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

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.

This can also work.

let arr = [ [1,2,3], [2,3,4] ];
console.log([].concat(...arr))

Or for older browsers,

[].concat.apply([], arr);

Array.flat is not supported by 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 and eventually returned.

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;
}

As a Polyfill, extending Array.prototype if you prefer the arr.flat() syntax:

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;
        }
    });
}