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Introduction to Javascript

Keywords​

Keywords are words that are built into the JavaScript language, so the computer will recognize them and treats them specially.

The Console​

In JavaScript, the console keyword refers to an object, a collection of data and actions, that we can use in our code.

One action, or method, that is built into the console object is the .log() method.

Data Types​

Data types are the classifications we give to the different kinds of data that we use in programming. In JavaScript, there are seven fundamental data types:

  • Number (integers or floating point)
  • String
  • Boolean
  • Null: intentional absence of a value
  • Undefined: the absence of a value
  • Symbol: unique identifiers, useful in more complex coding
  • Object

The first 6 types are considererd primitive data types.

Arithmetic Operators​

An operator is a character that performs a task in our code. JavaScript has several built-in in arithmetic operators, that allow us to perform mathematical calculations on numbers. These include the following operators and their corresponding symbols:

  • Add (+)
  • Substract (-)
  • Multiplay (*)
  • Divide (/)
  • Remainder (%)

Built-in Objects​

In addition to console, there are other objects built into JavaScript. A list of them can be found here.

Variables​

In short, variables label and store data in memory. It is important to distinguish that variables are not values; they contain values and represent them with a name.

var​

There were a lot of changes introduced in the ES6 version of JavaScript in 2015. One of the biggest changes was two new keywords, let and const, to create, or declare, variables. Prior to the ES6, programmers could only use the var keyword to declare variables.

var myName = "Arya";
console.log(myName);
// Output: Ary

var, short for variable, is a JavaScript keyword that creates, or declares, a new variable.

let​

The let keyword was introduced in ES6. The let keyword signals that the variable can be reassigned a different value. we can declare a variable without assigning the variable a value. In such a case, the variable will be automatically initialized with a value of undefined.

const​

The const keyword was also introduced in ES6, and is short for the word constant. A const variable cannot be reassigned because it is constant. If you try to reassign a const variable, you’ll get a TypeError. You need to initialize a const variable with a value.

The difference between var and let​

let allows you to declare variables that are limited to the scope of a block statement, or expression on which it is used, unlike the var keyword, which declares a variable globally, or locally to an entire function regardless of block scope. The other difference between var and let is that the latter is initialized to a value only when a parser evaluates it.

String Interpolation​

In the ES6 version of JavaScript, we can insert, or interpolate, variables into strings using template literals:

const myPet = "armadillo";
console.log(`I own a pet ${myPet}.`);
// Output: I own a pet armadillo.

One of the biggest benefits to using template literals is the readability of the code.

typeof operator​

The typeof operator returns a string indicating the type of the unevaluated operand.

let newVariable = "Playing around with typeof.";

console.log(typeof newVariable);
// Output: string

newVariable = 1;

console.log(typeof newVariable);
// Output: number

Note: The type of a variable is not fixed in JavaScript!

Truthy and Falsy​

In JavaScript, a truthy value is a value that is considered true when encountered in a Boolean context. All values are truthy unless they are defined as falsy (i.e., except for false, 0, -0, 0n, "", null, undefined, and NaN).

A falsy (sometimes written falsey) value is a value that is considered false when encountered in a Boolean context.

Complete list of falsy values: MDN

Assignment via truthy/falsy values​

In a boolean condition, JavaScript assigns the truthy value to a variable if you use the || operator in your assignment:

let defaultName = username || "Stranger";