Does JavaScript support arraylist comprehensions like Python? - Stack Overflow

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I'm trying to get an array from each individual integer from the string. In Python, I would do:

string = '1234-5'
forbidden = '-'

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

Does JavaScript have something similar?

I'm trying to get an array from each individual integer from the string. In Python, I would do:

string = '1234-5'
forbidden = '-'

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

Does JavaScript have something similar?

• javascript

Share Improve this question Follow edited Feb 8, 2024 at 10:59 Mateen Ulhaq 27.2k2121 gold badges118118 silver badges152152 bronze badges asked Jul 11, 2015 at 3:49 A KA K 1,66422 gold badges1313 silver badges1818 bronze badges 2

• No it does not. Although JavaScript does support higher-ordered functions and ES5 adds support for some basic map/filter support in Array.prototype (there is no direct flatmap, although that can be emulated map-concat). There are additional libraries like underscore which add to this set of 'primitives' that is all Python is doing under the list-comprehension syntax guise. – user2864740 Commented Jul 11, 2015 at 4:14
• See stackoverflow.com/questions/33872615/… – Ben Creasy Commented Nov 17, 2016 at 2:15

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11 Answers 11

Sorted by: Reset to default 97

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]);

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
• unpacking string to list -> spread operator
• list comprehension 'if' -> Array.filter
• list comprehension 'for' -> Array.map
• substr in str? -> string.includes

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);
    }
});

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.

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)

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.

JavaScript doesn't need list comprehensions because map and filter work better in the language compared to Python.

In Python:

[int(i) for i in '1234-5' if i != '-'] 
  
# is equivalent to the ugly
list(map(lambda _: int(_),filter(lambda _: _!='-','1234-5')))

Whereas in JavaScript, map and filter are Array methods, so:

[...'1234-5'].filter(_=> _!='-').map(_=> parseInt(_))

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(""));

You could have a look at CoffeeScript. CoffeeScript adds missing features to java-script and allows you to write cleaner, more readable code. https://coffeescript.org/#coffeescript-2

You write a .coffee file and the coffeScript-compiler compiles your coffee file into a JavaScript file. Because the translation into JavaScript happens by compiling, the script should not run any slower.

So your code would look like the following in coffee script:

string = '1234-5'

forbidden = '-'

alert(JSON.stringify(+i for i in string when i isnt forbidden))

Honestly, this is even easier to read then the python counterpart. And it compiles quickly to the fallowing JavaScript:

var forbidden, i, string;

string = '1234-5';

forbidden = '-';

alert(JSON.stringify((function() {
  var j, len, results;
  results = [];
  for (j = 0, len = string.length; j < len; j++) {
    i = string[j];
    if (i !== forbidden) {
      results.push(+i);
    }
  }
  return results;
})()));

You don’t even need to install anything. On their website you can play around with it, and it will show you the translated JavaScript code.

A bit late for the party, but as of 2024 I would do:

console.log(parseInt(string.split('').filter(i => !forbidden.includes(i))))

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.

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