An ES2019 spec-compliant Array.prototype.flat shim/polyfill/replacement that works as far down as ES3.
Array.prototype.flat
^1.0.8
^1.2.1
^1.23.5
^1.0.2
^2.5.1
^21.1.1
^2.5.0
^1.1.1
^0.1.13
=8.8.0
^0.0.19
^1.1.2
^1.0.1
^2.0.1
^0.3.1
^10.3.2
^1.13.3
^2.0.0
^5.9.0