Arrays  

Accessing array elements  

You can get elements out of arrays if you know their index. Array elements' indexes start at 0 and increment by 1, so the first element's index is 0, the second element's index is 1, the third element's is 2, etc.

Syntax
array[index]
Example
var primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37];
primes[0]; // 2
primes[3]; // 7
primes[150]; // undefined

Array literals  

You can create arrays in two different ways. The most common of which is to list values in a pair of square brackets. JavaScript arrays can contain any types of values and they can be of mixed types.

Syntax
var arrayName = [element0, element1, ..., elementN]
Example
var primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37];

var junk = [10, "Hello", Math, console, { lots: "of different types" }];

Multi-dimensional Arrays  

A two-dimensional array is an array within an array. If you fill this array with another array you get a three-dimensional array and so on.

Example
var multidimensionalArray = [[1,2,3],[4,5,6],[7,8,9]] // two dimensions, 3x3

Array constructor  

You can also create an array using the Array constructor.

Example
var myArray = new Array(45 , "Hello World!" , true , 3.2 , undefined);
console.log(myArray);

// output: [ 45, 'Hello World!', true, 3.2, undefined ]
Example
var stuff = new Array();

stuff[0] = 34;
stuff[4] = 20;

stuff    // [34, undefined, undefined, undefined, 20]

Accessing nested array elements  

Accessing multi dimensional array elements is quite similar to one-dimension arrays . They are accessed by using [index][index]..... (number of them depends upon the number of arrays deep you want to go inside).

Syntax
array[index][index]....
Example
var myMultiArray = [
            [1,2,3,4,5, [1,2,3,4,5] ],
            [6,7,8,9,10 , [1,2,3,4,6] ],
            [11,12,13,14,15 , [1,2,3,4,5] ],
            [16,17,18,19,20, [1,2,3,4,5] ]
            ];

console.log( myMultiArray[1][5][4] ); //Outputs 6 , the value in the last element of the last element of the second element of myMultiArray.

Booleans  

Boolean literals  

Syntax
true
false

Boolean logical operators  

Syntax
expression1 && expression2 //returns true if both the expressions evaluate to true

expression3 || expression4 // return true if either one of the expression evaluates to true

!expression5 // returns the opposite boolean value of the expression
Example
if ( true && false )alert("Not executed!");
//because the second expression is false

if( false || true )alert("Executed!");
//because any one of the expression is true

if( !false )alert("Executed!");
// because !false evaluates to true

!!true // remains true
Example
if(!false && ( false || (false && true) ))alert("Guess what...");

/* not executed because 
!false && ( false || (false && true) ) - becomes
!false && ( false || false) - becomes
true && false , which is false.*/
Example
/* An important thing to note here is the Operator Precedence - which determines the order in which operators are evaluated. Operators with higher precedence are evaluated first. Thus among the four - () , && , || , ! */

// Brackets - have the highest precedence
// ! - lower than Brackets
// && - lower than !
// || - the lowest

if(true && !!false || true)alert("Guess again ??");

/* Executed , here is the evaluation process-
true && !!false || true - becomes
true && false || true - (no brackets present , so ! evaluated ) becomes
false || true - (then && evaluated) which becomes true */
Example
/* Next important thing is the Associativity - which determines the order in which operators of the same precedence are processed. For example, consider an expression: a * b * c . Left-associativity (left-to-right) means that it is processed as (a * b) * c, while right-associativity (right-to-left) means it is interpreted as a * (b * c). */

// Brackets , && , || have left to right associativity 
// ! has right to left associativity
// So ,

!false && !!false //false
// evaluated in the manner - !false && false - true && false - false


Comparison operators  

Syntax
x === y // returns true if two things are equal
x !== y // returns true if two things are not equal
x <= y // returns true if x is less than or equal to y
x >= y // returns true if x is greater than or equal to y
x < y // returns true if x is less than y
x > y // returns true if x is greater than y

Note:  

An important thing to note here is that not only Boolean literals (true and false) assert truth or false , but there are some other ways too to derive true or false.Have a look at the examples.

Example
if(1)console.log("True!"); // output True! , since any non-zero number is considered to be true

if(0)console.log("I doubt if this gets executed"); // not executed , since 0 is considered to be false

if("Hello")alert("So, any non-empty String is also true."); //Gets executed

if("")alert("Hence , an empty String is false"); // Not executed

== vs. ===  

A simple explanation would be that == does just value checking ( no type checking ) , whereas , === does both value checking and type checking . Seeing the examples may make it all clear. It is always advisable that you never use == , because == often produces unwanted results

Syntax
expression == expression
expression === expression
Example
'1' == 1 //true (same value)
'1' === 1 // false  (not the same type)

true == 1 // true (because 1 stands for true ,though it's not the same type)
true === 1 // false (not the same type)

Code Comments  

Definition  

Code comments are used for increasing the readability of the code.If you write 100 lines of code and then forget what each function did , it's not useful at all. Comments are like notes , suggestions , warnings ,etc. that you can put for yourself. Code comments are not executed

Single Line Comment  

Anything on the line following // will be a comment while anything before will still be code.

Syntax
console.log("This code will be run")

//console.log("Because this line is in a comment, this code will not be run.")
Syntax
// This is a single line comment.

Multi-Line Comment  

Anything between /* and */ will be a comment.

Syntax
/* This is
   a multi-line


   comment!
*/
Example
/* 
alert("Hello,I won't be executed.");
console.log("Hello ,I also will not be executed");
*/

console  

console.log  

Prints text to the console. Useful for debugging.

Example
console.log('Poker night!');
Example
var name = "Codecademy";
console.log(name);

console.time  

This function starts a timer which is useful for tracking how long an operation takes to happen.You give each timer a unique name, and may have up to 10,000 timers running on a given page.When you call console.timeEnd() with the same name, the browser will output the time, in milliseconds, that elapsed since the timer was started.

Syntax
console.time(timerName);
Example
console.time("My Math");
var x = 5 + 5;
console.log(x);
console.timeEnd("My Math");
console.log("Done the math.");
/* Output:
10
My Math: (time taken)
Done the math.

console.timeEnd  

Stops a timer that was previously started by calling console.time().

Syntax
console.timeEnd(timerName);
Example
console.time("My Math");
var x = 5 + 5;
console.log(x);
console.timeEnd("My Math");

/* Output :
10
My Math: (time taken)

Functions  

Function definition  

A function is a JavaScript procedure—a set of statements that performs a task or calculates a value.It is like a reusable piece of code. Imagine , having 20 for loops ,and then having a single function to handle it all . To use a function, you must define it somewhere in the scope from which you wish to call it. A function definition (also called a function declaration) consists of the function keyword, followed by the name of the function, a list of arguments to the function, enclosed in parentheses and separated by commas, the JavaScript statements that define the function, enclosed in curly braces, { }.

Syntax
function name(argument1 , argument2 .... argumentN){
    statement1;
    statement2;
    ..
    ..
    statementN;
} 
Example
function greet(name) {
  return "Hello" + name + "!";
}

Function calling  

Syntax
functionName(argument1, argument2, ..., argumentN);
Example
greet("Anonymous");
// Hello Anonymous!

Function hoisting  

The two ways of declaring functions produce different results. Declaring a function one way "hoists" it to the top of the call, and makes it available before it's actually defined.

Example
hoistedFunction(); // Hello! I am defined immediately!
notHoistedFunction(); // ReferenceError: notHoistedFunction is not defined

function hoistedFunction () {
  console.log('Hello! I am defined immediately!');
}

var notHoistedFunction = function () {
  console.log('I am not defined immediately.');
}

If statement  

if  

It simply states that if this condition is true , do this , else do something else ( or nothing ) . It occurs in varied forms.

Syntax
// Form : Single If
if (condition) {
  // code that runs if the condition is true
}
Example
if (answer === 42) {
  console.log('Told you so!');
}

else  

A fallback to an if statement. This will only get executed if the previous statement did not.

Syntax
// If the condition is true, statement1 will be executed.
// Otherwise, statement2 will be executed.

if (condition) {
  // statement1: code that runs if condition is true
} else {
  // statement2: code that runs if condition is false
}
Example
if (gender == "male") {
  console.log("Hello, sir!");
} else {
  console.log("Hello, ma'am!");
}

else if  

This is like an else statement, but with its own condition. It will only run if its condition is true, and the previous statement's condition was false.

Syntax
// Form : else if . If the condition is true, statement1 will be executed. Otherwise, condition2 is checked . if it is true , then statement2 is executed. Else , if nothing is true , statement3 is executed.
if (condition1) {
  statement1;
} else if (condition2) {
  statement2;
} else {
  statement3;
}
Example
if (someNumber > 10) {
  console.log("Numbers larger than 10 are not allowed.");
} else if (someNumber < 0) {
  console.log("Negative numbers are not allowed.");
} else {
  console.log("Nice number!");
}

Loops  

For Loops  

You use for loops, if you know how often you'll loop. The most often used varName in loops is "i".

Syntax
for ([var i = startValue];[i < endValue]; [i+=stepValue]) {
    // Your code here
}
Example
for (var i = 0; i < 5; i++) {
  console.log(i); // Prints the numbers from 0 to 4
}
Example
var i;  // "outsourcing" the definition
for (i = 10; i >= 1; i--) {
    console.log(i); // Prints the numbers from 10 to 1
}
Example
/* Note that all of the three statements are optional, i.e. , */
var i = 9;
for(;;){
    if(i === 0)break;
    console.log(i);
    i--;
}

//This loop is perfectly valid.

While Loops  

You use while loops, if you don't know how often you'll loop.

Syntax
while (condition) {
  // Your code here
}
Example
var x = 0;
while (x < 5) {
  console.log(x); // Prints numbers from 0 to 4
  x++;
}
Example
var x = 10;
while (x <= 5) {
    console.log(x); // Won't be executed
    x++;
}

Do While Loops  

You use do while loops, if you have to loop at least once, but if you don't know how often.

Syntax
do {
  // Your code here
} while (condition);
Example
var x = 0;
do {
    console.log(x);  // Prints numbers from 0 to 4
    x++;
} while (x < 5);
Example
var x = 10;
do {
    console.log(x); // Prints 10
    x++;
} while (x <= 5);

Math  

random  

Returns a random number between 0 and 1.

Syntax
Math.random()
Example
Math.random(); // A random number between 0 and 1.

floor  

Returns the largest integer less than or equal to a number.

Syntax
Math.floor(expression)
Example
Math.floor(9.99); // 9
Math.floor(1 + 0.5); // 1
Math.floor(Math.random() * X + 1); // Returns a random number between 1 and X

ceil  

Returns the smallest integer greater than or equal to a number.

Syntax
Math.ceil(expression)
Example
Math.ceil(45.4); //  46
Math.ceil(4 - 1.9); //  3

PI  

Returns the ratio of the circumference of a circle to its diameter, approximately 3.14159 or in better terms, the value of PI (π). Note in syntax , we do not put `()` at the end of `Math.PI` because `Math.PI` is not a function.

Syntax
Math.PI
Example
Math.round(Math.PI); // rounds the value of PI ,gives 3
Math.ceil(Math.PI); // 4

sqrt  

Returns the square root of a number.

Syntax
Math.sqrt(expression)
Example
Math.sqrt(5+4); // 3

Math.sqrt(Math.sqrt(122+22) + Math.sqrt(16)); //4

Numbers  

% (Modulus)  

it returns the remainder left after dividing the left hand side with the right hand side.

Syntax
number1 % number2
Example
14 % 9 // returns 5

isNaN  

Returns true if the given number is not a number , else returns false.

Syntax
isNaN([value])
Example
var user_input = prompt("Enter a number"); // Enter "a number"

if(isNaN(user_input))
    alert("I told you to enter a number.");

//alert executed , since "a number" is not a number

//Another important thing:

if( isNaN("3") )
    alert("bad");

//Not executed , because the string "3" gets converted into 3 ,and 3 is a number

Basic Arithmetic  

Doing basic arithmetic is simple.

Syntax
4 + 5;  // 9
4 * 5;  // 20
5 - 4;  // 1
20 / 5; // 4

Prefix and Postfix increment/decrement operators  

Prefix increment / decrement operators are operators that first increase the value of the variable by 1 (increment) or decrease the value of an expression / variable by 1 (decrement) and then return this incremented / decremented value. They are used like ++(variable) [increment] or --(varaible) [decrement] On the other hand , Postfix increment / decrement operators are operators that first return the value of the variable and then increase the value of thAT variable by 1 (increment) or decrease the value of the variable by 1 (decrement) . They are used like (variable)++ [increment] or (varaible)-- [decrement]

Syntax
--variable   //Prefix Decrement
++variable   //Prefix Increment
variable--   //Postfix Decrement
variable++  //Postfix Increment
Example
//The examples will make it clear

var x = 15; // x has a value of 15
var y = x++;
// since it is postfix , the value of x (15) is first assigned to y and then the value of x is incremented by 1
console.log(y); //15
console.log(x); //16

var a = 15; // a has a value of 15
var b = ++a;
// since it is prefix , the value of a (15) is first incremented by 1 and then the value of x is assigned to b
console.log(b); //16
console.log(a); //16
Example
//Guess the output in each, and then check your answer on labs.codecademy.com

//Question 1

var x = 43;
var y = 34;
var z = x++ + --y;
console.log(z);
console.log(y);
console.log(x); 

//Question 2

var a = 1;
var b = 2;
var c = ++a * b++ / 2 + 1 - b + a;
console.log(c);
console.log(b);
console.log(a);

//Question 3

var p = 6;
var q = 3;
var r = Math.pow((--p - 4) , (q++)) + Math.sqrt(q) + --q;
console.log(r);
console.log(q);
console.log(p);

Objects  

Object Literals  

Syntax
{
  "property 1": value1,
  property2: value2,
  number: value3
}
Example
var obj = {
  name: "Bob",
  married: true,
  "mother's name": "Alice",
  "year of birth": 1987,
  getAge: function () {
    return 2012 - obj["year of birth"];
  },
  1: 'one'
};

Property Access  

Syntax
name1[string]
name2.identifier
Example
obj['name'];  // 'Bob'
obj.name;     // 'Bob'
obj.getAge(); // 24

OOP  

Classes  

A class can be thought of as a template to create many objects with similar qualities. Classes are a fundamental component of object-oriented programming (OOP).

Syntax
SubClass.prototype = new SuperClass();
Example
var Lieutenant = function (age) {
  this.rank = "Lieutenant";
  this.age = age;
};

Lieutenant.prototype = new PoliceOfficer();

Lieutenant.prototype.getRank = function () {
  return this.rank;
};

var John = new Lieutenant(67);

John.getJob(); // 'Police Officer'
John.getRank(); // 'Lieutenant'
John.retire(); // true

Strings  

Strings are text. They are denoted by surrounding text with either single or double quotes.

Syntax
"string of text"
'string of text'

Concatenation  

Syntax
string1 + string2
Example
"some" + "text"; // returns "sometext"
var first = "my";
var second = "string";
var union = first + second; // union variable has the string "mystring"

length  

Returns the length of the string.

Syntax
string.length
Example
"My name".length // 7 , white space is also counted
"".length // 0

toUpperCase, toLowerCase  

Changes the cases of all the alphabetical letters in the string.

Example
"my name".toUpperCase(); // Returns "MY NAME"
"MY NAME".toLowerCase(); // Returns "my name"

trim()  

Removes whitespace from both ends of the string.

Syntax
string.trim()
Example
"       a        ".trim(); // 'a'
"     a  a       ".trim(); // 'a  a'

replace()  

Returns a string with the first match substring replaced with a new substring.

Example
"original string".replace("original", "replaced"); // returns "replaced string"

charAt()  

Returns the specified character from a string. Characters in a string are indexed from left to right. The index of the first character is 0, and the index of the last character in a string called stringName is stringName.length - 1. If the index you supply is out of range, JavaScript returns an empty string.

Syntax
string.charAt(index) // index is an integer between 0 and 1 less than the length of the string.
Example
"Hello World!".charAt(0); // 'H'
"Hello World!".charAt(234); // ''

substring()  

Returns the sequence of characters between two indices within a string.

Syntax
string.substring(indexA[, indexB])
//indexA : An integer between 0 and the length of the string
// indexB : (optional) An integer between 0 and the length of the string.
Example
"adventures".substring(2,9); // Returns "venture"
// It starts from indexA(2) , and goes up to but not including indexB(9)
"hello".substring(1); // returns "ello"
"Web Fundamentals".substring(111); // returns ''
"In the market".substring(2,999); // returns ' the market'
"Fast and efficient".substring(3,3); // returns ''
"Go away".substring("abcd" , 5); // returns 'Go aw'
// Any non-numeric thing is treated as 0

indexOf()  

Returns the index within the calling String object of the first occurrence of the specified value, starting the search at fromIndex, Returns -1 if the value is not found. The indexOf method is case sensitive.

Syntax
string.indexOf(searchValue[, fromIndex]) //fromIndex is optional.It specifies from which index should the search start.Its default value is 0.
Example
"My name is very long.".indexOf("name"); // returns 3
"My name is very long.".indexOf("Name"); // returns -1 , it's case sensitive
"Where are you going?".indexOf("are",11); //returns -1
"Learn to Code".indexOf(""); //returns 0 
"Learn to Code".indexOf("",3); //returns 3
"Learn to Code".indexOf("",229); returns 13 , which is the string.length

Switch statements  

switch  

Acts like a big if / else if / else chain. Checks a value against a list of cases, and executes the first case that is true. It goes on executing all other cases it finds after the first true case till it finds a breaking statement,after which it breaks out of the switch If it does not find any matching case , it executes the default case.

Example
var gender = "female";

switch (gender) {
  case "female":
    console.log("Hello, ma'am!");
  case "male":
    console.log("Hello, sir!");
    break;
  default:
    console.log("Hello!");
}

/* Output:
Hello, ma'am!
Hello, sir!
because break statement is found at the second case so execution stops there */
Syntax
switch (expression) {
  case label1:
    statements1
    [break;]
  case label2:
    statements2
    [break;]
  ...
  case labelN:
    statementsN
    [break;]
  default:
    statements_def
    [break;]
}
Example
var gender = "female";

switch (gender) {
  case "female":
    console.log("Hello, ma'am!");
  case "male":
    console.log("Hello, sir!");
  default:
    console.log("Hello!");
}

/* Output:
Hello, ma'am!
Hello, sir!
Hello!
because no breaking statement is found it executes all of the cases after the true case (which is case 1) */

Ternary Operator  

The ternary operator is usually used as a shortcut for the if statement.

Syntax
condition ? expr1 : expr2
Example
var grade = 85;
console.log("You " + (grade > 50 ? "passed!" : "failed!"));

//Output: You passed!

/* The above statement is same as saying:
if(grade > 50){
    console.log("You " + "passed!");  //or simply "You passed!"
}
else{
    console.log("You " + "failed!"); 
}
*/
Syntax
condition ? (condition ? expr1 : expr2) : expr3
Example
var grade = 90;
console.log( "Your grade is: " + (grade > 50 ? (grade >= 90? "Excellent!":"Average"):"Need to be improved") );

// Output: Your grade is: Excellent!

/* This is a nested if statement ,same as writing:
if(grade > 50){
    if(grade >= 90){
        console.log("Your grade is: Excellent!"):
    }
    else{
        console.log("Your grade is: Average");
    }
}
else{
    console.log("Your grade is: Need to be improved"); 
}
*/
Syntax
condition ? expr1 : (condition( expr2 : expr3 ))
Example
var grade = 51;

console.log( "Your grade is: " + (grade >= 90 ? "Excellent!": (grade > 50? "Average" : "Needs to be improved")) );

/* Output: Your grade is: Average

This is in the form of an else if statements , similar to the following:

if(grade >= 90){
    console.log("Your grade is: Excellent!");
}
else if (grade > 50){
    console.log("Your grade is: Average"); 
}
else{
    console.log("Your grade is: Needs to be improved"); 
}
*/

Variables  

Variable Assignment  

Syntax
var name = value;
Example
var x = 1;
var myName = "Bob";
var hisName = myName;

Variable changing  

Syntax
varname = newValue
Example
var name = "Michael" //declare variable and give it value of "Michael"
name = "Samuel" //change value of name to "Samuel"

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