Variables

Starting from ES2015, there are two recommended way to declare variables:

let myVar = 2;
const numberOfDaysInWeek = 7;

const actually declares a constant (not variable), which cannot be reassigned.

Previously, javascript used the keyword var to declare variables. Once you define a variable, we can initialize it. We do not need to explicitly declare the data type (like whether integer, float or string). The data type is determined when you assign a value. We can check the datatype following way:

> let pi = 3.14;
  console.log(typeof pi)

number

Common arithmetic operations

let a = 5;
let b = 7;
let sum = 5 + 7;
console.log(sum)

12

let a = 3;
let b = 9;
let c = 6;
let result = a + b*c
console.log(result)

57

Some shorthand notations

a = a + 4;
a += 4;

The above works with +, -, *, /

a = a + 1;
a++;

a = a -1;
a--;

String concatenation

let greeting = "Hello ";
let name = "Pranab";

console.log(greeting + name)

Hello Pranab

Be careful to check the data type while using operators. Notice the following:

let a = "4"; // this a string
let b = 5;

let sum = a + b;
let sub = a - b;
let mult = a * b;

console.log("sum = " + sum + ", sub =" + sub + ", mult =" + mult)

sum = 45, sub =-1, mult =20

Notice the string concatenation in case of addition, however subtraction and multiplication carried out properly even though a was a string. Obviously, it works only when the string contains only numbers.

Please also see Variable scope.