JavaScript Flashcards
What is the purpose of variables?
To store bits of data
How do you declare a variable?
With a variable keyword (var, let, const) followed by the variable name
How do you initialize (assign a value to) a variable?
With an assignment operator (=)
What characters are allowed in variable names?
letters, digits, underscores and dollar signs (CANNOT START WITH NUMBER)
What does it mean to say that variable names are “case sensitive”?
upper and lowercase matter
What is the purpose of a string?
To store text data
What is the purpose of a number?
To store number data
What is the purpose of a boolean?
To identify true or false
What does the = operator mean in JavaScript?
Assignment operator
How do you update the value of a variable?
By calling again with the assignment operator followed by the new value
What is the difference between null and undefined?
Null usually a placeholder
Undefined points at valueless variables
Why is it a good habit to include “labels” when you log values to the browser console?
Describes the variable or value being logged
Give five examples of JavaScript primitives.
String
Boolean
Object
Undefined
Null
What data type is returned by an arithmetic operation?
Number
What is string concatenation?
When two or more lines of strings are combined using the addition operator
What purpose(s) does the + plus operator serve in JavaScript?
Adds one value to another
What data type is returned by comparing two values (<, >, ===, etc)?
Boolean
What does the += “plus-equals” operator do?
Adds the value of the right operand to a variable and assigns the result to the variable
What are objects used for?
To represent real world objects
What are object properties?
Information about the object
How do you remove a property from an object?
With the delete operator
What are the two ways to get or update the value of a property?
Dot and Bracket notation
What are arrays used for?
To store list data
Describe array literal notation.
var arrayName = [’’]
How are arrays different from “plain” objects?
Arrays have indexes objects dont
What number represents the first index of an array?
array[0]
What is the length property of an array?
Represents the number of properties in that array
How do you calculate the last index of an array?
array[array.length -1]
What is a function in JavaScript?
A pack of code to reuse
Describe the parts of a function definition.
function funcName(parameter,/…/,parameter) {
code …/
return;
code …/
}
Describe the parts of a function call.
funcName(parameter,/…/,parameter);
When comparing them side-by-side, what are the differences between a function call and a function definition?
No code block
What is the difference between a parameter and an argument?
Arguments are real values being passed to the function, while parameters are just placeholder names
Why are function parameters useful?
Specify what the argument should be
What two effects does a return statement have on the behavior of a function?
Returns the function when being called
Stops the code from running
Why do we log things to the console?
To check our work
What is a method?
Function of an object
How is a method different from any other function?
It’s a property of an object
How do you remove the last element from an array?
With the array.pop(); method
How do you round a number down to the nearest integer?
With the Math.floor() method
How do you generate a random number?
With the Math.random(); method
How do you delete an element from an array?
With the array.splice(index,amount);
How do you append an element to an array?
With the push(); method
How do you break a string up into an array?
With the split() method
Do string methods change the original string? How would you check if you weren’t sure?
They dont
Roughly how many string methods are there according to the MDN Web docs?
Around 50
Is the return value of a function or method useful in every situation?
No, you may use it later on
Roughly how many array methods are there according to the MDN Web docs?
40-50
What three-letter acronym should you always include in your Google search about a JavaScript method or CSS property?
mdn
What data type do comparison expressions evaluate to?
Booleans
What is the purpose of an if statement?
To execute a block of code when the condition is met or not
Is else required in order to use an if statement?
else
How do you compare two different expressions in the same condition?
&& ||
What is the purpose of a loop?
Easy way to do something repeatedly
What is the purpose of a condition expression in a loop?
To let it know when to start and stop
What does “iteration” mean in the context of loops?
To repeat
When does the condition expression of a while loop get evaluated?
At the beginning
When does the initialization expression of a for loop get evaluated?
At the very start
When does the condition expression of a for loop get evaluated?
After every initialization and after every loop
When does the final expression of a for loop get evaluated?
After the iteration
Besides a return statement, which exits its entire function block, which keyword exits a loop before its condition expression evaluates to false?
The Break; Key
What does the ++ increment operator do?
Increment the value by one
How do you iterate through the keys of an object?
With a for in loops
for ( var prop in obj ) {
code
}
Why do we log things to the console?
To keep track that everything is working.
What is a “model”?
A “model” is a memory saved model created with nodes representing an HTML element (DOM tree)
Which “document” is being referred to in the phrase Document Object Model?
The HTML document currently being worked on
What is the word “object” referring to in the phrase Document Object Model?
DOM Tree nodes
Give two examples of document methods that retrieve a single element from the DOM.
getElementById()
querySelector()
Give one example of a document method that retrieves multiple elements from the DOM at once.
querySelectorAll()
Why might you assign the return value of DOM query to a variable?
To refer back to it
What console method allows you to inspect the properties of a DOM element object?
console.dir()
Why would
tag need to be placed at the bottom of the HTML content instead of at the top?
The HTML needs to be able to run first
what does document.querySelector() take as its argument and what does it return?
Uses a CSS class selector and returns the first matching element
What does document.querySelectorAll() take as its argument and what does it return?
Uses a CSS selector and returns all matching elements
What is a DOM Tree
A tree model of all the elements of an HTML document
Why do we log things to the console?
To make sure everything is working
What is the purpose of event handling?
They allow you to indicate which event you are waiting for on any particular elements.
Are all possible parameters required to use a Javascript method or function?
No
What method of element objects lets you set up a function be called when a specific type of event occurs?
element.onevent = functionName;
What is a callback function?
A function being 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 event.target? If you weren’t su re, how would you check? Where could you get more information about it?
Refers to the event target
What is the className property of element objects?
Class Attribute
How do you update the CSS class attribute of an element using JavaScript?
By setting the new value to the element.className property using the assignment operator
What is the textContent property of element objects?
Text content of an HTML element
How do you update the text within an element using JavaScript?
By setting the new text to the element.textContent property using the assignment operator
Is the event parameter of an event listener callback always useful?
Yes it allows you to know that the listener is working but not always required
Would this assignment be simpler or more complicated if we didnt use a variable to keep track of the number of clicks?
Harder, there wouldnt be an easier way to compare the number of clicks to the required clicks so the button can change
Why is storing information about a program in variables better than only storing it in the DOM?
Storing them in variables allows for easy access when referring back to them and it makes it easier to keep track of.
What is a method?
A method is a function which is a property of an object. There are two types of methods: instance methods (built in) and static methods (called directly on an object constructor)
How can you tell the difference between a method definition and a method call?
A me
Describe method definition syntax (structure).
First have an anonymous function being assigned to an object property
Describe methods call syntax (structure)
object.property(parameter)
How is a method different from any other function?
Method is associated to an object
What is the defining characteristic of Object-Oriented programming?
A programming paradigm based on the concept of “objects”, which can contain data and code. The data is in the form of fields (attributes/properties) and the code is in. the form of procedures (methods)
What are the four “principles” of Object-Oriented Programming?
Abstraction
Encapsulation
Inheritance
Polymorphism
What is “abstraction”?
Being able to work with (possible) complex things in simple ways.
What does API stand for?
Application Programming Interface.
What is the purpose of an API?
It’s a way for two or more computers to talk to one another.
What is this in JavaScript?
The value of this depends on in which context it appears: function, class, or global
What does it mean to say that this is an “implicit parameter”?
It is included in a function’s code block even though it was never included in the function’s parameter list or declared with var.
When is the value of this determined in a function; call time or definition time?
Call Time
What does this refer to in the following code snippet?
var character = {
firstName: ‘Mario’,
greet: function () {
var message = ‘It's-a-me, ‘ + this.firstName + ‘!’;
console.log(message);
}
};
The Window because the function hasn’t been called yet
Given the above character object, what is the result of the following code snippet? Why?
character.greet();
The result would be “It’s-a-mi, Mario!”
Given the above character object, what is the result of the following code snippet? Why?
var hello = character.greet;
hello();
The result is “It’s-a-me, undefined!”
How can you tell what the value of this will be for a particular function or method definition?
You won’t know only assume
How can you tell what the value of this is for a particular function or method call?
By looking at the left of the dot
What kind of inheritance does the JavaScript programming language use?
Prototype inheritance
What is a prototype in JavaScript?
Contains functionally to reuse
How is it possible to call methods on strings, arrays, and numbers even though those methods don’t actually exist on string, arrays, and numbers?
Inherited from the prototype chain
If an object does not have it’s own property or method by a given key, where does JavaScript look for it?
Prototype
What does the new operator do?
Lets developers create an instance of user-defined object type of one of the built-in object types that has a constructor function.
What property of JavaScript functions can store shared behavior for instances created with new?
The Prototype
What dies the instanceof operator do?
Tests to see if the prototype property of a constructor appears anywhere in the prototype chain of an object.
What is a “callback” function?
A callback function is a function passed into another function as an argument.
Besides adding an event listener callback function to an element or the document, what us one way to delay the execution of a JavaScript function until some point in the future?
setTimeout()
How can you set up a function to be called repeatedly without using a loop?
setInterbal()
What is the default time delay if you omit the delay parameter from setTimeout() or setInterval()?
0
What do setTimeout() and setInterval() return?
The code/function in a delayed time
in JavaScript, when is scope determined?
When it parses through
What allows JavaScript to “remember” values from their surroundings?
A closure
What values does a closure contain?
All variables and functions in its scope
When is a closure created?
Every time a function is created
How can you tell if a function will be created as a closure?
If it needs any variables outside of itself
In React, what is one important case where you need to know when a closure is created?
When there’s a useEffect.
if you have a function that is a dependency in your useEffect, you have to care if a closure was created.
What is a closure?
The key to remember is that when a function gets declared, it contains a function definition and a closure. The closure is a collection of all the variables in scope at the time of creation of the function.
