Declaring Variables and Setting Their Values - jpjohnsonjr/learning-notes GitHub Wiki

const and let

There are two basic ways to declare variables:

const — Short for constant, a JavaScript keyword that creates a new variable with a value that cannot change. The typical naming convention for constants is myName. This is known as camelCasing because the capital letters look like camel humps. A declaration might look like this:

const myName = 'Arya';
console.log(myName);

The = is the assignment operator. The variable value is Arya in this case. Other examples:

const entree = 'Enchiladas';
const price = 12;

let — Creates a new variable with a value that can be changed.

Example: The following will throw an error message:

const entree = 'Enchiladas';
entree = 'Tacos';

This will return the error `TypeError: Assignment to constant variable'. The following would not:

let meal = `Enchiladas`
console.log(meal);
meal = `Tacos`;
console.log(meal);

Setting a Boolean Value

Examples:

• let changeMe = true;
• changeMe = false;

Creating an Undefined Variable

It is also possible to create a variable but not assign a value; JavaScript will create space for the variable in memory and set it to undefined, which is the fifth and final primitive data type. Example:

let notDefined;
console.log(notDefined);

Mathematical Assignment Operators

Example:

let x = 4;
x = x + 1;

The above creates the variable x and then increases its value by 1. Other operators:

let x = 4;
x += 2; // x now equals 6

let y = 4;
y -= 2; // y now equals 2

let z = 4;
z *= 2; // z now equals 8

let r = 4;
r++; // r now equals 5 (incremented by 1)

let t = 4;
t--; // t now equals 3 (decremented by 1)

String Interpolation

String interpolation in the term in JavaScript for inserting data saved to a variable into a string. The + operator is used to interpolate, as shown:

let myPet = 'armadillo';
console.log('I own a pet ' + myPet + '.');
// Output: 'I own a pet armadillo.'

Newest version of JavaScript also allows this to be done this way:

console.log(`I own a pet ${myPet}.`);

Not necessary to use the + operator or close and re-open the string using this technique, but must use backticks instead of single quotes (`);