Where can I see deno downloaded packages?

You can use deno info to get the cache directory of remote modules.

Sample output (Windows 10):

> deno info
DENO_DIR location: "C:\\Users\\ford\\AppData\\Local\\deno"
Remote modules cache: "C:\\Users\\ford\\AppData\\Local\\deno\\deps"
TypeScript compiler cache: "C:\\Users\\ford\\AppData\\Local\\deno\\gen"

To get info about a single (remote) module:

deno info --unstable https://deno.land/std/fs/mod.ts # --unstable needed as of 1.0.3

Sample output:

local: C:\Users\ford\AppData\Local\deno\deps\https\deno.land\434fe4a7...8f300799a073e0
type: TypeScript
compiled: C:\Users\ford\AppData\Local\deno\gen\https\deno.land\std\fs\mod.ts.js
map: C:\Users\ford\AppData\Local\deno\gen\https\deno.land\std\fs\mod.ts.js.map 
deps:
https://deno.land/std/fs/mod.ts
  ├─┬ https://deno.land/std/fs/empty_dir.ts
  ...

local: Local path of source file (TS or JS)
compiled: Local path of compiled source code (.js, for TS only)
map: Local path of source map (TS only)
deps: Dependency tree of source file

The imports are cached in $DENO_DIR

From the docs:

Deno caches remote imports in a special directory specified by the $DENO_DIR environmental variable. It defaults to the system's cache directory if $DENO_DIR is not specified. The next time you run the program, no downloads will be made. If the program hasn't changed, it won't be recompiled either. The default directory is:

• On Linux/Redox: $XDG_CACHE_HOME/deno or $HOME/.cache/deno
• On Windows: %LOCALAPPDATA%/deno (%LOCALAPPDATA% = FOLDERID_LocalAppData)
• On macOS: $HOME/Library/Caches/deno If something fails, it falls back to $HOME/.deno

Relying on external servers is convenient for development but brittle in production. Production software should always bundle its dependencies. In Deno this is done by checking the $DENO_DIR into your source control system, and specifying that path as the $DENO_DIR environmental variable at runtime.

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what is the deno command to install all the dependencies mentioned in dep.ts file

To install just import dep.ts in one of your files and run:

deno run index.js