More Basics

Question 1

How do you get all the keys in an object?

Answer 1

Object.getOwnPropertyNames() or Object.keys() - returns an array of object keys

Question 2

How do you get all the values in an object?

Answer 2

Object.values() - returns an array of object values

Question 3

How do you get all keys and values in an object?

Answer 3

Object.entries() returns an array of [key, value]

Question 4

What is hoisting?

Answer 4

Hoisting is JavaScript’s default behavior of moving all declarations to the top of the current scope (to the top of the current script or the current function).

Question 5

When are hoisted variables initialized?

Answer 5

JavaScript only hoists declarations, not initializations. Initializations occur as written in the code.

Question 6

What happens due to hoisting when you declare a variable with let?

Answer 6

Variables defined with let and const are hoisted to the top of the block, but not initialized.

Meaning: The block of code is aware of the variable, but it cannot be used until it has been declared.

Using a let variable before it is declared will result in a ReferenceError.

Question 7

What happens due to hoisting when you declare a variable with const?

Answer 7

Variables defined with let and const are hoisted to the top of the block, but not initialized.

Meaning: The block of code is aware of the variable, but it cannot be used until it has been declared.

Using a const variable before it is declared, is a syntax errror, so the code will simply not run.

Question 8

What does Javascript evaluate as a boolean False?

Answer 8

false, 0, ‘ ‘, “ “, null, undefined, NaN

Question 9

What does Javascript evaluate as a boolean True?

Answer 9

‘0’, ‘false’, [ ], { }, function() { }

Question 10

What is the rule of precision for decimal numbers?

Answer 10

Always treat them as approximations.

Question 11

What does the template literal ${} do inside a string?

Answer 11

Evaluates whatever is inside the {}

Question 12

What is one value in javascript that doesn’t equal itself?

Answer 12

NaN

Question 13

How do you test whether a variable has a real value?

Answer 13

== or != null

Question 14

What does the || operator do?

Answer 14

Returns the left value if true and the right value otherwise.

Question 15

What are legal characters for binding names?

Answer 15

Letters, numbers, $, and _

Question 16

What functions convert data to different types?

Answer 16

Number, String, Boolean

Question 17

What function call evaluates if the input is a number?

Answer 17

Number.isNan (use ! to determine if the input is a number)

Question 18

What is lexical scoping?

Answer 18

Each local scope can see the local scopes that contain it, and all scopes can see the global scope.

Question 19

What happens when you make a function call with extra parameters?

Answer 19

Nothing, the function ignores the extra values.

Question 20

How do you copy all properties from one object to another?

Answer 20

Object.assign(Original, Copy)

Question 21

What does == do when applied to Objects?

Answer 21

Checks whether they point to the same Object, not whether the contents are equivalent.

Question 22

What are the two ways to access a value’s properties?

Answer 22

Bracket and dot notation

Question 23

What’s the difference between the bracket and dot notation?

Answer 23

The word after the dot is the literal name of the property, the data in brackets is evaluated. For dot notation, the word should be a valid binding (not a number or variable).

Question 24

What happens if you read an object property that doesn’t exist?

Answer 24

You get undefined.

Question 25

What does the binary in operator do on an object?

Answer 25

It returns true or false if the object has or doesn’t have a property.

Question 26

How do you get all property names in an object?

Answer 26

Object.keys(objName) returns an array of object keys.

Question 27

What types of values are immutable and what does that mean?

Answer 27

Numbers, Strings, booleans. You can’t change them but you can combine them and derive new values from them. For example, you can’t change the string “cat” to “rat.”

Question 28

What happens if you pass concat an argument that isn’t an array?

Answer 28

It treats the object like a one element array.

Question 29

What is unique about using indexOf on a string?

Answer 29

It can search for multiple-character patterns - i.e “three”.indexOf(“ee”);

Question 30

What do string.split and string.join do?

Answer 30

split the string into an array strings every time (arg) appears and join an array of strings into string with (arg) as a separator.

Question 31

how do you add a string to itself?

Answer 31

string.repeat(x) - repeats a string x times

Question 32

What are restrictions on JSON bindings?

Answer 32

All property names have to be surrounded by double quotes and only simple expressions are allowed (no function calls, bindings, or anything requiring computation.