ffmpeg unable to find a suitable output format for 'subs.srt'

I am trying to extract the subtitles

ffmpeg -i filename -map 0:s:0 subs.srt

but I get

ffmpeg version n3.1.2 Copyright (c) 2000-2016 the FFmpeg developers
  built with emcc (Emscripten gcc/clang-like replacement) 1.36.7 ()
  configuration: --cc=emcc --enable-cross-compile --target-os=none --arch=x86 --disable-runtime-cpudetect --disable-asm --disable-fast-unaligned --disable-pthreads --disable-w32threads --disable-os2threads --disable-debug --disable-stripping --disable-all --enable-ffmpeg --enable-avcodec --enable-avformat --enable-avutil --enable-swresample --enable-swscale --enable-avfilter --disable-network --disable-d3d11va --disable-dxva2 --disable-vaapi --disable-vda --disable-vdpau --enable-decoder=vp8 --enable-decoder=vp9 --enable-decoder=theora --enable-decoder=mpeg2video --enable-decoder=mpeg4 --enable-decoder=h264 --enable-decoder=hevc --enable-decoder=png --enable-decoder=mjpeg --enable-decoder=vorbis --enable-decoder=opus --enable-decoder=mp3 --enable-decoder=ac3 --enable-decoder=aac --enable-decoder=ass --enable-decoder=ssa --enable-decoder=srt --enable-decoder=webvtt --enable-demuxer=matroska --enable-demuxer=ogg --enable-demuxer=avi --enable-demuxer=mov --enable-demuxer=flv --enable-demuxer=mpegps --enable-demuxer=image2 --enable-demuxer=mp3 --enable-demuxer=concat --enable-protocol=file --enable-filter=aresample --enable-filter=scale --enable-filter=crop --enable-filter=overlay --disable-bzlib --disable-iconv --disable-libxcb --disable-lzma --disable-sdl --disable-securetransport --disable-xlib --disable-zlib --enable-encoder=libvpx_vp8 --enable-encoder=libopus --enable-encoder=mjpeg --enable-muxer=webm --enable-muxer=ogg --enable-muxer=null --enable-muxer=image2 --enable-filter=subtitles --enable-libass --enable-libopus --enable-libvpx --extra-cflags=-I../libvpx/dist/include --extra-ldflags=-L../libvpx/dist/lib
  libavutil      55. 28.100 / 55. 28.100
  libavcodec     57. 48.101 / 57. 48.101
  libavformat    57. 41.100 / 57. 41.100
  libavfilter     6. 47.100 /  6. 47.100
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  1.100 /  2.  1.100
[h264 @ 0x7d7510] Warning: not compiled with thread support, using thread emulation
[aac @ 0x7d81c0] Warning: not compiled with thread support, using thread emulation
[ssa @ 0x7d8e30] Warning: not compiled with thread support, using thread emulation
Input #0, matroska,webm, from 'censored filename.mkv':
  Metadata:
    encoder         : no_variable_data
    creation_time   : 1970-01-01 00:00:00
  Duration: 00:23:40.13, start: 0.000000, bitrate: 2789 kb/s
    Stream #0:0: Video: h264 (High), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
    Metadata:
      BPS             : 2658044
      BPS-eng         : 2658044
      DURATION        : 00:23:40.045000000
      DURATION-eng    : 00:23:40.045000000
      NUMBER_OF_FRAMES: 34047
      NUMBER_OF_FRAMES-eng: 34047
      NUMBER_OF_BYTES : 471817808
      NUMBER_OF_BYTES-eng: 471817808
      _STATISTICS_WRITING_APP: no_variable_data
      _STATISTICS_WRITING_APP-eng: no_variable_data
      _STATISTICS_WRITING_DATE_UTC: 1970-01-01 00:00:00
      _STATISTICS_WRITING_DATE_UTC-eng: 1970-01-01 00:00:00
      _STATISTICS_TAGS: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
      _STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
    Stream #0:1(jpn): Audio: aac (LC), 44100 Hz, stereo, fltp (default)
    Metadata:
      BPS             : 128000
      BPS-eng         : 128000
      DURATION        : 00:23:40.109000000
      DURATION-eng    : 00:23:40.109000000
      NUMBER_OF_FRAMES: 61159
      NUMBER_OF_FRAMES-eng: 61159
      NUMBER_OF_BYTES : 22721748
      NUMBER_OF_BYTES-eng: 22721748
      _STATISTICS_WRITING_APP: no_variable_data
      _STATISTICS_WRITING_APP-eng: no_variable_data
      _STATISTICS_WRITING_DATE_UTC: 1970-01-01 00:00:00
      _STATISTICS_WRITING_DATE_UTC-eng: 1970-01-01 00:00:00
      _STATISTICS_TAGS: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
      _STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
    Stream #0:2(eng): Subtitle: ass (default)
    Metadata:
      BPS             : 110
      BPS-eng         : 110
      DURATION        : 00:23:25.280000000
      DURATION-eng    : 00:23:25.280000000
      NUMBER_OF_FRAMES: 298
      NUMBER_OF_FRAMES-eng: 298
      NUMBER_OF_BYTES : 19407
      NUMBER_OF_BYTES-eng: 19407
      _STATISTICS_WRITING_APP: no_variable_data
      _STATISTICS_WRITING_APP-eng: no_variable_data
      _STATISTICS_WRITING_DATE_UTC: 1970-01-01 00:00:00
      _STATISTICS_WRITING_DATE_UTC-eng: 1970-01-01 00:00:00
      _STATISTICS_TAGS: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
      _STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
    Stream #0:3: Attachment: ttf
    Metadata:
      filename        : OpenSans-Semibold.ttf
      mimetype        : application/x-truetype-font
[NULL @ 0x9eac90] Unable to find a suitable output format for 'subs.srt'
subs.srt: Invalid argument

I am using javascript to do this but I want to make sure that there is no problem in how I pass the arguments first