let nums = [3, 7, 5, 9];

    let sum = nums.reduce ( (accum, el) => {
        return accum + el;
    });

    //sum is now 24.

Or, in arrow function syntax:

    let sum = nums.reduce ( (accum, el) => return accum + el, 100);

Or ...

    Array.reduce( fn(), accum);

Answer 66

accum = 3; elt = 7;

Answer 67

This would be the 2nd argument to the reduce function:

    let nums = [3, 7, 5, 9]

    let sum = nums.reduce(function(accum, el) {
        return accum + el;
    }, 100);

Answer 68

    let nums = [3, 7, 5, 9]

    let product = nums.reduce( (accum, elt) => {
        return accum * el;
    });

    // returns

Answer 69

Because you're doubling each number, you'll need to double the first number. This requires that the accumulator doubles the number - but the accumulator will not automatically do this. So you'll need to pass in an initial value for the accumulator so that it doesn't use the first element of the array as the initial value.

    let nums = [3, 7, 5, 9];

    let doubleProduct = nums.reduce( (accum, el) => {
        return accum * 2 * el
    }, 1);

Now on the first pass accumulator will be set to 1, and each element will be doubled before being multiplied -- including the first element. Remember: If there is no initial value given for the accumulator, the initial value will be set to the first value in the array, and the second element in the array will be used in the first iteration of the reducer's function.

Answer 70

The accumulator.

Answer 71

    let nums = [3, 7, 5, 9, 2]

    let max = nums.reduce( (accum, el) => {
        if (el > accum) {
            return el;
        } else {
            return accum
        }});

Answer 72

    let nums = [1, 2, 3]
    let newArray = []

    for (let i = 0, i < nums.length; i++) {
        newArray.push(2 * nums[i]);
    }

or ...

    let nums = [1, 2, 3];

    let newArray = nums.forEach( (elt, index) => {
        return elt[i] * 2
    }

Answer 73

When you write code to avoid throwing exceptions. eg:

    if (value !== undefined) {}
    if (value !== null) {}

Answer 74

The function that is passed into Array#forEach may accept the current element, index, and array

Answer 75

The Array#reduce method accepts a function as it's first argument and a value to be used as the initial accumulator as an optional second argument.

Answer 76

The function that is passed into Array#filter should return boolean.

Answer 77

The function that is passed should accept the current accumulator and current element as its arguments.

Answer 78

The Array#filter method accepts a function as an argument and returns a new array containing elements of the original array that result in `true` when passed into the function.

Answer 79

The Array#map method accepts a function as an argument returns a new array containing the results of calling that function on each element of the original array.

Answer 80

Array#shift

Answer 81

The String#split method accepts a "separator" string as an argument and returns an array containing the resulting substrings when the string is cut along instances of the "separator".

For example:

    let sentence = "follow the yellow brick road";
    let words = sentence.split(" ");
    console.log(words); // [ 'follow', 'the', 'yellow', 'brick', 'road' ]

Answer 82

Both methods accept a start index and end index representing the range of indices to be sliced as a new array or string. The start index is inclusive, while the end index is exclusive.

Answer 83

"Mutability" refers to an object's ability to undergo changed. Mutable objects can be modified, while immutable objects cannot be changed after they are created.

Answer 84

The Array#join method accepts a "separator" string as an argument and returns a string where elements of the array are concatenated together with the "separator" between each element.

For example:

let words = ["run", "around", "the", "block"];
console.log(words.join("_")); // 'run_around_the_block'

Answer 85

Arrays are mutable.

Numbers, strings, and booleans are all immutable. Don't confuse mutability with the variable types: var, let and const. When you define a variable with let, then yes. You can change the value of the variable. Similarly with var. However, when you change the value of the variable the variable is re-created in a different location in memory. This makes the data types shown immutable. Arrays, on the other hand, you can change the contents of the array without changing where the array points in memory. The variable containing the array will never change its memory address.

Answer 86

The Array#unshift method accepts one or more values as arguments and will add those values to the front of the array, mutating the array. The method returns the new length of the array.

let array = ['a', 'b', 'c', 'd' ]
let arrayLength = array.unshift('e', 'd', 'f', 'g')

arrayLength  //=> 8
array  //=> :
            [
            'e', 'd', 'f',
            'g', 'a', 'b',
            'c', 'd'
            ]

Answer 87

The Array#push method accepts one or more values as arguments and adds those values as new elements to the end of the array, mutating the array. The method will return the new length of the array.

let array = [1, 2, 3, 4]
array.push(6, 7, 8, 9, 10)  //=> 9
array      // => :
                [
                1, 2, 3,  4, 6,
                7, 8, 9, 10
                ]

Answer 88

Array#splice has two required arguments: a target index and a number of elements to remove

Array#splice can additionally accept any number of additional arguments representing values to be inserted

array  // => :
            [
            1, 2, 3,  4, 6,
            7, 8, 9, 10
            ]
array.splice(-1)  //=> [ 10 ]  -- removes 10 from the arrray and returns it as an array
array    // =>  array now does not have 10 in it
                [
                1, 2, 3, 4,
                6, 7, 8, 9
                ]
array.splice(4, 1) // returns [ 6 ] from the array, while removing it from the array
[ 6 ]
array  // Array no longer has 6 in it
            [
            1, 2, 3, 4,
            7, 8, 9
            ]
array.splice(0, 1, 'a', 'b', 'c')  //removes 1 from the array and returns [ 1 ], then adds 'a', 'b', 'c', in its place
array  // contents of the array has removed 1, and in its place put 'a', 'b', 'c'
        [
        'a', 'b', 'c', 2, 3,
        4,   7,   8,   9
        ]

Answer 89

['a', 'b', 'f'] : Array#splice mutates the input array, removing 3 elements starting at index 2.

Answer 90

A mutuble object can be changed. An immutable object cannot be changed.

Answer 91

Arrays are mutable. Booleans, Numbers, and Strings are immutable.

Answer 92

['a', 'b', 'c', 'd', 'e'] : Array#slice does not mutate the input array, so arr is unchanged.

Answer 93

A primitive (primitive value, primitive data type) is data that is not an object, and has no methods.

Answer 94

Six: string, number, bigint, boolean, undefined and symbol.

We haven't learned about symbols yet.

Answer 95

Null means "the ABSENCE of data". Any data type can be set to null.

Answer 96

Use the built in "typeof" operator

    console.log(typeof null); // 'object'
    console.log(typeof 'c'); // 'string'
    console.log(typeof 1); // 'number'
    console.log(typeof true); // 'boolean'

    let a;
    console.log(typeof a); //'undefined'

Answer 97

A first-class object is something that can be stored in a variable.

Answer 98

Yes! You can store a function in a variable and pass it to another function

Answer 99

Use a try/catch block

Answer 100

try, catch, finally

Answer 101

Provides a mechanism to evalute statements and throw errors if the statements do not conform.

    try {
        if (array.length === 0) {
            do something ...
        }
    }

if array is undefined, checking the length of an undefined object will throw an exception.

Answer 102

You use a catch block to gracefully process the error, rather than allowing your code to break and throw an exception to the user.

    try {
        if (array.length === 0) {
            ... do something
        }
    } catch (e) {
        if (e === instanceOf ReferenceError) {
            console.log("array is not defined");
        } else {
            console.log("error:  ", e)
        }
    }

Answer 103

Function Expression Syntax

Answer 104

throw Error('this error');

throw new Error('this error');

Answer 105

The finally block runs whether or not an error occurs. It always runs.

    function sumArray(array) {
        let sum = 0;

        try {
            for (let i = 0; i < array.length; i += 1) {
                sum += array[i];
            }
        } catch (e) {
            console.log(e);
            return null;
        } finally {
            console.log("This wil aways print.");
        }
        return sum;
    }

Week 1 Quiz

Learning Objectives:

Functions

Comments

Simple debugging

The Number data type

REPL

Variables: let, const, var

Strings

Arithmetic Expressions, Grouping and Order of Precedence

Loops and iteration: for and while

Arrays

Mutability

Data Types

First Class Objects

try/catch blocks