Data Structures

const arr = ["a", "b", "c"];

console.log(arr[10]); // undefined
arr.push("d");        // adds "d" to the end

lst = ["a", "b", "c"]

print(lst[10])    # IndexError
lst.append("d")   # adds "d" to the end

const lst = [0, 1, 2, 3, 4, 5];

console.log(lst.slice(2, 5));
// [2, 3, 4]

lst = [0, 1, 2, 3, 4, 5]

print(lst[2:5])
# [2, 3, 4]

const lst = [0, 1, 2, 3, 4, 5];

console.log(lst.indexOf(3));  // 3
console.log(lst.includes(3)); // true

lst = [0, 1, 2, 3, 4, 5]

print(lst.index(3))  # 3
print(3 in lst)      # True

const lst = [0, 1, 2, 3, 4, 5];

console.log(lst.at(-1)); // 5

lst = [0, 1, 2, 3, 4, 5]

print(lst[-1])  # 5

const lst = [0, 1, 2, 3, 4, 5];

const copy1 = [...lst];

console.log(copy1 === lst); // false
console.log(copy1);
// [0, 1, 2, 3, 4, 5]

nums = [0, 1, 2, 3, 4, 5]

copy1 = [*nums]

print(copy1 is nums)  # False
print(copy1)
# [0, 1, 2, 3, 4, 5]

const obj = { a: 1, b: 2, c: 3 };

console.log(obj.a);   // 1
obj.d = 4;           // adds new property
delete obj.b;        // removes property

obj = { "a": 1, "b": 2, "c": 3 }

print(obj["a"]) # 1
obj["d"] = 4 # adds new property
del obj["b"] # removes property
# print(obj["z"]) # KeyError: 'z'  
# (missing key raises, not undefined)
print(obj["a"]) # 1   — always brackets, no dot access
print(obj.get("z", "hello")) # 'hello' — avoids KeyError

for (const [key, val] of Object.entries(dict)) {
  console.log(key, val);
}

for key, val in d.items():
    print(key, val)

tup = (1, 2, 3)
print(tup[1])    # 2
# tup[0] = 5 # TypeError: 
#'tuple' object does not support item assignment

# GOTCHA: a trailing comma makes a tuple 
#(often a typo!)
a = 1
print(type(a)) # class 'int'
a = 1,
print(type(a)) # class 'tuple'