Compare Python and JavaScript array loops

I mainly use Python, but when I touch JavaScript, there are various array operations and I get confused, so I organized it.

theme

This time, the theme is a program that loops an array containing strings and returns an array with __ at the end of all elements.

Python

for in In Python, there is only one basic for statement. You can iterate with for i in <list>.

from typing import List
array = ["hoge", "fuga", "foo", "bar"]

def add_suffix(text: str) -> str:
    return text + "__"

def for_in(array: List[str]) -> List[str]:
    ls: list = []
    for item in array:
        ls.append(add_suffix(item))
    return ls

print(for_map(array))
# ['hoge__', 'fuga__', 'foo__', 'bar__']

map
You can use map to apply a function to all arrays. However, the return value of the map method is an iterator called a map object, so it needs to be converted to a list.

def for_map(array: List[str]) -> List[str]:
    return list(map(add_suffix, array))

print(for_in(array))
# ['hoge__', 'fuga__', 'foo__', 'bar__']

enumerate
If you need an index, you can use ʻenumerate`.

def for_enum(array: List[str]) -> List[str]:
    ls: list = []
    for idx, item in enumerate(array):
        # idx = 0, 1, 2, 3
        ls.append(add_suffix(item))
    return ls

print(for_enum(array))
# ['hoge__', 'fuga__', 'foo__', 'bar__']

JavaScript

for In JavaScript, there are various description methods, but the basic one is probably the same style as C language.

var array = ["hoge", "fuga", "foo", "bar"];

function add_suffix(text) {
  return text + "__";
}

function forI(array) {
  returnArray = new Array();
  for (var i = 0; i < array.length; i++) {
    // i = 0, 1, 2, 3
    returnArray.push(add_suffix(array[i]));
  }
  return returnArray;
}

console.log(forI(array));
// ['hoge__', 'fuga__', 'foo__', 'bar__']

for in
var i in array loops the properties of the object. Since the property of the ʻArray object is an array of integer values, the above counter can be retrieved. However, note that ʻidx here is a string type.

function forIn(array) {
  returnArray = new Array();
  for (var idx in array) {
    // idx = "0", "1", "2", "3"
    returnArray.push(add_suffix(array[idx]));
  }
  return returnArray;
}

console.log(forIn(array));
// ['hoge__', 'fuga__', 'foo__', 'bar__']

However, this method is not recommended for arrays where the order is important because the iteration order is implementation dependent. for...in - JavaScript _ MDN

Note: for ... in should not be used for array iterations where index order is important.

Because the order of the iterations is implementation-dependent, array iterations do not always refer to the elements in a consistent order. For this reason, it is better to use a for loop (or Array.prototype.forEach () or for ... of loop) with a numeric index when repeating an array where the order of access is important.

for of
You can use for of from ECMAScript6. This behavior is familiar to Python users. The contents of the array itself are retrieved.

function forOf(array) {
  returnArray = new Array();
  for (var item of array) {
    returnArray.push(add_suffix(item));
  }
  return returnArray;
}

console.log(forOf(array));
// ['hoge__', 'fuga__', 'foo__', 'bar__']

forEach
You can use ʻArray.prototype.forEach ()` to execute one function for each element. Array.prototype.forEach() - JavaScript _ MDN

The forEach () method executes the given function once for each element of the array.

function forEach(array) {
  returnArray = new Array();
  array.forEach(item => {
    returnArray.push(add_suffix(item));
  });
  return returnArray;
}

console.log(forEach(array));
// ['hoge__', 'fuga__', 'foo__', 'bar__']

The difference from for of is that you cannot break in the middle except to raise an exception. Also, if the element is ʻundefined`, the function will not be called. However, it can be used with IE9 or above.

map
The last is map, which applies the function to all the elements of the array. Unlike Python, the return value is ʻArray`.

function forMap(array) {
  return array.map(add_suffix);
}

console.log(forMap(array));
// ['hoge__', 'fuga__', 'foo__', 'bar__']