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]

You could use Array#reduce with Array#concat.

var arr = [[{ c: 1 }, { d: 2 }], [{ c: 2 }, { d: 3 }]],
    result = arr.reduce((r, a) => r.concat(a), []);

console.log(result)

ES5

var arr = [[{ c: 1 }, { d: 2 }], [{ c: 2 }, { d: 3 }]],
    result = arr.reduce(function (r, a) { return r.concat(a); }, []);

console.log(result)

Use reduce and concat

var output = arr.reduce( (a, c) => a.concat(c), []); //assuming arr is the input array

Edit

As @TJ mentioned in his comment, that above solution will create some intermediate arrays along the way, you can try (concat without spread)

var output = [].concat.apply([], arr);

or

var output = Array.prototype.concat.apply([], arr); //avoiding usage of another unnecessary array `[]`

Assuming your input data is an object containing several arrays, like so:

var val = {
    _id: ["5412fc1bd123cf7016674a92", "5412cf270e9ca9b517b43ca3"],
    _id2: ["5412cf5a6cc4f4bc151fd220"]
};

You can get the desired array structure pretty easily using concat:

var flat = [];
for (var key in val) {
    flat = flat.concat(val[key]);
}
console.log(flat);

Output:

[ '5412fc1bd123cf7016674a92',
  '5412cf270e9ca9b517b43ca3',
  '5412cf5a6cc4f4bc151fd220' ]

Try

var z = x.map( (s,i) => ({ x : x[i], y : y[i], w : w[i] }) );

Explanation

• iterate x using map
• for each index of x, return an object having keys as x, y and z with values from their respective index.

Demo

var x = ['a', 'b', 'c', 'd'];
var y = ['e', 'f', 'g', 'h'];
var w = ['i', 'j', 'k', 'l'];

var z = x.map((s, i) => ({
  x: x[i],
  y: y[i],
  w: w[i]
}));

console.log(z);

If you don't need the months, but just want to flatten it, flatMap and flat() it :

const arr =  [{JUNE: [{"iD": "67854", "event": " closed", "title": "test", "startDate": "2024-06-20", "endDate": "2024-07-25" } ] }, {MAY: [{"ID": "908765", "event": "closed", "title": "test", "startDate": "2024-05-21", "endDate": "2024-06-27" }, {ID: "12345", event: "open", title: "test123", startDate: "2024-05-21", endDate: "2024-06-27" } ] } ];

const res = arr.flatMap(c => Object.values(c).flat());

console.log(res)

If you want to add the month, thenreduce the array and flatMap each Object.keys with an deeper map for all the events.

const arr =  [{JUNE: [{"iD": "67854", "event": " closed", "title": "test", "startDate": "2024-06-20", "endDate": "2024-07-25" } ] }, {MAY: [{"ID": "908765", "event": "closed", "title": "test", "startDate": "2024-05-21", "endDate": "2024-06-27" }, {ID: "12345", event: "open", title: "test123", startDate: "2024-05-21", endDate: "2024-06-27" } ] } ];

const res = arr.reduce((p, c) => ([ ...p, 
    ...(Object.keys(c)
        .flatMap(month => c[month]
          .map(o => ({ ...o, month })) )) 
        ])
, []);

console.log(res)

Or as an one-liner":

const res = arr.reduce((p, c) => ([ ...p,...(Object.keys(c).flatMap(month => c[month].map(o => ({ ...o, month }))))]), []);

var arr = [{"1":"test1"},{"2":"test2"},{"3":"test3"}];
var newarr = arr.reduce(function(data,val){
  Object.assign(data[0],val);
  return data;
},[{}]);
console.log(newarr);