Transcode Webm to Audio file (fluent-ffmpeg) - node.js

Question

I have a video file and its properties are as the following when I check via the ffprobe command;
My purpose is to take the audio file only and its size should not be >100mb.
Is there any proper way of doing it in an NPM project?
ffprobe 2.webm
ffprobe version N-86175-g64ea4d1 Copyright (c) 2007-2017 the FFmpeg
developers built with gcc 6.3.0 (GCC) configuration: --enable-gpl --
nable-version3 --enable-cuda --enable-cuvid --enable-d3d11va --enable-dxva2
--
nable-libmfx --enable-nvenc --enable-avisynth --enable-bzlib --enable-
ontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --
nable-libbluray --enable-libbs2b --enable-libcaca --enable-libfreetype --
nable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-
ibmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-
ibopenh264 --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-
ibsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-
ibtwolame --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --
nable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-
ibx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-lzma --
nable-zlib libavutil 55. 63.100 / 55. 63.100 libavcodec 57.
96.101
7. 96.101
libavformat 57. 72.101 / 57. 72.101
libavdevice 57. 7.100 / 57. 7.100
libavfilter 6. 90.100 / 6. 90.100
libswscale 4. 7.101 / 4. 7.101
libswresample 2. 8.100 / 2. 8.100
libpostproc 54. 6.100 / 54. 6.100
Input #0, matroska,webm, from '2.webm':
Metadata:
encoder : libwebm-0.2.1.0
creation_time : 2017-05-04T14:59:01.639000Z
Duration: 00:02:35.16, start: 0.000000, bitrate: 32 kb/s
Stream #0:0(eng): Audio: opus, 48000 Hz, mono, fltp (default)
Stream #0:1(eng): Video: vp8, yuv420p(progressive), 640x480, SAR 1:1 DAR
4:3, 14.99 tbr, 1k tbn, 1k tbc (default)

Answer 1

Using fluent-ffmpeg's API
You could use the noVideo() method which tells ffmpeg to produce output with "no video".
ffmpeg('/path/to/file.avi').noVideo();
Using ffmpeg's CLI API
I'm not sure of fluent-ffmpeg, but using ffmpeg, you would use the -map option to dictate what streams you want from you inputs in your output. So for your example, the command to get an MP3 file that contains only the first audio stream from your input would be:
ffmpeg -i 2.webm -map 0:a:0 out.mp3
Alternatively, you could tell ffmpeg to not include video streams using the -vn option; but I like to be explicit. Your command would then be:
ffmpeg -i 2.webm -vn out.mp3
You can also use the -acodec option to dictate what encoder you want to use such as AAC or whatever. ffmpeg will use the best guess by looking at the extension you gave your output if you don't give it enough options.
As to limiting filesize, I'm not sure of a clean way to do that, but this question on the Video Production StackExchange may help: How to limit file size with ffmpeg?. It suggests playing with your bitrate until you get the size you want. I would assume you could calculate correct bitrate ahead of time.