es-shimsstring.prototype.towellformed 
An ESnext spec-compliant String.prototype.toWellFormed shim/polyfill/replacement that works as far down as ES3.
This package implements the es-shim API interface. It works in an ES3-supported environment and complies with the expected ES2024 spec.
Because String.prototype.toWellFormed depends on a receiver (the this value), the main export takes the string to operate on as the first argument.
Getting started
npm install --save string.prototype.towellformed
Usage/Examples
var toWellFormed = require('string.prototype.towellformed');
var assert = require('assert');
var leadingPoo = '\uD83D';
var trailingPoo = '\uDCA9';
var wholePoo = leadingPoo + trailingPoo;
var replacementChar = '\uFFFD';
assert.equal(toWellFormed(wholePoo), wholePoo);
assert.equal(toWellFormed(leadingPoo), replacementChar);
assert.equal(toWellFormed(trailingPoo), replacementChar);
var toWellFormed = require('string.prototype.towellformed');
var assert = require('assert');
/* when String#toWellFormed is not present */
delete String.prototype.toWellFormed;
var shimmed = toWellFormed.shim();
assert.equal(shimmed, toWellFormed.getPolyfill());
assert.deepEqual(wholePoo.toWellFormed(), toWellFormed(wholePoo));
assert.deepEqual(leadingPoo.toWellFormed(), toWellFormed(leadingPoo));
assert.deepEqual(trailingPoo.toWellFormed(), toWellFormed(trailingPoo));
var toWellFormed = require('string.prototype.towellformed');
var assert = require('assert');
/* when String#at is present */
var shimmed = toWellFormed.shim();
assert.equal(shimmed, String.prototype.toWellFormed);
assert.deepEqual(wholePoo.toWellFormed(), toWellFormed(wholePoo));
assert.deepEqual(leadingPoo.toWellFormed(), toWellFormed(leadingPoo));
assert.deepEqual(trailingPoo.toWellFormed(), toWellFormed(trailingPoo));
Tests
Simply clone the repo, npm install, and run npm test