Programming in JavaScript - The Elara Handbook

Standard Library and Imports¶

JavaScript has a built-in standard library available in all environments. In Node.js, modules can be imported using require() or import. Some examples:

import fs from 'fs'; for reading and writing files
import path from 'path'; for working with file paths
import readline from 'readline'; for reading user input in the terminal

Comments¶

// single line comment
/* multi
line
comment */

Data Types and Variables¶

Each variable is a container for a value. JavaScript is dynamically typed, so the type is inferred from the value. Variables are declared using let, const, or var.

let variableName = value; — declares a block-scoped variable that can be reassigned
const variableName = value; — declares a block-scoped variable that cannot be reassigned
number: stores integers and decimals
string: stores text
boolean: stores either true or false
null: represents an intentional absence of value
undefined: a variable that has been declared but not assigned a value
object: stores collections of key-value pairs
array: stores ordered lists of values (a special kind of object)

User Input and Output¶

In a browser, console.log() prints to the developer console. In Node.js, the readline module is needed for user input.

console.log("Hello, World!");            // printing to the console

// Reading user input in Node.js
import readline from 'readline';

const rl = readline.createInterface({ input: process.stdin });

rl.question("What is your name? ", (name) => {
    console.log("Hello " + name);
    rl.close();
});

Operators¶

Operators are used to manipulate variables in different ways.

Arithmetic (mainly used for numbers):

+  // Addition
-  // Subtraction
*  // Multiplication
/  // Division
%  // Modulus (gives the remainder)
** // Exponentiation
++ // Increment (adds 1 to variable)
-- // Decrement (subtracts 1 from variable)

Assignment Operators (Shorthand version of doing an operation and then assigning it to the variable):

+=
-=
*=
/=
%=
**=

Comparison Operators (Compares two statements and returns a boolean):

===  // strict equality (checks value AND type)
!==  // strict inequality
>
<
>=
<=

Logical Operators (Can also compare two statements and returns a boolean):

&&  // and
||  // or
!   // not

Strings¶

To use strings, no imports are needed as they are built into JavaScript.

let a = "Hello";
let b = " World";

// String concatenation (adding two strings together will create a new string that contains both strings)
let c = a + b;

// Returns the length of the string
let length = c.length;

// Accessing a character of the string using an index
let firstChar = c[0];

// Template literals (another way to build strings using backticks, expressions go inside ${})
let d = `${a}${b}`;

// Escape characters (special characters that can be added to a string)
\' // Single quote
\" // Double quote
\\ // Backslash
\n // New line
\t // Tab

// Common string methods
c.toUpperCase();              // returns the string in all caps
c.toLowerCase();              // returns the string in all lowercase
c.includes("Hello");          // returns true if the string contains the given substring
c.slice(0, 5);                // returns a portion of the string (start index, end index)
c.replace("Hello", "Hi");     // replaces the first occurrence of a substring with another
c.trim();                     // removes whitespace from both ends of the string

Statements and Loops¶

// If statement
let a = 5;
let b = 3;
if (a > b) {
  console.log("a is greater");
} 
else if (b > a) {
  console.log("b is greater");
} 
else {
  console.log("a is equal to b");
}

// While loop
let i = 0;
while (i < 5) {
  console.log(i);
  i++;
}

// For loop
for (let i = 0; i < 5; i++) {
  console.log(i);
}

// For...of loop (loops over the elements of an array)
let fruits = ["apple", "banana", "orange"];
for (const fruit of fruits) {
  console.log(fruit);
}

// Break & continue statements
// when placed inside a loop and break is reached, the loop will stop and move to the next line of code after the loop
break;
// when placed inside a loop and continue is reached, the loop will skip the remaining lines of code after the continue statement and the loop will run its next iteration
continue;