Variables, Types, and Collections Flashcards
8 data types
string, bigint, symbol, object, null, and undefined
let and const
block scoped
block = anything inside curly braces
when we don’t assign a value to a variable
undefined
except with const (you will get SyntaxError: missing initializer in const declaration)
check typeof
if (typeof role == ‘undefined’)
which three data types have wrapper functions?
Boolean, Number, String
which two datatypes represent absence of data?
null, undefined
typecasting methods
Number()
String()
Boolean()
toString() - used to covered a number to a string
parseInt() - parse a string argument to an integer or NaN
parseFloat() - method used to parse an argument to floating point
array construction
var brands = ["Fender", "VOX", "Line6"]
var brands = new Array("Fender", "VOX", "Line6")Number type in JS
represents any number between -(2^53 - 1) and (2^53 - 1)
most common string methods
toLowerCase()
toUpperCase()
concat(str [,…str]) - combines two or more strings and returns a new string
includes(searchString, [,position])
indexOf(searchValue [, fromIndex]) = returns the index of a value or -1 if not found
replace(searchFor, replaceWith)
substring(indexStart [, indexEnd]) - the substring() method returns a part of the string between the start and end indexes or the start index to the end of the string
search()
padStart() & padEnd()
trim()
common methods for Number object
isNaN() - static method can be used to determine whether a value is NaN
isInteger()
parseFloat(str) & parseInt(str [,radix])
toFixed(digits) - returns number that has the specified number of digits after the decimal point
toString([radix])
valueOf() - returns primitive value of specified number object
common date methods
now() = returns current time in milliseconds parse() = parses a date string and returns milliseconds UTC() = returns milliseconds in UTC Get Methods (getDay, getMonth, getHours, etc) Set Methods (setFullYear, setMinutes, setSeconds, etc) UTC Methods (getUTCDate, setUTCdate)
toString() - returns a string value of a date object
toDateString() - returns the date value of the date object without the time in a string format
toUTCString() - converts a date to a string using the UTC time zone
toISOString() - convert UTC time into a simplified ISO-8601 format
toLocaleDateString() - return a date value or parts of date in a specified locale
getDate / setDate
type coercion with different operators
== or !=
coerces primitives to numbers except for null and undefined
comparison operators
coerces primitives to numbers
logical operators
coerce primitives to boolean
type coercion with other primitives
symbol: evaluated as true in boolean coercion / cannot be implicitly coerced to a string or number
null: evaluated as false / 0 / null (string coercion)
undefined: false / NaN in numeric / undefined in string
NaN: false / NaN in string coercion
falsy values
false 0 -0 0n (BigInt 0) '' null undefined NaN
double bang
coerces to the equivalent boolean value
for in loop
vs.
for of loop
for in loop - when concerned with index
for of loop - for simplicity when you are just trying to access the values
adding elements to arrays
push() - add one or more element to the end of an array; returns the new length of the array
unshift() - add one or more elements to the beginning of array; returns new length
splice() - allows modifying the contents of an array; used to add elements starting from a specific condition
removing array elements
pop() - can be used to remove the last element from an array, returns removed element
shift() - remove first element; returns removed element
splice() - used to remove or replace, returns an array containing the removed elements
SLICE()
used to make a copy of an array or a portion of an array; does not change original array
finding array elements
includes()
indexOf()
lastIndexOf() - returns the last index of the given element in an array
findIndex() - returns first index that satisfies the given condition
find() - returns the value of the first element that satisfies the condition; undefined otherwise
advanced data manipulation
map() - creates a new array based on a callback function that is invoked for every element of the original array
filter() - create a new array that contains elements which pass the given condition in a callback function
reduce() - uses the specified reducer function to return a single output value based on the reduction
flat() - used to create a new array containing all the sub-array elements concatenated into the original array recursively up to the specified path
every() - used to check whether all the elements of an array pass the given condition
some() - used to check if at least one element of an array passes the given condition in a callback function
spread / rest operator
…
spreads the elements of array instead of the array itself
in json…properties and any strings….
are enclosed within double quotes
how do we convert a js object to json
stringify
SERIALIZED
how to access JSON object like a JS object
parse()
DESERIALIZE
how to iterate over data received
for in loop to iterative over properties
or object.entries (gets an array of key:value pairs)
functions can be…
assigned to a variable
stored in an array
passed and returned
property of an object (method)
6 types of functions
named functions anonymous function recursive function outer function inner function immediately invoked (IIFE)
IIFE
put parenthesis immediately after defining it
arrow function considerations
this is treated differently
traditional functions: the this keyword is set to the object that invoked the function; otherwise, the this keyword defaults to the global variable
arrow functions: does not default to the global object; the value of this is determined by the context of where the function is called
closure
allows code to access parent scope even after code is finished
apply, call, bind
apply() - invokes a function and assigns the object passed in to the keyword ‘this’. arguments for the function are passed in as an array.
call() - invokes a function and assigns the object passed in to “this”. arguments are passed in as a comma separated list.
bind() - returns a new function; the object passed in is this when the function is invoked; arguments may be bound to the function as well
object class methods
assign() - used to copy all the enumerable own properties from one or more source objects to a target object
entries() - can be used to get an array of all the key-value pairs of an object’s own enumerable string properties
values() - get array of all obj’s values
defineProperties() - define new or modify existing (also, defineProperty for just one)
freeze() - changes can no longer be made
preventExtensions() - prevents the addition of new properties
seal() - prevents new properties and deletions
is() - used to determine whether two values are the same
object instance methods
hasOwnPropertyOf()
isPrototypeOf()
toString()
valueOf()
