If you are talking about gzip data compression, then that is handled at a much lower level than javascript as any browser element can use gzip compression. In that case, the document is automatically decompressed before being used by the browser. There are many, many gzip libraries available. This would not be exposed via a javascript API because there is no need - it's seamless and automatic in the browser. There is no need to do manually gzip decompress in a javascript app.
If you are talking about javascript minification where javascript files are reduced in size by doing variable substitution (using shorter variable names), removing whitespace, etc... then the browser does not need to "decompress" minified files and does not do so. They are still legal javascript files and it runs them as they are. Thus, there is no API for unminifying because the browser doesn't do it. Third party apps have been built to unminify, but this is mostly to make code more readable to humans - it is not needed or done by the browser.
Minification and compression can be used together where a JS file is first minified and can then be compressed by the web server during transfer to the browser. In this case, the browser only sees the compression and does it's automatic decompression as the file is received. It ignores the minification because the file is still legal and runnable javascript even after being minified. The browser does not need to unminify it and does not.