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Data types and variables

  • JavaScript provides eight different data types which are undefined,  null,  boolean (true and false),  string, symbol,  bigint,  number, and object.
  • Variables allow computers to store and manipulate data in a dynamic fashion.
  • Declare a variable by putting the keyword var like var name = "Jon Doe". A variable can be declared using the let and const keywords also.
  • When JavaScript variables are declared, they have an initial value of undefined. If you do a mathematical operation on an undefined variable your result will be NaN which means “Not a Number”. If you concatenate a string with an undefined variable, you will get a string of undefined.
  • In JavaScript all variables and function names are case sensitive.
  • Like python string can be written using both single or double quotes.

var vs let vs const

  • Using var a variable can be re-declared multiple times.
  • Unlike var, when using let, a variable with the same name can be declared once only. But the variable can be reassigned multiple times.
  • Variables declared using const can only be declared once and are read-only. These cannot be reassigned.

⌨️ Write variable names in JavaScript in camelCase. In camelCase, multi-word variable names have the first word in lowercase and the first letter of each subsequent word is capitalized.

⌨️ It is common to use UPPERCASE variable identifiers for immutable values and lowercase or camelCase for mutable values (objects and arrays).

Operators

JS supports all basic kind of operators. Some of the operators are listed below:

Operator Purpose Example
++ increment by 1 i++;
-- decrement by 1 i--;
+= Addition and assign a += 1
-= Subtraction and assign a -= 1
*= Multiply and assign a *= 8
/= Divide and assign a /= 4
\ Escapes certain characters. "he said, \"I love cricket\""
== Equality operator. Checks if two items are equal. Data type invariant. 1 == 1 // true also 1 == '1' // true
=== Strict equality, i.e. types of the comparing items must be same. 1 === 1 // true but 1 === '1' // false
!= Inequality operator. true when two items are not equal. 1 != 2 // true
!== Strict inequality operator. 1 !== '1' // true
> Greater than operator. Not strict. 4 > '2' // true
>= Greater than or equal operator. Not strict. 4 >= '4' // true
< Less than operator. Not strict. 1 < '2' // true
<= Less than or equal operator. Not strict. 4 <= '4' // true
&& Logical and operator.  
|| Logical or operator.  
typeof Returns the type of a variable. typeof "Jon"
a ? b : c Ternary operator. a > b ? "a is greater" : "b is greater"

📌 If the values being compared are not of the same type, the equality operator will perform a type conversion, and then evaluate the values. However, the strict equality operator will compare both the data type and value as-is, without converting one type to the other.

Type Coercion

In order for JavaScript to compare two different data types (for example, numbers and strings), it must convert one type to another. This is known as Type Coercion.

1   ==  1  // true
5   ==  2  // false
4   == '4' // true
"3" ==  3  // true

String operations

  • In JavaScript, String values are immutable. But a string literal can be reassigned.
let firstName = "Jon";
firstName[0] = "F";  // not permissable. [mutation]

firstName = "Fon"  // permissable. [reassignment]
  • Concatenation: + is used as the concatenation operator. Example:
let firstName = "Jon";
let lastName= "Doe";

let fullName = firstName + " " + lastName;  // string concatenation
console.log(fullName)
// >>> Jon Doe
  • The += operator can be used to concatenate and assign strings.
  • We can find the length of a String value by writing .length after the string variable or string literal.

Array

  • Using array variables, we can store several pieces of data in one place.
  • Uses zero based indexing.
  • Arrays are mutable.
  • An array can take values of different data types like arr = ["Nice", 17] contains both sting and number.
  • Nested arrays are called multi-dimensional array.

Array methods and properties

Method Purpose Example
push() Adds one or more items to the end. arr.push(4, 5, 6);
pop() Removes and returns the last element. arr.pop()
shift() Removes and returns the first element. arr.shift()
unshift() Adds one or more items at the beginning. arr.unshift(1, 2, 3)
toString() Converts array to comma separated string arr.toString()
length Returns the length of the array arr.length

Functions

  • A function can be declared using the keyword function with the following syntax:
function myAwesomeFunction (arg1, arg2) {
	// some awesome things here
	return SOMETHING;
}
  • A function can include the return statement but it does not have to.
  • When a function does not have a return statement, it returns undefined as the value.

Scopes

  • Scope refers to the visibility of variables.
  • Variables which are defined outside of a function block have Global scope.
  • Variables which are declared within a function, as well as the function parameters, have Local scope.
  • Variables which are declared without the var, let or const keywords are automatically created in the global scope.
let myGlobal = 10;  // global

function func() {
	var myLocal = 100;  // local
  oopsGlobal = 5;  // global
}
  • It is possible to have both local and global variables with the same name. When we do this, the local variable takes precedence over the global variable.

Conditional statements

if/else statements

if (CONDITION) {
	// do something
} else if (ANOTHER CONDITION) {
	// do something else
} else {
	// do other things
}

switch statements

If we need to match one value against many options, we can use a switch statement. A switch statement compares the value to the case statements which define various possible values. Any valid JavaScript statements can be executed inside a case block and will run from the first matched case value until a break is encountered.

In a switch statement we may not be able to specify all possible values as case statements. Instead, we can add the default statement which will be executed if no matching case statements are found.

case values are tested with strict equality (===). The break tells JavaScript to stop executing statements. If the break is omitted, the next statement will be executed.

switch (VALUE) {
    case CONDITION:
        // do something
        break;
    case CONDITION:
        // do something else
        break;
    // ...
    default:
        // do another thing
        break;
}

Objects

object is a collection of key-value pairs, where the keys, often called properties, map to the value.

const person = {
  firstName: "John",
  lastName: "Doe",
  age: 30,
  sayHello: function() {
    console.log(`Hello, my name is ${this.firstName} ${this.lastName}.`);
  }
};

// Delete a property
delete object.property

Key characteristics:

  • The key or property is always string. We can use a non-string key, but JavaScript will typecast to string.
  • Objects are unordered.
  • Objects are mutable, which means we can add, modify, or delete properties after creating an object.
  • Properties in an object can store data (such as strings, numbers, or other objects) or functions (methods) that can be called.
  • We can access object properties using dot notation (object.property) or square bracket notation (object["property"]).

Object methods

Method Purpose Example
hasOwnProperty() Checks if object contains a property. object.hasOwnProperty(property);

Loops

while loop

while (CONDITION) {
    // do something
}

for loop

for(INITIALIZATION; CONDITION; FINAL_EXPRESSION) {
    // do something
}
  • All of INITIALIZATION, CONDITION, and FINAL_EXPRESSION statements are optional.
  • The initialization statement is executed one time only before the loop starts.
  • The condition statement is evaluated at the beginning of every loop iteration and will continue as long as it evaluates to true. When the condition is false at the start of the iteration, the loop will stop executing.
  • The final expression is executed at the end of each loop iteration, prior to the next condition check and is usually used to increment or decrement the loop counter.

do...while loop

do {
    // do something
} while (CONDITION);
  • At first run an iteration then checks the condition.

Ternary Operator

CONDITION ? STATEMENT_IF_TRUE : STATEMENT_IF_FALSE

// example
// a < b ? "a is smaller" : "b is smaller";
  • It is considered best practice to format multiple conditional operators such that each condition is on a separate line.

Some methods

Method Description Example
Math.random() Returns a random number in range 0 (inclusive) to 1 (exclusive)  
Math.floor() Rounds number down  
parseInt(string, radix) Parses a string and returns an integer. radix indicates the base in which the string is written. parseInt("007", 10); // 7

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