Python vs JavaScript Data Types Comparison

Collection Data Types

Arrays vs Lists

// JavaScript
let arr = [1, 2, 3, "four", true];
arr.push(5);            // Add to end: [1, 2, 3, "four", true, 5]
arr.pop();              // Remove from end: [1, 2, 3, "four", true]
arr.unshift(0);         // Add to beginning: [0, 1, 2, 3, "four", true]
arr.shift();            // Remove from beginning: [1, 2, 3, "four", true]
let sliced = arr.slice(1, 3);   // [2, 3]
arr.splice(1, 1, "two");  // Replace: [1, "two", 3, "four", true]

# Python
arr = [1, 2, 3, "four", True]
arr.append(5)          # Add to end: [1, 2, 3, "four", True, 5]
arr.pop()              # Remove from end: [1, 2, 3, "four", True]
arr.insert(0, 0)       # Add to beginning: [0, 1, 2, 3, "four", True]
arr.pop(0)             # Remove from beginning: [1, 2, 3, "four", True]
sliced = arr[1:3]      # [2, 3]
arr[1:2] = ["two"]     # Replace: [1, "two", 3, "four", True]

Objects vs Dictionaries

// JavaScript
let person = {
    name: "Alice",
    age: 30,
    "likes python": true
};

// Accessing properties
person.name;            // "Alice"
person["likes python"];  // true

// Methods
Object.keys(person);    // ["name", "age", "likes python"]
Object.values(person);  // ["Alice", 30, true]

// Checking if a key exists
"name" in person;       // true

# Python
person = {
    "name": "Alice",
    "age": 30,
    "likes python": True
}

# Accessing properties
person["name"]          # "Alice"
person["likes python"]  # True
# person.name          # Error! No dot notation for arbitrary keys

# Methods
person.keys()           # dict_keys(["name", "age", "likes python"])
person.values()         # dict_values(["Alice", 30, True])
person.items()          # dict_items([("name", "Alice"), ("age", 30), ("likes python", True)])

# Getting with default (no KeyError)
person.get("height", 165)  # 165 (default value if key doesn't exist)

# Checking if a key exists
"name" in person        # True

JavaScript Sets vs Python Sets

// JavaScript
let set = new Set([1, 2, 3, 3]);  // Set(3) {1, 2, 3}
set.add(4);             // Set(4) {1, 2, 3, 4}
set.has(2);             // true
set.delete(3);          // Set(3) {1, 2, 4}

// Set operations (ES6+)
let set1 = new Set([1, 2, 3]);
let set2 = new Set([3, 4, 5]);
let union = new Set([...set1, ...set2]);  // Set(5) {1, 2, 3, 4, 5}

# Python
set_a = {1, 2, 3, 3}    # {1, 2, 3}
set_a.add(4)            # {1, 2, 3, 4}
2 in set_a              # True
set_a.remove(3)         # {1, 2, 4}

# Set operations
set1 = {1, 2, 3}
set2 = {3, 4, 5}
union = set1 | set2     # {1, 2, 3, 4, 5}
intersection = set1 & set2  # {3}
difference = set1 - set2    # {1, 2}
symmetric_diff = set1 ^ set2  # {1, 2, 4, 5}

Maps vs Python Dictionaries

// JavaScript Map
let map = new Map();
map.set("key1", "value1");
map.set(42, "value2");  // Keys can be any type
map.get("key1");        // "value1"
map.has(42);            // true
map.delete("key1");     // true

# Python Dictionary (Equivalent to JavaScript Map)
dict_a = {}
dict_a["key1"] = "value1"
dict_a[42] = "value2"   # Keys can be any hashable type
dict_a["key1"]          # "value1"
42 in dict_a            # True
del dict_a["key1"]      # Removes the key

Other Collection Types in Python

Python has several collection types that don't have direct JavaScript equivalents:

# Tuple - immutable sequence
coordinates = (10, 20)  # Can't be modified after creation
first_coord = coordinates[0]  # Access like lists

# Named Tuple - immutable with named fields
from collections import namedtuple
Person = namedtuple('Person', ['name', 'age'])
alice = Person('Alice', 30)
alice.name  # 'Alice' - access by name
alice[0]    # 'Alice' - access by index

# Deque - efficient append/pop at both ends
from collections import deque
queue = deque([1, 2, 3])
queue.append(4)      # Add to right: [1, 2, 3, 4]
queue.appendleft(0)  # Add to left: [0, 1, 2, 3, 4]

# Counter - count occurrences
from collections import Counter
c = Counter("hello")  # Counter({'l': 2, 'h': 1, 'e': 1, 'o': 1})