Programming in a new language can feel unfamiliar, intimidating, and tricky to navigate at first. There are new rules, syntax, and nuances to learn.

I recently started learningPython again after years of programming inJavaScript. After diving back in in 2020, I noticed that, as with many programming languages, Python and JavaScript share a lot of the same basic features, data structures, and design patterns.

If you’re familiar with JavaScript and wondering if you should learn Python, your knowledge of JavaScript can help you quickly get up to speed. This article covers some of the essential concepts, mechanics, and features of the two languages. I’ll provide just enough of the basics to pique your curiosity and help kick off your polyglot learning journey.

Python vs. JavaScript Language Basics

JavaScript and Python areinterpreted programming languages, meaning their runtime environments use an interpreter (or engine) that parses and executes code one statement at a time.

The two languages are also “object-based” — everything is (or can be treated as) an object: strings, numbers, data structures, functions, etc.

Primitive Types

First up, JavaScript and Python have similar built-in data types. For example, both use numeric data types (integers and floats), strings and Booleans.

// JavaScript data types  const pi = 3.14;  const age = 31;  const greeting = "good morning";const isAdmin = true;

# Python data types  pi = 3.14  age = 13  greeting = "good morning"  is_admin = True

Type Checking and Conversion

Python and JavaScript are “dynamically typed” languages, which means you do not have to set the type of a variable explicitly. The data type is set when you assign a value to a variable.

In JavaScript, you use thetypeof operator to verify the data type of a variable. Python provides a similar built-in function,type().

// JavaScript  const greeting = "good morning";  typeof greeting; // "string"

# Python  pi = 3.14  type(pi) # float

You can convert from one type to another, like a string to a number, in Python with theint() andfloat() functions:

# Python  input = input('Enter a number: ') #'2'# convert string to int  int(input) # 2

JavaScript includes the methodsparseInt() andparseFloat() for the same purpose:

// JavaScript  const input = prompt('Enter a number:'); // '4'  parseInt(input); // 4

Built-in String Methods

To convert cased characters in a string from uppercase to lowercase (and the reverse), use Python’supper() andlower() functions:

# Python  user_name = input('What is your name? ') # GUIL  user_name.lower() # 'guil'greeting = "good evening"  greeting.upper() # 'GOOD EVENING'

JavaScript supplies thetoUpperCase() andtoLowerCase() methods to convert strings:

// JavaScript  const greeting = "good evening";  const userName = prompt("What is your name?"); // "gUiL"greeting.toUpperCase(); // "GOOD EVENING"  userName.toLowerCase(); // "guil"

String Interpolation

Template literals in JavaScript let you replace${} placeholders with values inside of a string literal. This process is called string interpolation:

// JavaScript  const greeting = "Good evening";  const name = "Guil";  console.log(`${greeting}, ${name}!`); // Good evening, Guil!

The Python stringformat() method inserts values into a template string containing{} replacement fields. You pass the method the values to interpolate. For example:

# Python strings  greeting = "Good evening"  name = "Guil"  print( "{}, {}!".format(greeting, name) ) # Good evening, Guil!

Each set of curly braces gets replaced with the values passed toformat() in sequential order.

Python’s formatted string literal (f-String) offers a more concise syntax to accomplish the same. It looks like a regular string that’s prepended by the characterf, and you include the value to interpolate directly inside the string.

# Python strings  greeting = "Good evening"  name = "Guil"  print(f"{greeting}, {name}!") # Good evening, Guil!

Learn more built-in string methods in our Python Basics course.

Python vs. JavaScript Data Structures

JavaScript and Python give you comparable structures to store and organize your data.

Arrays and Lists

Like a JavaScript array, a Python list stores a collection of values in a single container. The values can be different data types like strings, integers, Booleans, etc.

# Python list  students = ['Lee', 'Toni', 'Marie', 'Agata']  # return length of list  len(students) # 4  students[2] # 'Marie'

// JavaScript array  const students = ['Lee', 'Toni', 'Marie', 'Agata'];  students.length; // 4  students[0]; // Lee

Notice how both languages have similar ways of returning the length of a list and retrieving a value by index.

Array and List Methods

Since arrays and lists are considered objects in their respective language, there are various properties and methods you can use on them. For example, a common way to add elements to the end of an array in JavaScript is with thepush() method:

// JavaScript  const instruments = ['piano', 'drums', 'trumpet'];  instruments.push('guitar');  // ['piano', 'drums', 'trumpet', 'guitar']

You add an item to the end of a Python list with theappend() method:

# Python  instruments = ['piano', 'drums', 'trumpet']  instruments.append('guitar') # ['piano', 'drums', 'trumpet', 'guitar']

JavaScript arrays and Python lists have apop() method for removing and returning items. Callingpop() on a JavaScript array removes the last element:

// JavaScript arrays  const instruments = ['piano', 'drums', 'trumpet'];  instruments.pop();  // "trumpet"

While JavaScript’spop() method does not accept arguments, Python’s does! For instance, passpop() the index of the item you want to remove:

# Python lists  instruments = ['piano', 'drums', 'trumpet']  instruments.pop(1) # 'drums'

Spreading and Unpacking

In JavaScript, you use the spread operator (...) to copy, combine and manipulate arrays:

// JavaScript arrays  const studentsA = ['Lee', 'Toni', 'Marie'];  const studentsB = ['Meg', 'Jesse', 'Anwar'];const students = [...studentsA, ...studentsB];  // ['Lee', 'Toni', 'Marie', 'Meg', 'Jesse', 'Anwar']

In a similar way, you can “unpack” items from one Python list into another using an asterisk (*):

# Python lists  students_a = ['Lee', 'Toni', 'Marie']  students_b = ['Meg', 'Jesse', 'Anwar']students = [*students_a, *students_b]  # ['Lee', 'Toni', 'Marie', 'Meg', 'Jesse', 'Anwar']

Both create a copy of a list or array, preserving the original values.

One handy aspect of the JavaScript spread operator is that you can pass arrays as arguments to functions.

// JavaScript  const numbers = [10, 20, 30, 40];  Math.max(...numbers); // 40

Likewise, you have the ability to unpack items in a Python list for function calls:

# Python  numbers = [1, 10]  list(range(*numbers)) # [1, 2, 3, 4, 5, 6, 7, 8, 9]

Both expand an array or list into separate arguments.

Objects and Dictionaries

If you’re familiar with JavaScript objects, you’ll recognize Python dictionaries. You write both using curly brackets holding related data in the form of key/value pairs.

// JavaScript object  const pet = {      name: 'Joey',      type: 'Dog',      breed: 'Australian Shepherd',      age: 8  };

# Python dictionary  pet = {      'name': 'Joey',      'type': 'Dog',      'breed': 'Australian Shepherd',      'age': 8  }

Lists and dictionaries, like arrays and objects aremutable, which means that you can change the data inside them without changing their identity. Once you create an object, its type and identity (or the address in memory it’s pointing to) does not change.

Copying/Merging Objects and Dictionaries

JavaScript’s spread operator copies key/value pairs from one object literal to another. It’s comparable to the double asterisks (**) operator in Python, which copies and merges dictionaries:

// JavaScript objects  const name = {      firstName: 'Reggie',      lastName: 'Williams'  };const developer = {      ...name, // place the 'name' key/values here      title: 'Software developer',      skills: ['JavaScript', 'HTML', 'CSS']  };

# Python dictionaries  name = {      'firstName': 'Reggie',      'lastName': 'Williams'  }developer = {      **name, # place the 'name' key/values here      'title': 'Software developer',      'skills': ['JavaScript', 'HTML', 'CSS']  }

Functions

Both languages take full advantage of functions for code reuse. Python uses thedef keyword compared tofunction in JavaScript.

# Python function  def add(a, b=10):      val = a + b      return val

// JavaScript function  function add(a, b = 10) {      const val = a + b;      return val;  }

Notice how both use thereturn keyword to return a value, and you’re able to specify default parameters in each function definition.

Single Line Functions

Arrow functions in JavaScript offer a concise syntax for creating functions. More so, if your function body is only one line of code, you can omit thereturn keyword and place everything on one line:

// JavaScript arrow function  const add = (a, b) => a + b;

In Python, thelambda keyword provides syntactic sugar for defining functions as single-line expressions:

# Python lambda  add = lambda a,b : a + b

These single-line functions are common when you want to pass an anonymous function as an argument to another (higher-order) function. For example, they’re used with the built-in Python and JavaScript functionsmap(),filter(), andreduce().

// JavaScript arrow function  const states = ['ca', 'fl', 'hi', 'ny'];  states.map( s => s.toUpperCase() );  // ["CA", "FL", "HI", "NY"]

# Python lambda  states = ['ca', 'fl', 'hi', 'ny']  list( map(lambda s: s.upper(), states) )  # ['CA', 'FL', 'HI', 'NY']

Conditional Statements

Python’s flow control statements also look and work similarly to theif/else you know from #"">// JavaScript conditional let score = 4;if ( score === 5 ) { console.log("Gold Medal!"); } else if ( score >= 3 ) { console.log("Silver Medal"); } else if ( score >= 1 ) { console.log("Bronze Medal"); } else { console.log("No Medal :("); }

# Python conditional  score = 5if score == 5:      print("Gold Medal!")  elif score >= 3:      print("Silver Medal")  elif score >= 1:      print("Bronze Medal")  else:      print("No Medal :(")

# Python  password = input("Enter the secret password: ")  while password != 'sesame':      password = input("Invalid password. Try again: ")

// JavaScript  let password = prompt("Enter the secret password:");  while (password !== 'sesame') {      password = prompt("Invalid password. Try again: ");  }

# Python for loop  students = ['Lee', 'Toni', 'Marie', 'Jesse', 'Anwar']  for student in students:      print(student)

// JavaScript for...of loop  const students = ['Lee', 'Toni', 'Marie', 'Jesse', 'Anwar']  for (let student of students) {      console.log(student);  }

# Python  scores = [50, 20, 30, 0, 10, 15, 35]  for score in scores:      print(f"Score: {score}");      if score == 0:          print("You may not continue if you have a 0 score.")          break

// JavaScript  while (true) {      let response = prompt("Type 'exit' to make this stop.");      if (response === 'exit') {          break;      }  }