btoa() (binary-to-ASCII) encodes a string to Base64 — a text-safe representation using only ASCII characters. Used for HTTP headers, embedding images as data URLs, and Basic Auth credentials. Only accepts Latin-1 characters; for Unicode strings (Cyrillic, emoji) pre-process with encodeURIComponent.

Originally browser-specific, standardized in HTML5 (2014). Available as a global in Node.js since v16. Supported in all modern browsers and IE10+.

• For Unicode strings use: btoa(unescape(encodeURIComponent(str)))
• Base64 increases data size by ~33%
• btoa() is not encryption — Base64 is trivially reversible with atob().