6 basic types of values: numbers, strings, booleans, objects, functions, and undefined

Infinity and NaN (not a number) are both numbers

NaN !== NaN

Typeof: get the type of a value or variable. Ex. typeof 4.5, typeof 'x', typeof x

Comparison of alphabetic strings is by unicode value: 'Z' < 'a', 'a' < 'z'

Null and undefined are values but carry no information

Null == undefined, null != 0

0, Nan, and '' count as false, 0 == false, '' == false

Us break; to jump out of a loop

Use continue; to go to the next iteration of a loop

Lexical scope: all variables surrounding a local scoped function are available in the function

In JavaScript, functions are the only things that create a new scope, not blocks

Let creates a variable that is local to the enclosing block, not the enclosing function

Function variables can be passed around. Ex: let square = function(x) { return x * x; };

Function declarations define the variable and point it at the function. The variables are moved to the top of their scope. Ex. function square(x) { return x * x; };

Function calls get put in a call stack

Closure: a function that closes over local variables. A function within another function. The inner function remembers the environment in which it was created and has access to the outer function's variables and parameters. The function 'freezes' the code in its body and wraps it into a package

Recursion: a function that calls itself

Running through a loop is a lot cheaper than calling a function multiple times

Pure function: when called with the same arguments, it always produces the same value, zero side effects

Dot notation allows us to access the property of a value

Reading a property that doesn't exist will produce the value undefined

Delete anObject.left will remove a named property from the object

Numbers, strings, and booleans are all immutable

== operator will return true only if both objects are precisely the same value. Comparing different objects will return false, even if they have identical contents

For (let event in map) maps over the keys in a map

Array: length, push, pop, unshift, shift, indexOf, lastIndexOf, slice(start, end), concat

String: slice(start, end), indexOf, trim, length, charAt

A special arguments variable/object is added to the function environment whenever a function is called. arguments.length is the # of parameters passed into the function

Math: random (a random number between 0 and 1), floor, ceil, round

ForEach is a standard method on arrays. Only takes one argument, the function to be executed for each element

Higher-order functions: functions that operate on other functions, either by taking them as arguments or returning them

Filter: returns a new array of values that pass the test

Map: transforms the array by applying a function to all of its elements and builds a new array from the returned values

This: special variable which refers to the object it is called on

Prototype: the original type of an object which has properties. Object.prototype is the ancestral prototype behind all objects

Implement defensive programming to ignore nonsense input

Throw new Error('Something went wrong.');

Try { ... } catch(error) { .... };

Tye/finally or try/catch/finally: finally runs no matter what, usually contains clean up

Regular expressions: new RegExp('abc'); or /abc/;

Put a backslash before any character that isn't a letter, number, or whitespace

Characters between square brackets match any of the characters: /[0123456789]/.test(' in 1993') // true. Within square brackets, a dash (-) between characters represents a range

Common character groups: d (any digit, [0-9]), w (any alphanumeric character), s (any whitespace character, space, tab, newline), D (any char not a digit), W (non-alphanumeric character), S (non-whitespace character), . (any character except for newline)

Using a carat (^) after the opening square bracket is invert or except

A plus sign after something in a regular expression indicates the element may be repeated more than once

The star (*) means the same as + but also may match zero times

Question mark means optional

Curly braces indicate that a pattern should occur a precise number of times, can also use a range, ranges may be open ended

I: makes the regular expression case insensitive

^ matches the start of the string, $ matches the end

: is a word boundary, word must start or end with a nonword character

Pipe (|) represents a choice between the pattern on the left or right

g means all matches will be replaced

The search method is like indexOf

Month numbers start at 0 for JavaScript Date, but days start at 1

Date: getTime, getFullYear, getMonth, getDate, getHours, getMinutes, getSeconds

Node.js is a browserless JavaScript environment

Modules create an interface for external code to use

Functions are the only things in JavaScript that create a new scope

Network protocol: style of communication over a network

TCP: transmission control protocol, most communication on the internet is built on this

HTTP: hypertext transfer protocol allows computers to request documents over the network

Each document on the web is named by a url (uniform resource locator) made up of a protocol, server, and path

Each machine connected to the internet gets a unique IP address

HTML (hypertext markup language) is the document format used for web pages

Href: hypertext reference

Each document node has a parentNoode, childNodes, firstChild, lastChild, previousSibling, nextSibling, nodeType, and nodeValue

OffsetWidth and offsetHeight give you the space the element takes up

ClientWidth and clientHeight is the size of space iinside the element, ignoring the border

GetBoundingClientRect: returns an object with top, bottom, left, and right properties giving the pixel positions of the sides of the element relative to the top left of the screen

Browsers do not update their display when a JavaScript program is running

Browsers allow us to register functions as handlers for specific events

Event handler functions are passed an event object as an argument. It has information about the event. Events have a target property which refers to the node it was originated

Modifier keys: shiftKey, ctrlKey, altKey, metaKey

Whenever an element is scrolled, a 'scroll' event fires on it

InnerHeight global variable gives us the height of the window

When a page finishes loading, the 'load' event fires on the window and document objects

When a page is navigated away from, a 'beforeunload' event fires to prevent losing work

Browsers provide web works to do something time-consuming in the background

SetTimeout schedules another function to be called later. If you want to cancel a function you have scheduled, store it in a variable and call clearTimeout(variable)

HTTP methods: GET, DELETE, PUT, POST

Status codes: 200s success, 400s request error, 500s server error

A query string on a URL starts with a question mark and contains names and values separated by &s

Promises: wrap an asynchronous action in an object, which can be passed around and told to do certain things when the action finishes or fails

A button with a type attribute submit will submit forms, same as pressing enter when a form is active

Long pulling: client continuously asks the server for new information

Web sockets: open connection for data exchange between client and server

Router: component that helps dispatch a request to the function that can handle it