JavaScript: Basic, Advanced, & More Cheat Sheet by acwinter

the purpose of this cheat sheet is to briefly describe the core elements of the JavaScript language for those of studying it who have taken in much more than we can hold onto well. nothing here is explained in full but rather meant to get you on the right track. also, this document purposely does not cover browse­r-s­pecific methods / syntax / objects and the like.

 this cheat sheet is a work in progress and may be updated -- check back on occasion!

someNum.toFixed(num)
• shortens someNum to have only num decimal places
num.toExponential()
• converts num to expone­ntial notation (i.e. 5.569e+0)
num.toString()
• converts num to a string
num.toPrecision(#)
• converts num to a num with # places starting with whole numbers
String(someValue)
• converts or coerces someValue to a string - someValue can be any type, ie "­Boo­lea­n(1­)" returns true
parseInt(string, radix)
parseFloat(string, radix)
• converts a string into an integer. the optional radix argument defines the base -- i.e., base 10 (decimal) or base 16 (hexad­eci­mal).

Math.round(num)
• rounds num to nearest integer
Math.ceil(num)
• rounds num up to nearest integer
Math.floor(num)
• rounds num down to nearest integer
Math.max(num1, num2)
• returns larger num
Math.min(num1, num2)
Math.pow(num1, num2)
• returns num1 to the power num2
Math.sqrt(num)
Math.random()
• returns decimal between 0 (inclu­sive) and 1(excl­usive)
Math.abs(num)
• returns absolute value of num•

Note: these "­extra method­s," which are "­hig­her­-or­der­" functions, ignore holes in the array (i.e.: [“apples”, , , , “orang­es”]). they also have more arguments than shown here -- best to look them up for more info!
Note: array-­like objects, for example argum­ents and NodeL­ists, can also make use of these methods.

arr.some(callback)
arr.every(callback)
• returns a boolean value. returns true if some or every element in the array meets the evalua­tion. example:
 ­ ­ var a = [1,2,3];
 ­ ­ var b = a.ever­y(f­unc­tio­n(i­tem){
 ­ ­ ­ ­  return item > 1;
 ­ ­ }); // false

arr.reduce(function(prev, next){..}, startVal)
arr.reduceRight(function(prev, next){..}, startVal)
• returns a value. reduce employs a callback to run through the elements of the array, returning "­pre­v" to itself with each iteration and taking the next "­nex­t" value in the array. for it's first "­pre­v" value it will take an optional "­sta­rtV­al" if supplied. an intere­sting example:
 ­ ­ var arr = ["ap­ple­", "­pea­r", "­app­le", "­lem­on"];
 ­ ­ var c = arr.re­duc­e(f­unc­tio­n(prev, next) {
 ­ ­ ­ ­ ­pr­ev[­next] = (prev[­next] += 1) || 1;
 ­ ­ ­ ­ ­return prev;
 ­ ­ }, {});
 ­ ­ // objCount = { apple: 2, pear: 1, lemon: 1 }

arr.filter(function(){..})
• returns an array. filter returns an array of elements that satisfy a given callback. example:
 ­ ­ var arr2 = ["ji­m", "­nan­cy", "­ned­"];
 ­ ­ var letter3 = arr2.f­ilt­er(­fun­cti­on(­item) {
 ­ ­ ­ ­ ­return (item.l­ength === 3);
 ­ ­ ­});
 ­ ­ ­co­nso­le.l­og­(le­tter3); // ['jim', 'ned']

arr.sort(function(){..})
• returns the original array, mutated. sort returns the elements sorted with a given criteria. for example:
 ­ ­ var stock = [{key: “r”, num: 12}, {key: “a”, num: 2}, {key: “c”, num: 5}];
 ­ ­ var c = stock.s­or­t(f­unc­tio­n(a,b) {
 ­ ­ ­ ­ ­return a.num - b.num;
 ­ ­ } ); // [ { key: ‘a', num: 2 }, { key: ‘c', num: 5 }, { key: ‘r', num: 12 } ]

arr.map()
• returns an array. map goes over every element in the array, calls a callback on the element, and sets an element in the new array to be equal to the return value the callback. for example:
 ­ ­ var stock = [{key: "­red­", num: 12}, {key: "­blu­e", num: 2}, {key: "­bla­ck", num: 2}];
 ­ ­ var b = stock.m­ap­(fu­nction (item){
 ­ ­ ­ ­ ­return item.key;
 ­ ­ }) // ["re­d","b­lue­"­,"bl­ack­"]

arr.forEach()
• no return value. forEach performs an operation on all elements of the array. for example:
 ­ ­ var arr = [“jim”, “mary”];
 ­ ­ ­a.f­orEach (function (item) {
 ­ ­ ­ ­  consol­e.l­og(­"I simply love “ +item);
 ­ ­ }); // “I simply love jim”, “I simply love mary"

Note: you can combine array methods in a chain where the result of the leftmost operation is passed to the right as such:
array.so­rt(­).r­eve­rse­()...

Callbacks: placing ( ) after a function call executes it immedi­ately. leaving these off allows for a callback.

Function Declar­ation
 function aFunct­ionName (args) {...
• functions created in this manner are evaluated when the code is parsed and are 'hoisted' to the top and are available to the code even before they're formally declared. Note: Due to JS's odd constr­uction, using function declar­ations within a flow control statement can be wonky and is best avoided.

Function Expression / Anonymous Functi­ons
 var bar = function (args) {...
• (also referred to as 'Function Operat­ors') anonymous functions are evaluated at 'runtime' and are therefore less memory intensive. they must be provided a variable name but need not have a function name (there­fore: anonym­ous). [these are ]

Named Function Expres­sion
 var bar = function foo (args) {...
• confus­ingly, this is still an 'anonymous function.' assigning a name is useful for debugging purposes and also allows for self-r­efe­rential / recursive calls

Function Constr­uctor
 var anothe­rFu­nction = new Function (args, function () {... }) {...}
• equivalent to a functional expression

Self­-In­voking Anonymous Functi­ons
 ( function (args) { doSome­thing; } ) ( );
• (also known as IIFEs / 'Immed­iately Invoked Function Expres­sions') and invokes immedi­ately

if .. else if .. else
 ­ ­ if (consi­dtion1) {
 ­ ­ ­ ­  doSome­thing;
 ­ ­ } else if {
 ­ ­ ­  doSome­thi­ngE­lse;
 ­ ­ } else {
 ­ ­ ­  doSome­thi­ngM­ore;
 ­ ­ }

for loop
 ­ ­ for (var i = 0; i < someNu­mber; i++) {
 ­ ­  doSome­thing;
 ­ ­ }

switch loop
 ­ ­ ­switch (someE­val­uation) {
 ­ ­  case "­eva­lua­tes­AsT­his­" :
 ­ ­ ­ ­ ­  doSome­thing;
 ­ ­  case "­eva­lua­tes­AsT­hat­" :
 ­ ­ ­   ­  doSome­thi­ngE­lse;
 ­ ­ }

while loop
 ­ ­ ­while (someE­val­uation === true) {
 ­ ­ ­  doSome­thing;
 ­ ­ }

do .. while
 ­ ­ do {
 ­ ­ ­  doSome­thing;
 ­ ­ }
 ­ ­ ­while (someE­val­uation === true);

for .. in (objec­ts)
 ­ ­ for (anItem in anObject) {
 ­ ­ ­ ­  doSome­thing With anItem;
 ­ ­ ­ ­ ­ ­ ­ ­ // will be the key
 ­ ­ ­ ­  doSome­thi­ngWith Object­[an­Item];
 ­ ­ ­ ­ ­ ­  // will be the value of that key
 ­ ­ }

 a string can be coerced into an array so many array methods are applicable as well

str.charAt(num)
• returns the character in str at index num
str.charCodeAt(num)
• returns the unicode value of the char
String.fr­omC­har­Cod­e(num)`
• returns the character with unicode's num
str.indexOf(char)
• returns -1 if char not found in str
str.lastIndexOf(subString)
• returns the index of the last occurrence of subString
str.length
• returns length of str starting at 1
str.match(pattern)
• returns null if not found. returns an array of all matches
str.match(/pattern/g)
• provides global search of string
str.replace(old, new)
str.search(pattern)
• returns index of first match or -1 if not found
str.substring(index1, index2)
• char at index1 is returned, index2 is not
str.split(char)
• returns an array of str split on char
str.substr(index1, num)
• returns substring starting at index1 and running num letters
str.toLowerCase()
str.toUpperCase()
str.toLocaleLowerCase()
• takes local language settings into account
str.toLocaleUpperCase()
• ibid
Number(var/string/object)
• converts to number. "­tru­e" converts to 1, etc
one.concat(two)
• concat­enates string­/array one with two
JSON.stringify( )
• converts a javascript value/­object into a string
JSON.parse ( )
• converts a JSON string into a javascript object

Note: Unix epoch is January 1, 1970

var today = new Date();
• creates date object for now
var someDate = new Date("june 30, 2035");
• creates date object for arbitrary date
var today = Date.n­ow();
• returns number of millis­econds since epoch
parse()
• returns millis­econds between date and Unix epoch.
toDateString()
toTimeString()
toLocalTimeString()

break;
• breaks out of the current loop
continue;
• stops current loop iteration and increments to next

isNaN(someVar)
• returns true if not a number
isFinite(someVar)

var aVar = anObje­ct[­anA­ttr­ibute] || "­non­esu­ch";
• assigns a default value if none exists
var aVar = anEval­uation ? trueVal : falseVal;
• ternary operator. assigns trueVal to aVar if anEval­uation is true, falseVal if not

delete anObje­ct[­anA­ttr­ibute]
(aProperty in anObject)
• returns true or false if aProperty is a property of anObject

eval(someString)
• evaluates a someString as if it was JavaSc­ript. i.e. eval("var x = 2+3") returns 5

Note: index numbers for arrays start at 0

arr.length()
arr. push(val)
• adds val to end of arr
arr. pop()
• deletes last item in arr
arr. shift()
• deletes first item in arr
arr.unshift(val)
• adds val to front of arr

arr.reverse ()
arr1.concat(arr2)
• concat­enates arr1 with arr2
arr.join(char)
• returns string of elements of arr joined by char
arr.slice(index1, index2)
• returns a new array from arr from index1 (inclu­sive) to index2 (exclu­sive)
arr.splice(index, num, itemA, itemB,..)
• alters arr. starting at index and through index+num, overwr­ite­s/adds itemsA..