base64 encoder and decoder

base64.ts

function base64ToBytes(base64: string): Uint8Array {
  const binString = atob(base64);
  return Uint8Array.from(binString, (m) => m.codePointAt(0) || 0);
}

function bytesToBase64(bytes: Uint8Array): string {
  const binString = String.fromCodePoint(...bytes);
  return btoa(binString);
}

function ensureWellFormed(str: string | Uint8Array): void {
  if (typeof str === "string" && !isWellFormed(str)) {
    throw new Error("Cannot process a string with lone surrogates");
  }
}

function isWellFormed(str: string) {
  /// @ts-expect-error Missing type definition
  // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/isWellFormed
  if (typeof (str.isWellFormed) != "undefined") {
    /// @ts-expect-error Missing type definition
    return str.isWellFormed();
  } else {
    try {
      encodeURIComponent(str);
      return true;
    } catch (error) {
      return false;
    }
  }
}

const encoder = new TextEncoder();

export function validateBinaryLike(source: unknown): Uint8Array {
  if (typeof source === "string") {
    return encoder.encode(source);
  } else if (source instanceof Uint8Array) {
    return source;
  } else if (source instanceof ArrayBuffer) {
    return new Uint8Array(source);
  }
  throw new TypeError(
    `The input must be a Uint8Array, a string, or an ArrayBuffer. Received a value of the type ${typeof source}
    }.`,
  );
}

export const base64 = {
  encode(binaryData: Uint8Array | ArrayBuffer | string): string {
    const bytes = validateBinaryLike(binaryData);
    ensureWellFormed(bytes);
    return bytesToBase64(bytes);
  },
  decode(value: string): Uint8Array {
    ensureWellFormed(value);
    return base64ToBytes(value);
  },
};

export const base64url = {
  encode(binaryData: Uint8Array | ArrayBuffer | string): string {
    const bytes = validateBinaryLike(binaryData);
    ensureWellFormed(bytes);
    return bytesToBase64(bytes)
      .replace(/\+/g, "-")
      .replace(/\//g, "_")
      .replace(/=/g, "");
  },
  decode(value: string): Uint8Array {
    ensureWellFormed(value);
    return base64ToBytes(
      value
        .replace(/-/g, "+")
        .replace(/_/g, "/")
        .padEnd((value.length + 3) & ~3, "="),
    );
  },
};