[Javascript] Variable

Reference
Codecademy. Retrieved from https://www.codecademy.com/

A variable is a container for data that is stored in computer memory. It is referenced by a descriptive name that a programmer can call to assign a specific value and retrieve it.

let name = "Tammy";
const found = false;
var age = 3;
console.log(name, found, age);
// Tammy, false, 3

To declare a variable in JavaScript, any of these three keywords can be used along with a variable name:

• var is used in pre-ES6 versions of JavaScript.
• let is the preferred way to declare a variable when it can be reassigned.
• const is the preferred way to declare a variable with a constant value.

let Keyword

let creates a local variable in JavaScript & can be re-assigned. Initialization during the declaration of a let variable is optional. A let variable will contain undefined if nothing is assigned to it.

let count; 
console.log(count); // Prints: undefined
count = 10;
console.log(count); // Prints: 10

const Keyword

A constant variable can be declared using the keyword const. It must have an assignment. Any attempt of re-assigning a const variable will result in JavaScript runtime error.

const numberOfColumns = 4;
numberOfColumns = 8;
// TypeError: Assignment to constant variable.

Assignment Operators

An assignment operator assigns a value to its left operand based on the value of its right operand. Here are some of them:

• += addition assignment
• -= subtraction assignment
• *= multiplication assignment
• /= division assignment

let number = 100;

// Both statements will add 10
number = number + 10;
number += 10;

console.log(number); 
// Prints: 120

String Interpolation

String interpolation is the process of evaluating string literals containing one or more placeholders (expressions, variables, etc).

It can be performed using template literals: text ${expression} text.

let age = 7;

// String concatenation
'Tommy is ' + age + ' years old.';

// String interpolation
`Tommy is ${age} years old.`;