.flat() is not a function, what's wrong?

Question

The following code

function steamrollArray(arr) {
  // I'm a steamroller, baby
  return arr.flat();
}

steamrollArray([1, [2], [3, [[4]]]]);

returns

arr.flat is not a function

I tried it in Firefox and Chrome v67 and the same result has happened.

What's wrong?

Answer 1

The flat method is not yet implemented in common browsers (only Chrome v69, Firefox Nightly and Opera 56). It’s an experimental feature. Therefore you cannot use it yet.

You may want to have your own flat function instead:

Object.defineProperty(Array.prototype, 'flat', {
    value: function(depth = 1) {
      return this.reduce(function (flat, toFlatten) {
        return flat.concat((Array.isArray(toFlatten) && (depth>1)) ? toFlatten.flat(depth-1) : toFlatten);
      }, []);
    }
});

console.log(
  [1, [2], [3, [[4]]]].flat(2)
);

The code was taken from here by Noah Freitas originally implemented to flatten the array with no depth specified.

Answer 2

Array.flat is not supported in your browser. Below are two ways to implement it.

As a function: The depth variable specifies how deep the input array structure should be flattened (defaults to 1; use Infinity to go as deep as it gets) while the stack is the flattened array, passed by reference on recursive calls.

function flat(input, depth = 1, stack = [])
{
    for (let item of input)
    {
        if (item instanceof Array && depth > 0)
        {
            flat(item, depth - 1, stack);
        }
        else {
            stack.push(item);
        }
    }

    return stack;
}

Polyfill, extending Array.prototype:

if (!Array.prototype.flat)
{
    Object.defineProperty(Array.prototype, 'flat',
    {
        value: function(depth = 1, stack = [])
        {
            for (let item of this)
            {
                if (item instanceof Array && depth > 0)
                {
                    item.flat(depth - 1, stack);
                }
                else {
                    stack.push(item);
                }
            }

            return stack;
        }
    });
}

Answer 3

Not sure if it is a valid answer however in my attemp to flat an array I employed the destructuring_assignment introduced in ES6.

// typeScriptArray:Array<Object> = new Array<Object>();
let concatArray = [];

let firstArray = [1,2,3];
let secondArray = [2,3,4];

concatArray.push(...firstArray);
concatArray.push(...secondArray);

console.log(concatArray);

It works like a charm even though I'm not sure if any broswer compatibily issues may arise.