Does JavaScript support array/list comprehensions like Python?

Question

I'm practicing/studying both JavaScript and Python. I'm wondering if Javascript has the equivalence to this type of coding.

I'm basically trying to get an array from each individual integer from the string for practice purposes. I'm more proficient in Python than JavaScript

Python:

string = '1234-5'

forbidden = '-'

print([int(i) for i in str(string) if i not in forbidden])

Does Javascript have something similar for me to do above?

Answer 1

Update: Array comprehensions were removed from the standard. Quoting MDN:

The array comprehensions syntax is non-standard and removed starting with Firefox 58. For future-facing usages, consider using Array.prototype.map, Array.prototype.filter, arrow functions, and spread syntax.

See this answer for an example with Array.prototype.map:

let emails = people.map(({ email }) => email);

Original answer:

Yes, JavaScript will support array comprehensions in the upcoming EcmaScript version 7.

Here's an example.

var str =  "1234-5";
var ignore = "-";

console.log([for (i of str) if (!ignore.includes(i)) i]);

Answer 2

Given the question's Python code

print([int(i) for i in str(string) if i not in forbidden])

this is the most direct translation to JavaScript (ES2015):

const string = '1234-5';
const forbidden = '-';

console.log([...string].filter(c => !forbidden.includes(c)).map(c => parseInt(c)));
// result: [ 1, 2, 3, 4, 5 ]

Here is a comparison of the Python and JavaScript code elements being used: (Python -> Javascript):

• print -> console.log
• iterate over characters in a string -> spread operator
• list comprehension 'if' -> Array.filter
• list comprehension 'for' -> Array.map
• substr in str? -> string.includes

Answer 3

Reading the code, I assume forbidden can have more than 1 character. I'm also assuming the output should be "12345"

var string = "12=34-5";

var forbidden = "=-";

console.log(string.split("").filter(function(str){
    return forbidden.indexOf(str) < 0;
}).join(""))

If the output is "1" "2" "3" "4" "5" on separate lines

var string = "12=34-5";

var forbidden = "=-";

string.split("").forEach(function(str){
    if (forbidden.indexOf(str) < 0) {
        console.log(str);
    }
});

Answer 4

Not directly, but it's not hard to replicate.

var string = "1234-5";

var forbidden = "-";

string.split("").filter(function(str){
    if(forbidden.indexOf(str) < 0) {
        return str;
    }
}).forEach(function(letter) { console.log(letter);});

I guess more directly:

for(var i=0 ; i < str.length ; i++) {
    if(forbidden.indexOf(str) < 0) {
        console.log(str[i]);
    }
}

But there's no built in way to filter in your for loop.

Answer 5

You could easily achieve this behavior using an application functor.

Array.prototype.ap = function(xs) {
  return this.reduce((acc, f) => acc.concat(xs.map(f)), []) 
}


const result = [x => x +1].ap([2])
console.log(result)

Answer 6

JavaScript no longer supports array comprehensions.

I too was looking for the JavaScript equivalent. Mozilla Developer's Network indicates that this functionality is no longer supported. The preferred syntax is referenced in the aforementioned link.

Answer 7

For "completeness"-sake, here's a shorter regexp version.

var str =  "1234-5";
var ignore = "-=";

console.log(str.replace(new RegExp(ignore.split("").join("|")), "").split(""));

EDIT: To make sure that RegExp does not "choke" on special characters, ignore can be implemented as regexp literal, instead of a string:

var str =  "1234-5";
var ignore = /[\+=-]/;
console.log(str.replace(ignore, "").split(""));

Answer 8

It does have a poor mans version

const string = '1234-5'

const forbidden = '-'

print([int(i) for i in str(string) if i not in forbidden])
const result = string.split('').filter(char => char !== forbidden);
console.log(result)

In JS you can only iterate over single elements in array, so no extraction of multiple entries at a time like in Python.

For this particular case you should use a RegExp to filter the string though.