JavaScript Flashcards
what is the purpose of variables
Variables are containers for storing data
Variables store data values
How do you declare a varaible?
Ways to Declare a JavaScript Variable:
Using var
Using let
Using const
How do you initialize (assign a value to) a variable?
To initialize (assign a value to) a variable, you use the equal sign =
what characters are allowed in variable names?
variable names can’t contain spaces
variable names must begin with a letter, an underscore, or a dollar sign
variable names can contain letters, underscores, numbers, or dollar signs
variables are case sensitive
What does it mean to say that variable names are case sensitive?
It means that the case matters
cat =/= Cat
What is the purpose of a string?
JavaScript strings are for storing and manipulating text
what is the purpose of a number?
The purpose of numbers is to represent and manipulate numbers
what is the purpose of boolean?
A JavaScript Boolean represents one of two values: true or false.
Very often, in programming, you will need a data type that can only have one of two values, like
YES / NO
ON / OFF
TRUE / FALSE
For this, JavaScript has a Boolean data type. It can only take the values true or false.
what does the = operator mean in JavaScript?
the = operator is an assignment operator.
it assigns value to JavaScript variables
how do you update the value of a variable?
You update the value of a variable by reassigning it using the assignment operator =
what is the difference between null and undefined
The undefined property indicates that a variable has not been assigned a value, or not declared at all.
null is a special value that represents an empty or unknown value.
why is it a good idea to include labels when you log values to the browser console?
so you know when shit dont work
give five examples of JavaScript primitives
string, number, boolean, null, undefined
what data type is returned by an arithmetic operation
number data type is returned by arithmetic operation
what is string concatenation?
string concatenation is the combining of strings
what purpose does the addition (+) operator serve in JavaScript
the addition (+) can be used for arithmetic or concatenating strings
what data type is returned by comparing two values with logical operators?
boolean data type is returned when comparing two operands with logical operators
what does the addition assignment (+=) operator do?
the addition assignment (+=) operator assigns the result of the expression back to the first value
what are objects used for
An object is a collection of related data and/or functionality.
These usually consist of variables and functions
(which are called properties and methods when they are inside objects).
what are object properties?
An object is a collection of related data and/or functionality. These usually consist of several variables and functions (which are called properties and methods when they are inside objects).
describe object literal notation
{
key: value
}
How do you remove a property from an object?
To delete a property from an object, you can use the delete operator Delete Object.Property Delete person(object).name(property)
What are two ways to get or update the value of a property?
The two ways to get or update the value of a property are dot and bracket notation
What are arrays used for?
An array is a special variable, which can hold more than one value
Arrays are used to store data
Think of arrays as lists
Describe array literal notation
Array literal notation looks like the following: var newArr = []
How are arrays different from plain objects?
Objects represent things with properties and methods
Array create and store lists of data in a single variable
What number represents the first index of an array?
Zero is the number which represents the first index of an array
Arrays are based on zero index, meaning arrays start a 0 instead of 1
What is the length property of an array?
he length property of an array returns the length of an array
The length is 1 indexed, meaning the count starts at 1
The length is not 0 indexed
How do you calculate he last index of an array?
To calculate the last index of an array, you would use the .length property and subtract 1
You subtract 1 because .length property is 1 indexed, and arrays are 0 indexed
What is a function in JavaScript?
A JavaScript function is a block of code designed to perform a particular task
A JavaScript function is executed when something invokes/calls it
Describe the parts of a function definition (when a function is being created)
A JavaScript function is defined with the function keyword, followed by a name followed by parentheses
What characters are allowed for function names?
Function names can contain letters, digits, underscores, and dollar signs (same rule as variables)
What can be included within the parentheses of a function definition?
a function definition’s parentheses can contain parameters separated by commas
Describe the parts of a function call
The parts of a function call include the name of the function along with any arguments that may or may not be passed
When comparing them side-by-side, what are the differences between a function call and a function definition?
a function definition has the function keyword preceding (coming before) the name
Function definition: function sampleFunction()
Function call: sampleFunction()
Why are function parameters useful?
Function parameters are useful because they hint at what values should be passed in as arguments for the function
What two effects does a return statement have on he behavior of a function
When JavaScript reaches a return statement, the function will stop executing.
Functions often compute a return value. The return value is returned back to the caller
What is a method?
JavaScript methods are actions that can be performed on objects
A JavaScript method is a property containing a function definition
Methods are functions stored as object properties
You access an object method with the following syntax:
objectName.methodName()
How is a method different from any other function?
A function can be called without an object
A method is a function that is associated with a particular object
How do you remove the las element from an array?
You can use he array.pop() method to remove elements from the end of an array
How do you round a number down to the nearest integer?
You can round down to he nearest integer with the Math.floor() method
Example: Math.floor(1.6) returns 1
How do you generate a random number?
You can generate a random number with the Math.random() function
Example: Math..Random(100) returns a number from 1 - 100
How do you delete an element from an array?
You can use the array.splice() method to remove elements from an array
You can use the array..pop() method to remove elements from the end of an array
You can use the array.shift() method to remove elements from the front of an array
How do you append an element to an array?
You can use the array.push() method to append an element to an array
You can also use array[array.length] = value
How do you break a string up into an array?
You can use the string.split() method to break a string into an array
Do string methods change the original string? How would you check if you weren’t sure?
No, string methods do not change the original string
Strings are immutable
You could check by console.log(‘string’)
Is the return value of a function or method useful in every situation?
No, the return value of a function or a method is not useful in every situation, it can be excluded
What are comparison operators?
Comparison operators are used in logical statements to determine equality or difference between variables or values
Give 6 examples of comparison operators
=== !== >< >= <=
What data type do comparison expressions evaluate to?
Comparison expressions evaluate to boolean values (true or false)
What is the purpose of an if statement?
If statements are used to execute specific JavaScript code blocks if a condition is true
Is else required in order to use an if statement?
No
Describe the syntax of an if statement
if (condition) {
code to be executed if condition is true
}
What are the three logical operators?
&& (and)
|| (or)
! (not)
How do you compare two different expressions in the same condition?
You can compare two different expressions in the same condition by wrapping the expression in parenthesis
What is the purpose of a loop?
To repeat an action
What is the purpose of a condition expression in a loop?
the condition expression of a loop = the brakes for the loop
the purpose of the condition expression in a loop is to tell the loop when to stop (the brakes)
condition - a boolean expression to be evaluated before each loop iteration. If this expression evaluates to true, the inner commands will be executed
What does iteration mean in the context of loops?
Every time the loop runs through the code block
What is a condition within the context of a for loop?
A condition is an expression to be evaluated before each loop iteration
If this expression evaluates to true, statement is executed
What is a statement within the context of a for loop?
A statement is what’s contained within the opening curly brackets for the for loops’ code block
A statement ( code within the code block ) that is executed as long as the condition evaluates to true
When does the condition expression of a while loop get evaluated?
before each loop iteration
When does the initialization expression of a for loop get evaluaed?
The initialization expression of a for loop gets evaluated at the beginning ( initialization )
When does the condition expression of a for loop get evaluated?
before each loop iteration
When does the final expression of a for loop get evaluated?
The final expression of a for loop gets evaluated after the code block / at the end
Besides a return statement ( which exits its entire function block ) which keyword exits a loop before its condition expression evaluates to false?
The break statement keyword exits a loop before its condition expression evaluates to false
what does the ++ increment operator do?
The increment operator ( ++ ) increments ( adds one to ) its operand ( data value ) and returns a value
What is an operand?
an operand is a data value
how do you iterate through the keys of an object?
you iterate through the keys of an object with a for in loop
the for in loop iterates ( loops ) over the properties ( keys ) of an object
The code block inside the loop is executed once for each property
The properties ( keys ) are the condition. The code runs for however many keys there are
Which document is being referred to in the phrase Document Object Model ( DOM )
The document object
the html document
What is a DOM tree?
The HTML DOM model constructed as a tree of objects ( HTML elements represented as DOM elements )
Give two examples of document methods that retrieve a single element from the DOM
document.querySelector()
getElementByID()
Give one example of a document method that retrieves multiple elements from the DOM at once
document.querySelectorAll()
Why might you want to assign he return value of a DOM query to a variable
Why would you do this:
var pElement = document.querySelector(‘p’)
You might want to add/change/remove the element
You might want to add/change/remove the element’s attributes
You might want to add/change/remove the events associated with the element or element’s parent/descendants
what console method allows you to inspect the properties of a DOM element object?
console.dir()
why would a tag need to be placed at the bottom of the HTML content instead of at the top?
because when the browser loads the page, it must complete loading relevant JS scripts/files before loading the rest of the page. this causes a slow page load / bad UX
what does document.querySelector() take as its argument and what does it return?
document.querySelector() takes CSS selectors as its argument
What is the purpose of events and event handling?
to execute code when an event occurs
what is a callback function?
a callback function is a function passed into another function as an argument
what is the event.target?
the event.target is the location of the event trigger
.target is a property of the event object
what is the className proeprty of element objecs?
the className property sets or returns an element’s class attribute
