Variables, Types, and Collections - 23% Flashcards
array method - .sort
Sorts a-z by default w/no cb.
For numbers, need to pass cb:
For reverse, pass a cb that returns a neg num if the 1st is > the 2nd
ASC - use .sort( (a, b) => a - b)
DES - (or rev alphabetical), use .sort( a, b) => b - a)
array method - .forEach
Pass cb, the fn gets called for every element. Does not modify elements.
array1.forEach(element => console.log(element));
can also pass index (item, index)
array method - .map
Modify elements using a cb.
let modifiedArr = arr.map(function(element){
return element *3;
});
array methods - .pop, .shift
pop - removes last el of array and returns it
shift - removes first el of array and returns it
array methods - .splice, .slice
splice - removing, replacing, or adding els in specified positions. Modifies origin array.
slice - cuts out subset of array and returns it as a shallow copy, orig array is not modified
array methods - .push, .unshift
push - adds el to end of array
unshift - adds el to start of array
array methods - .every
Returns a Boolean value that represents whether all els pass the condition in the cb fn
array methods - .filter
Returns a shallow copy of a subset of the array, with only the els that pass the condition in the cb fn
array methods - .indexOf
Returns the first index at which a given el can be found, or -1
array methods - .reduce
Reduces the array to a single value by applying the cb to each el and passing the return to the next.
Takes cb w/2 args (accumulated val and the current val) and and initial value (0 below):
array1.reduce((acc, current) => acc + current, 0);
array methods - .reverse
Reverses the array in place, returns ref to same array
Data Types (8)
String, Number,
BigInt - create by appending n to a num, for numbers > 2^53-1
Boolean,
Null - nothing, empty, or value unknown
Undefined - variable is declared, but value is not assigned
Object - not primitive, used to store collections of data
Symbol - used to create unique identifiers for objects
Map object
collection of key/value pairs, the keys are unique
Const myMap = new Map( );
myMap.add(‘apple’, ‘red’);
myMap.get(‘apple’) // returns ‘red’
myMap.size( ) // returns 1
myMap.has(‘banana’) // returns false
myMap.delete(‘apple’) // removes apple/red pair from myMap
myMap.clear( ) // removes all pairs from myMap
Map obj v. reg Obj
Map - keys can be any type
Object - keys only strings or symbols
Map - easy to get size
Object - have to keep track manually
Map - iteration is insertion order
for (const [key, value] of myMap)
Set obj
Collection of values, similar to array, except all are unique.
Const mySet = new Set( );
mySet.add(‘orange’);
mySet.has(‘orange’) // returns true
mySet.size( ) // returns 1
mySet.delete(‘orange’) // removes orange from mySet
Can iterate in insertion order:
for (const item of mySet)
Set v Array
Set - can delete by value: mySet.delete(val);
Array - deletion by value is slow: (arr.splice(arr.indexOf(val), 1))
Array - value NaN cannot be found by indexOf
Set - only unique vals, don’t have to keep track of duplicates
Convert b/t Array and Set
Array.from(mySet) // creates array
[…mySet] // creates array
new Set(myArray); // creates set, removes duplicates
var, let, const
var - gets hoisted, can declare w/o initializing, can reassign
let - can declare w/o initializing, can reassign
const - must initialize at declaration, cannot reassign
template literal
Strings declared w/backticks.
- can embed vars/JS in them
- line breaks are respected, no need for \n
escape chars in strings
Use backslash \
string / number conversion
Number(<string here>)String(<num here>)
Types of Numbers
Integers - whole nums, can be positive or negative
Floating point numbers (floats) - have decimals
NaN (Not a Number): usually the result of an undefined or unrepresentable operation like division by zero.
Infinity: represents mathematical infinity. Can be positive or negative..
BigInt: can store and operate on integers beyond the safe integer limit for Numbers (15 places). e.g. 9007199254740991n
Binary, Octal, and Hexadecimal Numbers: JS supports binary (base 2), octal (base 8), and hexadecimal (base 16) numbers. For example, binary: 0b1010, octal: 0o755, hexadecimal: 0xABC.
In JS all nums are technically stored as 64-bit floating-point numbers, regardless of whether they are integers or decimals. This can lead to some precision issues for very large or very small numbers.
.toFixed(n) - Number method
Rounds a decimal to a specified number of places (n)
% operator
** operator
% = Remainder (or Modulo) - Returns the remainder after you’ve divided the left number by the right
8 % 3 // 2
** = Exponent - raises left num to right power
5 ** 2 // 25
equiv to Math.pow(5, 2)
