polyfill.js

const RequireObjectCoercible = O => {
	if (O === null || typeof O === 'undefined') {
		throw new TypeError('"this" value must not be null or undefined');
	}
	return O;
};
const MAX_SAFE_INTEGER = Number.MAX_SAFE_INTEGER || Math.pow(2, 53) - 1;
const ToLength = argument => {
	const len = Number(argument);
	if (Number.isNaN(len) || len <= 0) { return 0; }
	if (len > MAX_SAFE_INTEGER) { return MAX_SAFE_INTEGER; }
	return len;
};

if (!String.prototype.padStart) {
	String.prototype.padStart = function padStart(maxLength, fillString = ' ') {
		const O = RequireObjectCoercible(this);
		const S = String(O);
		const intMaxLength = ToLength(maxLength);
		const stringLength = ToLength(S.length);
		if (intMaxLength <= stringLength) { return S; }
		let filler = typeof fillString === 'undefined' ? ' ' : String(fillString);
		if (filler === '') { return S; }
		const fillLen = intMaxLength - stringLength;
		while (filler.length < fillLen) {
			const fLen = filler.length;
			const remainingCodeUnits = fillLen - fLen;
			if (fLen > remainingCodeUnits) {
				filler += filler.slice(0, remainingCodeUnits);
			} else {
				filler += filler;
			}
		}
		const truncatedStringFiller = filler.slice(0, fillLen);
		return truncatedStringFiller + S;
	};
}

if (!String.prototype.padEnd) {
	String.prototype.padEnd = function padEnd(maxLength, fillString = ' ') {
		const O = RequireObjectCoercible(this);
		const S = String(O);
		const intMaxLength = ToLength(maxLength);
		const stringLength = ToLength(S.length);
		if (intMaxLength <= stringLength) { return S; }
		let filler = typeof fillString === 'undefined' ? ' ' : String(fillString);
		if (filler === '') { return S; }
		const fillLen = intMaxLength - stringLength;
		while (filler.length < fillLen) {
			const fLen = filler.length;
			const remainingCodeUnits = fillLen - fLen;
			if (fLen > remainingCodeUnits) {
				filler += filler.slice(0, remainingCodeUnits);
			} else {
				filler += filler;
			}
		}
		const truncatedStringFiller = filler.slice(0, fillLen);
		return S + truncatedStringFiller;
	};
}