JavaScript Flashcards
What is the purpose of variables?
To store data needed for a script to do its job.
How do you declare a variable?
Variables are declared by writing the var keyword and variable name followed by a semicolon.
How do you initialize (assign a value to) a variable?
Variables are initialized by writing the assignment operator and value after the variable name.
What characters are allowed in variable names?
Letters, numbers, underscores (_), and dollar signs ($).
What does it mean to say that variable names are “case sensitive”?
Variable names that consist of the same characters but different capitalization are distinct (not the name).
(Also, capitalization should be consistent.)
What is the purpose of a string?
The purpose of a string is to store text (any set of characters).
What is the purpose of a number?
The purpose of a number is to store numerical values.
What is the purpose of a boolean?
The purpose of a boolean is to express true or false statements (decision making).
What does the = operator mean in JavaScript?
The = operator in Javascript means assignment (of a value or return to a variable).
How do you update the value of a variable?
Variables are updated by writing the variable name and assignment operator followed by the new value and semicolon.
What is the difference between null and undefined?
Null represents a nonexistent object (an intentional placeholder for “empty”).
Undefined is automatically assigned to variables that have just been declared (JavaScript’s way of saying a variable is empty - does not have access/cannot create null).
Developers should not used undefined.
Why is it a good habit to include “labels” when you log values to the browser console?
Labels assist with referencing what a value represents or where it is from in the code.
Give five examples of JavaScript primitives.
String, Number, Boolean, Null, and Undefined.
(Also Big Int and Symbol)
What data type is returned by an arithmetic operation?
An arithmetic operation returns a number.
*NaN - not a number (typeof number)
What is string concatenation?
String concatenation is joining two or more strings or characters.
Immutable - Unable to be mutated/changed
What purpose(s) does the + plus operator serve in JavaScript?
The plus operator in JavaScript is used for either addition for arithmetic operations or concatenation for joining strings or characters.
What data type is returned by comparing two values (<, >, ===, etc)?
Comparing two values returns the Boolean data type.
What does the += “plus-equals” operator do?
The plus-equals operator adds the value from the right side of the operator to the left side of the operator.
(x += 1; is the shorthand version of x = x + 1;)
What are objects used for?
Objects are used for grouping together a set of variables and functions to represent something in the real world.
What are object properties?
Object properties are variables that are part of an object.
Describe object literal notation.
A comma-separated list of key-value pairs inside curly braces.
How do you remove a property from an object?
Properties are removed from an object by writing the delete operator/keyword followed by the object name an property name.
What are the 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 notation or bracket notation.
What are arrays used for?
Arrays are used for listing a set of values that are related to each other.
Describe array literal notation.
A comma-separated list of values between square brackets.
How are arrays different from “plain” objects?
The key for each value in an array is its index number instead of a property.
Values are added to arrays using methods.
Arrays have a length property.
What number represents the first index of an array?
Zero.
What is the length property of an array?
The length property returns the number of characters in a string or the number of elements in an array.
How do you calculate the last index of an array?
The last index of an array is calculated by evaluating the length property of the array minus one.
What is a function in JavaScript?
A set of statements that performs a task or calculates a value. Functions are reusable and can be written once to handle many situations.
Describe the parts of a function definition.
- The function keyword
- A function name (optional)
- A comma-separated list of parameters surrounded by parenthesis
- Start of the function’s code block
- A return statement (optional)
-The end of the function’s code block
Describe the parts of a function call.
- A function’s name
- A comma-separated list of arguments surrounded by parenthesis
When comparing them side-by-side, what are the differences between a function call and a function definition?
A function call does not have a function keyword or code block.
What is the difference between a parameter and an argument?
A parameter is a placeholder variable in a function definition that has an unknown value.
An argument is a value passed when a function is called.
Why are function parameters useful?
Function parameters help indicate what can or should be passed into the function when it is called.
What two effects does a return statement have on the behavior of a function?
- The function produces a value that can be used in the program.
- Stops any subsequent code from running in the function’s code block
Why do we log things to the console?
Testing, debugging, checking status, etc..
What is a method?
A function that is a property of an object.
How is a method different from any other function?
A method must be called using dot notation.
How do you remove the last element from an array?
The pop method.
How do you round a number down to the nearest integer?
The floor method.
How do you generate a random number?
The random method.
How do you delete an element from an array?
The splice method.
How do you append an element to an array?
The push method.
How do you break a string up into an array?
The split method.
Do string methods change the original string? How would you check if you weren’t sure?
String methods do not change the original string. You can check by testing on the console or checking the documentation.
Roughly how many string methods are there according to the MDN Web docs?
50 string methods.
Is the return value of a function or method useful in every situation?
Return values may not always be necessary,
Roughly how many array methods are there according to the MDN Web docs?
“A lot”
What three-letter acronym should you always include in your Google search about a JavaScript method or CSS property?
MDN (Mozilla Developer Network)
Give 6 examples of comparison operators.
>
- greater than = - greater than or equal to < - less than <= - less than or equal to === - absolutely equal to !== - strictly not equal to
What data type do comparison expressions evaluate to?
Comparison expressions evaluate to the Boolean data type.
What is the purpose of an if statement?
The purpose of an if statement is to guide a program to make decisions based on specified conditions.
(Checks whether a condition is true.)
Is else required in order to use an if statement?
Else is not required to use an if statement.
Describe the syntax (structure) of an if statement.
The if keyword, condition in parenthesis, opening curly brace for the code block, code to execute (if any), and the closing curly brace for the code block.
What are the three logical operators?
&& - And
|| - Or
! - Not
How do you compare two different expressions in the same condition?
Each individual expression should be separated by a logical operator excluding not.
What is the purpose of a loop?
The purpose of a loop is to repeatedly execute a block of code as long as the specified condition is true.
What is the purpose of a condition expression in a loop?
The purpose of a condition expression in a loop is to check if the code block should be executed.
What does “iteration” mean in the context of loops?
“Iteration” in the context of loops means the repetition of the loop.
When does the condition expression of a while loop get evaluated?
The condition expression of a while loops gets evaluated before each iteration of the loop. If the expression evaluates to true, then the code block is executed.
When does the initialization expression of a for loop get evaluated?
The initialization expression of a for loop gets evaluated once before the first iteration.
When does the condition expression of a for loop get evaluated?
The condition expression of a for loop gets evaluated after the final expression (if the second iteration an on).
When does the final expression of a for loop get evaluated?
The final expression of a for loop gets evaluated at the end of each loop iteration. (Before the next evaluation of the condition expression.)
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 exits a loop before its condition expression evaluates to false.
What does the ++ increment operator do?
The increment operator increases a variable’s numerical value by 1.
How do you iterate through the keys of an object?
The keys of an object can be iterated by using a for-in loop.
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 is one way to delay the execution of a JavaScript function until some point in the future?
The setTimeout() function.
How can you set up a function to be called repeatedly without using a loop?
The setInterval() function.
What is the default time delay if you omit the delay parameter from setTimeout() or setInterval()?
0 milliseconds (no delay).
What do setTimeout() and setInterval() return?
They will return an ID that is a number data type.
What is Array.prototype.filter useful for?
Returning a new array of elements that meet a specific criteria based on another array.
What is Array.prototype.map useful for?
Performing a function on each array in an element.
What is Array.prototype.reduce useful for?
Getting a single value that is cumulative of an array.
To collapse an array of elements into a final value.
What must the return value of myFunction be if the following expression is possible?
myFunction()();
Another function which is immediately called.
“Function’s function is being called.”
“The return of myFunction is being called which is another function.”
What does this code do?
const wrap = value => () => value;
Wrap puts a function around value and is not evaluated until the inner function is called.
In JavaScript, when is a function’s scope determined; when it is called or when it is defined?
When a function is defined.
What allows JavaScript functions to “remember” values from their surroundings?
Lexical scoping.
