You can use btoa() and atob(). btoa() is like base64_encode() and atob() like base64_decode().
Here is an example:
btoa('Some text'); // U29tZSB0ZXh0
atob('U29tZSB0ZXh0'); // Some text
Keep in mind that this is not a secure way to keep secrets. Base64 is a binary-to-text encoding scheme that represents binary data in an ASCII string format by translating it into a radix-64 representation.