Javascript Flashcards
What is the purpose of variables
To store data that you assign to it
How do you declare a variable?
Var
How do you initialize (assign a value to) a variable?
Using the assignment operator of =
What characters are allowed in variable name?
Name must begin with a letter, dollar sign, or underscore. CANNOT begin with a number.
What does it mean to say that variable names are “case sensitive”?
That if a variable has the same name as another variable by one is capitalized, they are not considered the same
What is the purpose of a string?
To add text
What is the purpose of a number?
For task that involve counting or calculating sums
What is the purpose of a boolean?
Helpful when determining what steps are required next depending on what conditions are met
What does the = operator mean in Javascript?
The variable’s values is being assigned to the variable’s name
How do you update the value of a variable?
Once you have declared that variable with the var keyword and assigned it a variable name and value, you only need to call the variable name and assign it a new value
What is the difference between null and undefined?
Undefined means that a value was never assigned to the variable and null is essentially a placeholder for a value created by the developer. In null you can place values in, in undefined you cannot.
What is a good habit to include “labels” when you log values to the browser console?
It helps you from getting confused when logging; it is a short string that describes the variable being logged
Give five examples of JavaScript primitives
String, number, boolean, undefined, and null
What data type is returned by an arithmetic operation?
Numbers
What is string concatenation?
When you combine strings together
What purpose(s) does the + plus operator serve in JavaScript?
To perform arithmetic operations or to concatenate strings
What data type is returned by comparing two values (<, >, ===, etc)
Boolean
What does the += “plus-equals” operator do?
Adds a value to an existing variable and assigns the new value back to the same variable
What are objects used for?
They are used to group together a set of variables and methods
What are object properties?
Variables with a name and value
Describe object literal notation
IT is an array of (key:value) pairs that are are placed inside of curly brackets
How do you remove a property from an object?
by using the delete method nameofObject.property
What are the two ways to get or update the value of a property?
Using period notation or square brackets
What are arrays used for?
To store a list of values
Describe array literal notation
You define a new array with empty square brackets
How are arrays different from “plain” objects?
Array are index objects are not
What kind of data types are array?
Reference data types
What is a function in JavaScript?
A set of statements that perform a task or calculates a value
It should take some input and return an output
When comparing them side-by-side, what are the differences between a function call and a function definition?
When you call a function you are only using the name of the function and the parameter you want to put in it
What two effects does a return statement have on the behavior of a function?
Stops the function from running. Returns the function being called
What is the difference if you declare a variable outside vs inside a function?
- If a variable is being defined within a function you can only use it in the function
o If it is declared outside of the function that is fine because you can just give the function a new value in the function
Why do we log things to the console?
To constantly be informed on what our code is doing
what is a method?
Actions that can be performed on objects
How is a method different from any other function?
A method is associated with an object, while a function is not
A method is passed on the object it is called on
How do you remove the last element from an array?
Using the pop() method
How do you round a number down to the nearest integer?
Math.floor()
How do you generate a random number?
o Math.random()
How do you delete an element from an array?
splice
How do you append an element to an array?
By using .push() or shift()
How do you break a string up into an array?
By using split
Do string methods change the original string? How would you check if you weren’t sure?
No they are not able to change
Is the return value of a function or method useful in every situation?
No because you might not want it returned
Give 6 examples of comparison operators.
o <, > , >=, <=, !== , ===
What data type do comparison expressions evaluate to?
Boolean to see if it is true or false
What is the purpose of an if statement?
To guide a program to make decisions based on specific criteria
Is else required in order to use an if statement?
It is not required you can just write if statements
Describe the syntax (structure) of an if statement.
If (condition) { two sets of code for different outcomes}
What are the three logical operators?
&&, ||, and !
How do you compare two different expressions in the same condition?
By using &&, ||
What is the purpose of a loop?
Repeats a sequence of instructions until a specific condition is met
What is the purpose of a condition expression in a loop?
It test the condition each time the loop repeats
When does the condition expression of a while loop get evaluated?
At the beginning of each iteration
When does the initialization expression of a for loop get evaluated?
The very start of the loop
When does the condition expression of a for loop get evaluated
It after the initialization and before every iteration
When does the final expression of a for loop get evaluated?
At the end of the iteration
Besides a return statement, which exits its entire function block, which keyword exits a loop before its condition expression evaluates to false?
break
How do you iterate through the keys of an object?
For … in loop
What are the four components of “the Cascade”.
Source code, inheritance, specificity, and !important
What does the term “source order” mean with respect to CSS?
The order that your CSS rules are written in your stylesheet
How is it possible for the styles of an element to be applied to its children as well without an additional CSS rule?
Inherited properties which by default are set to the computed value of the parent element
List the three selector types in order of increasing specificity.
No value, type, class, id
Why is using !important considered bad practice?
It makes it difficult for other people to go in and make changes because they’ll have to be aware of what you put important on. It disrupts the natural flow in applying the css rules where properties are applied from top to bottom.
What event is fired when a user places their cursor in a form control?
The ‘focus’ event
What event is fired when a user’s cursor leaves a form control?
the ‘blur’ event
What event is fired as a user changes the value of a form control?
The ‘input’ event
What event is fired when a user clicks the “submit” button within a <form>?
The ‘submit’ event
What does the event.preventDefault() method do?
o Cancels the event if it is cancelable, meaning that the default action that belongs to the event will not occur
o Will allow us to submit manually
o Prevents the default behavior
What does submitting a form without event.preventDefault() do?
o The request that you inputted is submitting the form the request to the URL
o You might want to do more things to the data before actually sending a request
What property of a form element object contains all of the form’s controls.
o The “elements” property
o .elements
What property of a form control object gets and sets its value?
.value
What is one risk of writing a lot of code without checking to see if it works so far?
If you do not catch the errors early one then the validity of the code after that mistake is in question
What is an advantage of having your console open when writing a JavaScript program?
You can console log anything you wish to check into the console
What is the event.target?
Whatever element that we are interacting with
What is the affect of setting an element to display: none?
Removes whatever element that we currently on the screen
What does the element.matches() method take as an argument and what does it return?
Matches compares the attribute as an argument with the attribute of whatever element you are comparing it to
How can you retrieve the value of an element’s attribute?
o .getAttribute()
What is JSON?
It is a text based format for representing structured data based on Javascript object syntax
What are serialization and deserialization?
o Serialization is the process of turning an object in memory into a stream of bytes so you can send it over the network
Turning it into a string
o Deserialization is the reverse process of turning a stream of bytes into an object in memory
Back into an object
Why are serialization and deserialization useful?
o Gives us a way to transmit data across different devices
o Clean way of being able to transmit data
How do you serialize a data structure into a JSON string using JavaScript?
o JSON.stringify()
How do you deserialize a JSON string into a data structure using JavaScript?
o JSON.parse()
How do you store data in localStorage?
localStorage.setItem(keyName, KeyValue)
How do you retrieve data from localStorage?
localStorage.getItem()
pass in the keyName and it should return the value or null if the key does not exist
What data type can localStorage save in the browser?
JSON string
When does the ‘beforeunload’ event fire on the window object?
Before the page gets reloaded
