string length is broken

endsWithTest.js

const loEndsWith = require('lodash.endsWith')

console.log('naive:', endsWith('asdsds😀', '?'))
console.log('naive:', endsWith('asdsds😀?', '?'))
console.log('naive:', endsWith('asdsds😀', '😀'))

console.log('mozilla polyfill:', mozEndsWith('asdsds😀', '?'))
console.log('mozilla polyfill:', mozEndsWith('asdsds😀?', '?'))
console.log('mozilla polyfill:', mozEndsWith('asdsds😀', '😀'))

console.log('lodash polyfill:', loEndsWith('asdsds😀', '?'))
console.log('lodash polyfill:', loEndsWith('asdsds😀?', '?'))
console.log('lodash polyfill:', loEndsWith('asdsds😀', '😀'))

function endsWith(a, b) {
	return a[a.length - 1] === b
}

function mozEndsWith(subjectString, searchString, position) {	
      
      if (typeof position !== 'number' || !isFinite(position) || Math.floor(position) !== position || position > subjectString.length) {
        position = subjectString.length;
      }
      position -= searchString.length;
      var lastIndex = subjectString.lastIndexOf(searchString, position);
      return lastIndex !== -1 && lastIndex === position;
 
}