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I have a JavaScript array like:

[["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]]

How would I go about merging the separate inner arrays into one like:

["$6", "$12", "$25", ...]

I have a JavaScript array like:

[["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]]

How would I go about merging the separate inner arrays into one like:

["$6", "$12", "$25", ...]

• javascript
• arrays
• multidimensional-array
• merge
• flatten

Share Improve this question Follow edited Oct 9, 2023 at 8:08 Super Kai - Kazuya Ito 1 asked Jun 2, 2012 at 18:53 AndyAndy 19.3k1414 gold badges4747 silver badges5454 bronze badges 4

• 33 All of the solutions that use reduce + concat are O((N^2)/2) where as a accepted answer (just one call to concat) would be at most O(N*2) on a bad browser and O(N) on a good one. Also Denys solution is optimized for the actual question and upto 2x faster than the single concat. For the reduce folks it's fun to feel cool writing tiny code but for example if the array had 1000 one element subarrays all the reduce+concat solutions would be doing 500500 operations where as the single concat or simple loop would do 1000 operations. – user128511 Commented Jul 30, 2017 at 15:45
• 13 With the latest browsers that support ES2019: array.flat(Infinity) where Infinity is the maximum depth to flatten. – Timothy Gu Commented Apr 6, 2020 at 2:47
• I'm glad they put a maximum depth.. :D – Magne Commented Mar 14, 2022 at 14:52
• Flatmap works like champ, try this. var d = [[1], [2], [3], [4,5,6]].flatMap(x => x); – Vince Commented Feb 28, 2024 at 15:45

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91 Answers 91

Sorted by: Reset to default 1 2 3 4 Next 2596

---

ES2019

ES2019 introduced the Array.prototype.flat() method which you could use to flatten the arrays. It is compatible with most environments, although it is only available in Node.js starting with version 11, and not at all in Internet Explorer.

const arrays = [
      ["$6"],
      ["$12"],
      ["$25"],
      ["$25"],
      ["$18"],
      ["$22"],
      ["$10"]
    ];
const merge3 = arrays.flat(1); //The depth level specifying how deep a nested array structure should be flattened. Defaults to 1.
console.log(merge3);
    

---

Older browsers

For older browsers, you can use Array.prototype.concat to merge arrays:

var arrays = [
  ["$6"],
  ["$12"],
  ["$25"],
  ["$25"],
  ["$18"],
  ["$22"],
  ["$10"]
];
var merged = [].concat.apply([], arrays);

console.log(merged);

Using the apply method of concat will just take the second parameter as an array, so the last line is identical to this:

var merged = [].concat(["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]);

Here's a short function that uses some of the newer JavaScript array methods to flatten an n-dimensional array.

function flatten(arr) {
  return arr.reduce(function (flat, toFlatten) {
    return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
  }, []);
}

Usage:

flatten([[1, 2, 3], [4, 5]]); // [1, 2, 3, 4, 5]
flatten([[[1, [1.1]], 2, 3], [4, 5]]); // [1, 1.1, 2, 3, 4, 5]

There is a confusingly hidden method, which constructs a new array without mutating the original one:

var oldArray = [[1],[2,3],[4]];
var newArray = Array.prototype.concat.apply([], oldArray);
console.log(newArray); // [ 1, 2, 3, 4 ]

It can be best done by javascript reduce function.

var arrays = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"], ["$0"], ["$15"],["$3"], ["$75"], ["$5"], ["$100"], ["$7"], ["$3"], ["$75"], ["$5"]];

arrays = arrays.reduce(function(a, b){
     return a.concat(b);
}, []);

Or, with ES2015:

arrays = arrays.reduce((a, b) => a.concat(b), []);

js-fiddle

Mozilla docs

There's a new native method called flat to do this exactly.

(As of late 2019, flat is now published in the ECMA 2019 standard, and core-js@3 (babel's library) includes it in their polyfill library)

const arr1 = [1, 2, [3, 4]];
arr1.flat(); 
// [1, 2, 3, 4]

const arr2 = [1, 2, [3, 4, [5, 6]]];
arr2.flat();
// [1, 2, 3, 4, [5, 6]]

// Flatten 2 levels deep
const arr3 = [2, 2, 5, [5, [5, [6]], 7]];
arr3.flat(2);
// [2, 2, 5, 5, 5, [6], 7];

// Flatten all levels
const arr4 = [2, 2, 5, [5, [5, [6]], 7]];
arr4.flat(Infinity);
// [2, 2, 5, 5, 5, 6, 7];

Most of the answers here don't work on huge (e.g. 200 000 elements) arrays, and even if they do, they're slow.

Here is the fastest solution, which works also on arrays with multiple levels of nesting:

const flatten = function(arr, result = []) {
  for (let i = 0, length = arr.length; i < length; i++) {
    const value = arr[i];
    if (Array.isArray(value)) {
      flatten(value, result);
    } else {
      result.push(value);
    }
  }
  return result;
};

Examples

Huge arrays

flatten(Array(200000).fill([1]));

It handles huge arrays just fine. On my machine this code takes about 14 ms to execute.

Nested arrays

flatten(Array(2).fill(Array(2).fill(Array(2).fill([1]))));

It works with nested arrays. This code produces [1, 1, 1, 1, 1, 1, 1, 1].

Arrays with different levels of nesting

flatten([1, [1], [[1]]]);

It doesn't have any problems with flattening arrays like this one.

Update: it turned out that this solution doesn't work with large arrays. It you're looking for a better, faster solution, check out this answer.

---

function flatten(arr) {
  return [].concat(...arr)
}

Is simply expands arr and passes it as arguments to concat(), which merges all the arrays into one. It's equivalent to [].concat.apply([], arr).

You can also try this for deep flattening:

function deepFlatten(arr) {
  return flatten(           // return shalowly flattened array
    arr.map(x=>             // with each x in array
      Array.isArray(x)      // is x an array?
        ? deepFlatten(x)    // if yes, return deeply flattened x
        : x                 // if no, return just x
    )
  )
}

See demo on JSBin.

References for ECMAScript 6 elements used in this answer:

• Spread operator
• Arrow functions

---

Side note: methods like find() and arrow functions are not supported by all browsers, but it doesn't mean that you can't use these features right now. Just use Babel — it transforms ES6 code into ES5.

You can use Underscore:

var x = [[1], [2], [3, 4]];

_.flatten(x); // => [1, 2, 3, 4]

Generic procedures mean we don't have to rewrite complexity each time we need to utilize a specific behaviour.

concatMap (or flatMap) is exactly what we need in this situation.

// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
  xs.concat (ys)

// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
  xs.map(f).reduce(concat, [])

// id :: a -> a
const id = x =>
  x

// flatten :: [[a]] -> [a]
const flatten =
  concatMap (id)

// your sample data
const data =
  [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]]

console.log (flatten (data))

foresight

And yes, you guessed it correctly, it only flattens one level, which is exactly how it should work

Imagine some data set like this

// Player :: (String, Number) -> Player
const Player = (name,number) =>
  [ name, number ]

// team :: ( . Player) -> Team
const Team = (...players) =>
  players

// Game :: (Team, Team) -> Game
const Game = (teamA, teamB) =>
  [ teamA, teamB ]

// sample data
const teamA =
  Team (Player ('bob', 5), Player ('alice', 6))

const teamB =
  Team (Player ('ricky', 4), Player ('julian', 2))

const game =
  Game (teamA, teamB)

console.log (game)
// [ [ [ 'bob', 5 ], [ 'alice', 6 ] ],
//   [ [ 'ricky', 4 ], [ 'julian', 2 ] ] ]

Ok, now say we want to print a roster that shows all the players that will be participating in game …

const gamePlayers = game =>
  flatten (game)

gamePlayers (game)
// => [ [ 'bob', 5 ], [ 'alice', 6 ], [ 'ricky', 4 ], [ 'julian', 2 ] ]

If our flatten procedure flattened nested arrays too, we'd end up with this garbage result …

const gamePlayers = game =>
  badGenericFlatten(game)

gamePlayers (game)
// => [ 'bob', 5, 'alice', 6, 'ricky', 4, 'julian', 2 ]

---

rollin' deep, baby

That's not to say sometimes you don't want to flatten nested arrays, too – only that shouldn't be the default behaviour.

We can make a deepFlatten procedure with ease …

// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
  xs.concat (ys)

// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
  xs.map(f).reduce(concat, [])

// id :: a -> a
const id = x =>
  x

// flatten :: [[a]] -> [a]
const flatten =
  concatMap (id)

// deepFlatten :: [[a]] -> [a]
const deepFlatten =
  concatMap (x =>
    Array.isArray (x) ? deepFlatten (x) : x)

// your sample data
const data =
  [0, [1, [2, [3, [4, 5], 6]]], [7, [8]], 9]

console.log (flatten (data))
// [ 0, 1, [ 2, [ 3, [ 4, 5 ], 6 ] ], 7, [ 8 ], 9 ]

console.log (deepFlatten (data))
// [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

There. Now you have a tool for each job – one for squashing one level of nesting, flatten, and one for obliterating all nesting deepFlatten.

Maybe you can call it obliterate or nuke if you don't like the name deepFlatten.

---

Don't iterate twice !

Of course the above implementations are clever and concise, but using a .map followed by a call to .reduce means we're actually doing more iterations than necessary

Using a trusty combinator I'm calling mapReduce helps keep the iterations to a minium; it takes a mapping function m :: a -> b, a reducing function r :: (b,a) ->b and returns a new reducing function - this combinator is at the heart of transducers; if you're interested, I've written other answers about them

// mapReduce = (a -> b, (b,a) -> b, (b,a) -> b)
const mapReduce = (m,r) =>
  (acc,x) => r (acc, m (x))

// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
  xs.reduce (mapReduce (f, concat), [])

// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
  xs.concat (ys)

// id :: a -> a
const id = x =>
  x

// flatten :: [[a]] -> [a]
const flatten =
  concatMap (id)
  
// deepFlatten :: [[a]] -> [a]
const deepFlatten =
  concatMap (x =>
    Array.isArray (x) ? deepFlatten (x) : x)

// your sample data
const data =
  [ [ [ 1, 2 ],
      [ 3, 4 ] ],
    [ [ 5, 6 ],
      [ 7, 8 ] ] ]

console.log (flatten (data))
// [ [ 1. 2 ], [ 3, 4 ], [ 5, 6 ], [ 7, 8 ] ]

console.log (deepFlatten (data))
// [ 1, 2, 3, 4, 5, 6, 7, 8 ]

To flatten an array of single element arrays, you don't need to import a library, a simple loop is both the simplest and most efficient solution :

for (var i = 0; i < a.length; i++) {
  a[i] = a[i][0];
}

To downvoters: please read the question, don't downvote because it doesn't suit your very different problem. This solution is both the fastest and simplest for the asked question.

You can also try the new Array.flat() method. It works in the following manner:

let arr = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]].flat()

console.log(arr);

The flat() method creates a new array with all sub-array elements concatenated into it recursively up to the 1 layer of depth (i.e. arrays inside arrays)

If you want to also flatten out 3 dimensional or even higher dimensional arrays you simply call the flat method multiple times. For example (3 dimensions):

let arr = [1,2,[3,4,[5,6]]].flat().flat().flat();

console.log(arr);

Be careful!

Array.flat() method is relatively new. Older browsers like ie might not have implemented the method. If you want you code to work on all browsers you might have to transpile your JS to an older version. Check for MDN web docs for current browser compatibility.

A solution for the more general case, when you may have some non-array elements in your array.

function flattenArrayOfArrays(a, r){
    if(!r){ r = []}
    for(var i=0; i<a.length; i++){
        if(a[i].constructor == Array){
            flattenArrayOfArrays(a[i], r);
        }else{
            r.push(a[i]);
        }
    }
    return r;
}

Another ECMAScript 6 solution in functional style:

Declare a function:

const flatten = arr => arr.reduce(
  (a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
);

and use it:

flatten( [1, [2,3], [4,[5,[6]]]] ) // -> [1,2,3,4,5,6]

 const flatten = arr => arr.reduce(
         (a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
       );


console.log( flatten([1, [2,3], [4,[5],[6,[7,8,9],10],11],[12],13]) )

Consider also a native function Array.prototype.flat() (proposal for ES6) available in last releases of modern browsers. Thanks to @(Константин Ван) and @(Mark Amery) mentioned it in the comments.

The flat function has one parameter, specifying the expected depth of array nesting, which equals 1 by default.

[1, 2, [3, 4]].flat();                  // -> [1, 2, 3, 4]

[1, 2, [3, 4, [5, 6]]].flat();          // -> [1, 2, 3, 4, [5, 6]]

[1, 2, [3, 4, [5, 6]]].flat(2);         // -> [1, 2, 3, 4, 5, 6]

[1, 2, [3, 4, [5, 6]]].flat(Infinity);  // -> [1, 2, 3, 4, 5, 6]

let arr = [1, 2, [3, 4]];

console.log( arr.flat() );

arr =  [1, 2, [3, 4, [5, 6]]];

console.log( arr.flat() );
console.log( arr.flat(1) );
console.log( arr.flat(2) );
console.log( arr.flat(Infinity) );

What about using reduce(callback[, initialValue]) method of JavaScript 1.8

list.reduce((p,n) => p.concat(n),[]);

Would do the job.

const common = arr.reduce((a, b) => [...a, ...b], [])

You can use Array.flat() with Infinity for any depth of nested array.

var arr = [ [1,2,3,4], [1,2,[1,2,3]], [1,2,3,4,5,[1,2,3,4,[1,2,3,4]]], [[1,2,3,4], [1,2,[1,2,3]], [1,2,3,4,5,[1,2,3,4,[1,2,3,4]]]] ];

let flatten = arr.flat(Infinity)

console.log(flatten)

check here for browser compatibility

ES6 One Line Flatten

See lodash flatten, underscore flatten (shallow true)

function flatten(arr) {
  return arr.reduce((acc, e) => acc.concat(e), []);
}

or

function flatten(arr) {
  return [].concat.apply([], arr);
}

Tested with

test('already flatted', () => {
  expect(flatten([1, 2, 3, 4, 5])).toEqual([1, 2, 3, 4, 5]);
});

test('flats first level', () => {
  expect(flatten([1, [2, [3, [4]], 5]])).toEqual([1, 2, [3, [4]], 5]);
});

ES6 One Line Deep Flatten

See lodash flattenDeep, underscore flatten

function flattenDeep(arr) {
  return arr.reduce((acc, e) => Array.isArray(e) ? acc.concat(flattenDeep(e)) : acc.concat(e), []);
}

Tested with

test('already flatted', () => {
  expect(flattenDeep([1, 2, 3, 4, 5])).toEqual([1, 2, 3, 4, 5]);
});

test('flats', () => {
  expect(flattenDeep([1, [2, [3, [4]], 5]])).toEqual([1, 2, 3, 4, 5]);
});

Using the spread operator:

const input = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]];
const output = [].concat(...input);
console.log(output); // --> ["$6", "$12", "$25", "$25", "$18", "$22", "$10"]

Please note: When Function.prototype.apply ([].concat.apply([], arrays)) or the spread operator ([].concat(...arrays)) is used in order to flatten an array, both can cause stack overflows for large arrays, because every argument of a function is stored on the stack.

Here is a stack-safe implementation in functional style that weighs up the most important requirements against one another:

• reusability
• readability
• conciseness
• performance

// small, reusable auxiliary functions:

const foldl = f => acc => xs => xs.reduce(uncurry(f), acc); // aka reduce

const uncurry = f => (a, b) => f(a) (b);

const concat = xs => y => xs.concat(y);


// the actual function to flatten an array - a self-explanatory one-line:

const flatten = xs => foldl(concat) ([]) (xs);

// arbitrary array sizes (until the heap blows up :D)

const xs = [[1,2,3],[4,5,6],[7,8,9]];

console.log(flatten(xs));


// Deriving a recursive solution for deeply nested arrays is trivially now


// yet more small, reusable auxiliary functions:

const map = f => xs => xs.map(apply(f));

const apply = f => a => f(a);

const isArray = Array.isArray;


// the derived recursive function:

const flattenr = xs => flatten(map(x => isArray(x) ? flattenr(x) : x) (xs));

const ys = [1,[2,[3,[4,[5],6,],7],8],9];

console.log(flattenr(ys));

As soon as you get used to small arrow functions in curried form, function composition and higher order functions, this code reads like prose. Programming then merely consists of putting together small building blocks that always work as expected, because they don't contain any side effects.

I recommend a space-efficient generator function:

function* flatten(arr) {
  if (!Array.isArray(arr)) yield arr;
  else for (let el of arr) yield* flatten(el);
}

// Example:
console.log(...flatten([1,[2,[3,[4]]]])); // 1 2 3 4

If desired, create an array of flattened values as follows:

let flattened = [...flatten([1,[2,[3,[4]]]])]; // [1, 2, 3, 4]

If you only have arrays with 1 string element:

[["$6"], ["$12"], ["$25"], ["$25"]].join(',').split(',');

will do the job. Bt that specifically matches your code example.

I have done it using recursion and closures

function flatten(arr) {

  var temp = [];

  function recursiveFlatten(arr) { 
    for(var i = 0; i < arr.length; i++) {
      if(Array.isArray(arr[i])) {
        recursiveFlatten(arr[i]);
      } else {
        temp.push(arr[i]);
      }
    }
  }
  recursiveFlatten(arr);
  return temp;
}

A Haskellesque approach

function flatArray([x,...xs]){
  return x ? [...Array.isArray(x) ? flatArray(x) : [x], ...flatArray(xs)] : [];
}

var na = [[1,2],[3,[4,5]],[6,7,[[[8],9]]],10];
    fa = flatArray(na);
console.log(fa);

if you use lodash, you can just use its flatten method: https://lodash.com/docs/4.17.14#flatten

The nice thing about lodash is that it also has methods to flatten the arrays:

i) recursively: https://lodash.com/docs/4.17.14#flattenDeep

ii) upto n levels of nesting: https://lodash.com/docs/4.17.14#flattenDepth

For example

const _ = require("lodash");
const pancake =  _.flatten(array)

ES6 way:

const flatten = arr => arr.reduce((acc, next) => acc.concat(Array.isArray(next) ? flatten(next) : next), [])

const a = [1, [2, [3, [4, [5]]]]]
console.log(flatten(a))

ES5 way for flatten function with ES3 fallback for N-times nested arrays:

var flatten = (function() {
  if (!!Array.prototype.reduce && !!Array.isArray) {
    return function(array) {
      return array.reduce(function(prev, next) {
        return prev.concat(Array.isArray(next) ? flatten(next) : next);
      }, []);
    };
  } else {
    return function(array) {
      var arr = [];
      var i = 0;
      var len = array.length;
      var target;

      for (; i < len; i++) {
        target = array[i];
        arr = arr.concat(
          (Object.prototype.toString.call(target) === '[object Array]') ? flatten(target) : target
        );
      }

      return arr;
    };
  }
}());

var a = [1, [2, [3, [4, [5]]]]];
console.log(flatten(a));

I was goofing with ES6 Generators the other day and wrote this gist. Which contains...

function flatten(arrayOfArrays=[]){
  function* flatgen() {
    for( let item of arrayOfArrays ) {
      if ( Array.isArray( item )) {
        yield* flatten(item)
      } else {
        yield item
      }
    }
  }

  return [...flatgen()];
}

var flatArray = flatten([[1, [4]],[2],[3]]);
console.log(flatArray);

Basically I'm creating a generator that loops over the original input array, if it finds an array it uses the yield* operator in combination with recursion to continually flatten the internal arrays. If the item is not an array it just yields the single item. Then using the ES6 Spread operator (aka splat operator) I flatten out the generator into a new array instance.

I haven't tested the performance of this, but I figure it is a nice simple example of using generators and the yield* operator.

But again, I was just goofing so I'm sure there are more performant ways to do this.

just the best solution without lodash

let flatten = arr => [].concat.apply([], arr.map(item => Array.isArray(item) ? flatten(item) : item))

I would rather transform the whole array, as-is, to a string, but unlike other answers, would do that using JSON.stringify and not use the toString() method, which produce an unwanted result.

With that JSON.stringify output, all that's left is to remove all brackets, wrap the result with start & ending brackets yet again, and serve the result with JSON.parse which brings the string back to "life".

• Can handle infinite nested arrays without any speed costs.
• Can rightly handle Array items which are strings containing commas.

var arr = ["abc",[[[6]]],["3,4"],"2"];

var s = "[" + JSON.stringify(arr).replace(/\[|]/g,'') +"]";
var flattened = JSON.parse(s);

console.log(flattened)

• Only for multidimensional Array of Strings/Numbers (not Objects)

Ways for making flatten array

• using Es6 flat()
• using Es6 reduce()
• using recursion
• using string manipulation

[1,[2,[3,[4,[5,[6,7],8],9],10]]] - [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

// using Es6 flat() 
let arr = [1,[2,[3,[4,[5,[6,7],8],9],10]]]
console.log(arr.flat(Infinity))

// using Es6 reduce()
let flatIt = (array) => array.reduce(
  (x, y) => x.concat(Array.isArray(y) ? flatIt(y) : y), []
)
console.log(flatIt(arr))

// using recursion
function myFlat(array) {
  let flat = [].concat(...array);
  return flat.some(Array.isArray) ? myFlat(flat) : flat;
}
console.log(myFlat(arr));

// using string manipulation
let strArr = arr.toString().split(','); 
for(let i=0;i<strArr.length;i++)
  strArr[i]=parseInt(strArr[i]);

console.log(strArr)

I think array.flat(Infinity) is a perfect solution. But flat function is a relatively new function and may not run in older versions of browsers. We can use recursive function for solving this.

const arr = ["A", ["B", [["B11", "B12", ["B131", "B132"]], "B2"]], "C", ["D", "E", "F", ["G", "H", "I"]]]
const flatArray = (arr) => {
    const res = []
    for (const item of arr) {
        if (Array.isArray(item)) {
            const subRes = flatArray(item)
            res.push(...subRes)
        } else {
            res.push(item)
        }
    }

    return res
}

console.log(flatArray(arr))

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