JavaScript Flashcards
What is the purpose of variables?
They are used to store data to utilize later on.
How do you declare a variable?
With the “var” keyword and the variable name.
How do you initialize (assign a value to) a variable?
With the “=” assignment operator and the variable value.
What characters are allowed in variable names?
Letters, dollar sign, underscore, numbers (cannot start with one)
What does it mean to say that variable names are “case sensitive”?
Variable names have to match exactly with correct capitalization.
What is the purpose of a string?
They hold text, letters and other characters.
What is the purpose of a number?
For counting or calculating values.
What is the purpose of a boolean?
- To determine which part of a script should run.
- Logic with binary states.
What does the = operator mean in JavaScript?
Assignment operator, used to assign a value.
How do you update the value of a variable?
The name of the variable with the new value.
What is the difference between null and undefined?
- Null is a nonexistent or invalid object or address
- Null has to be assigned.
- Undefined are for variables that have just been declared or to formal arguments for which there are no actual arguments.
Why is it a good habit to include “labels” when you log values to the browser console?
Labels in the console are good for knowing where the value stems from.
Give five examples of JavaScript primitives.
String, number, boolean, null, and undefined.
What data type is returned by an arithmetic operation?
A number.
What is string concatenation?
Combining string values.
What purpose(s) does the + plus operator serve in JavaScript?
- Adding together numbers, strings, and variables.
What data type is returned by comparing two values (, ===, etc)?
Boolean
What does the += “plus-equals” operator do?
Adds the value and assigns the result to the variable.
What are objects used for?
- Grouping together a set of variables and functions to create a model.
- A box of data related to each other grouped together.
What are object properties?
Properties are variables that tell us about the object.
Describe object literal notation.
Curly braces and its contents.
How do you remove a property from an object?
Use the delete keyword.
What are the two ways to get or update the value of a property?
Using dot and bracket notation.
What are arrays used for?
Lists or set of values that are related to each other.
Describe array literal notation.
Square brackets with each value separated by a comma.
How are arrays different from “plain” objects?
Arrays have index numbers, an order, and square brackets.
What number represents the first index of an array?
0
What is the length property of an array?
A true count of the amount of items within an array.
How do you calculate the last index of an array?
One less than the length of the array.
Why do we log things to the console?
It’s a debugging tool used to check errors.
What is a method?
Methods are functions which is a property of an object.
How is a method different from any other function?
Property of an object.
How do you remove the last element from an array?
pop method
How do you round a number down to the nearest integer?
floor method
How do you generate a random number?
random method
How do you delete an element from an array?
splice method
How do you append an element to an array?
push method
How do you break a string up into an array?
split method
Do string methods change the original string? How would you check if you weren’t sure?
- They do not change the original string.
- To check just log it into a console.
Roughly how many string methods are there according to the MDN Web docs?
Around 30 string methods.
Is the return value of a function or method useful in every situation?
No.
Roughly how many array methods are there according to the MDN Web docs?
Around 25 array methods.
What three-letter acronym should you always include in your Google search about a JavaScript method or CSS property?
MDN
Give 6 examples of comparison operators.
===, !=, less than, greater than, less than or equal to, greater than or equal to, &&
What data type do comparison expressions evaluate to?
Boolean
What is the purpose of an if statement?
Evaluates or checks a condition to see if its true, if it is then the code block executes.
Is else required in order to use an if statement?
No, if you don’t need a fallback.
Describe the syntax (structure) of an if statement.
if keyword - condition - code block
What are the three logical operators?
and operator, or operator, logical not operator
How do you compare two different expressions in the same condition?
logical operator: and, or
What is the purpose of a loop?
Repeating an action some number of times, under a certain condition.
What is the purpose of a condition expression in a loop?
To see if the loop will execute or not. If it’s true, then it will, if it’s false then it will terminate.
What does “iteration” mean in the context of loops?
One repetition for the code block.
When does the condition expression of a while loop get evaluated?
Evaluated before the code block runs.
When does the initialization expression of a for loop get evaluated?
It happens first and only once.
When does the condition expression of a for loop get evaluated?
After the initialization, and before each iteration.
When does the final expression of a for loop get evaluated?
At the end of each iteration, before the condition.
Besides a return statement, which exits its entire function block, which keyword exits a loop before its condition expression evaluates to false?
Break.
What does the ++ increment operator do?
Adds 1 and assigns it to a variable.
How do you iterate through the keys of an object?
For…in loop.
What is a “model”?
A model is representation of something.
Which “document” is being referred to in the phrase Document Object Model?
The HTML page.
What is the word “object” referring to in the phrase Document Object Model?
The different parts of a page.
What is a DOM Tree?
An element and all of its children.
Give two examples of document methods that retrieve a single element from the DOM.
getElementByID() and querySelector().
*use querySelector for everything
Give one example of a document method that retrieves multiple elements from the DOM at once.
querySelectorAll()
Why might you want to assign the return value of a DOM query to a variable?
If you need to reuse the element more than once it is much easier to access.
What console method allows you to inspect the properties of a DOM element object?
console.dir()
Why would a script tag need to be placed at the bottom of the HTML content instead of at the top?
HTML and CSS needs to be ran and loaded before the script can run.
What does document.querySelector() take as its argument and what does it return?
Takes a string of a CSS selector.
What does document.querySelectorAll() take as its argument and what does it return?
Also takes a string of a CSS selector.
What is the purpose of events and event handling?
Events are used to trigger a particular function.
Event handling are the steps involved in getting it to trigger.
Are all possible parameters required to use a JavaScript method or function?
No.
What method of element objects lets you set up a function to be called when a specific type of event occurs?
addEventListener()
What is a callback function?
A function passed into another function as an argument.
What object is passed into an event listener callback when the event fires?
The event object.
What is the event.target? If you weren’t sure, how would you check? Where could you get more information about it?
The target property of the event object into which the event was dispatched.
What is the className property of element objects?
It gets and sets the value of the class attribute of the specified element.
How do you update the CSS class attribute of an element using JavaScript?
The className property with a string value with the name of the class.
What is the textContent property of element objects?
Represents the text content of the node and its descendants.
How do you update the text within an element using JavaScript?
The textContent property and the value of a string.
Is the event parameter of an event listener callback always useful?
No.
Why is storing information about a program in variables better than only storing it in the DOM?
Variables allow you to make adjustments easily.
They are more dynamic.
