js1.md

JavaScript - Basics

This is a very short introduction to JavaScript. It is not meant to be a complete guide, but rather a quick overview of the basics, mainly for Python developers.

Variables

Variables in JavaScript are declared using the let keyword. The syntax is very similar to Python, but there are some differences.

let x = 5;
let y = 10;
let z = x + y;

JavaScript is a dynamically typed language, so you don't need to specify the type of the variable. The type is inferred from the value.

let x = 5; // x is a number
let y = "10"; // y is a string
let z = x + y; // z is a string

If-else

The syntax for if-else statements is very similar to Python, but there are some differences.

let x = 5;
if (x > 10) {
    console.log("x is greater than 10");
} else if (x > 5) {
    console.log("x is greater than 5");
} else {
    console.log("x is less than or equal to 5");
}

The main difference is the use of parentheses () around the condition, and the curly braces {} around the code block. Unlike Python, JavaScript does not use indentation to define code blocks.

Functions

Functions in JavaScript are declared using the function keyword. The syntax is very similar to Python, but again, code blocks are defined using curly braces {}.

function add(x, y) {
    return x + y;
}

Arguments in functions

Differently from Python, JavaScript will allow you to call a function with any number of arguments, regardless of the number of arguments defined in the function.

function print(a, b) {
    console.log(a);
    console.log(b);
}

print(); // undefined, undefined
print(1); // 1, undefined
print(1, 2); // 1, 2
print(1, 2, 3); // 1, 2

Undefined vs. null

In JavaScript, undefined is a value that means "no value". It is different from null, which is a value that means "nothing". This can be confusing, because in Python, None is used for both cases. Another way to think of the difference is that undefined means "not defined", while null means "defined, but empty".

let x; // x was "declared" but its value is yet undefined
let y = null; // y's IS defined, it's value is null

Arrays

Arrays are very similar to Python lists:

let x = [1, 2, 3];
console.log(x[0]); // 1
console.log(x[1]); // 2
console.log(x[2]); // 3

Getting the length of an array is done using the length property:

let x = [1, 2, 3];
console.log(x.length); // 3

Objects

Objects are very similar to Python dictionaries (dict):

let x = {
    "name": "John",
    "age": 30
};
console.log(x["name"]); // John
console.log(x["age"]); // 30

However, JavaScript allows you to access the values using the dot notation:

let x = {
    "name": "John",
    "age": 30
};
console.log(x.name); // John
console.log(x.age); // 30

Loops

While loop

The syntax for while loops is very similar to Python (but again, code blocks are defined using curly braces {}, and conditions are wrapped in ()).

let x = 0;
while (x < 10) {
    console.log(x);
    x++;
}

x++ is a shorthand for x = x + 1. This can also be written as x += 1.

For loop

A for loop is just a shorthand for a while loop. The following two code snippets are equivalent:

let x = 0;
while (x < 10) {
    console.log(x);
    x++;
}

for (let x = 0; x < 10; x++) {
    console.log(x);
}

Notice that in both, we create an index variable x with a starting value of 0, and increment it by 1 in each iteration, while the condition is x < 10.

For...of loop

A for...of loop is used to iterate over the values of an array. It is similar to Python's for item in item_list loop.

let x = [1, 2, 3];
for (let item of x) {
    console.log(item);
}

This is basically the same as:

let x = [1, 2, 3];
for (let i = 0; i < x.length; i++) {
    console.log(x[i]);
}

Except usually shorter and easier to read and write. This is the preferred way of iterating over or generators in JavaScript.

Resources

This guide is very short and incomplete. For a more complete guide, check out the MDN JavaScript Guide. It's also recommended to check out the MDN JavaScript Reference for a complete reference of all JavaScript features.

W3Schools also has a good JavaScript tutorial, but it's not as complete as the MDN guide. This is also recommended for a quick overview of the basics, or as a quick "cheat sheet" for the syntax or different features and operations.