A mostly-complete pure Javascript re-implementation of webpmux.
Can load "simple" lossy/lossless images as well as animations.

npm install node-webpmux

const WebP = require('node-webpmux');
let img = new WebP.Image();
// Load an animation
await img.load('img.webp');
// Extract the (unprocessed) fourth frame
await img.demux('.', { frame: 3 });
// Replace the fourth frame with a new image from disk
await img.replaceFrame(3, 'different.webp'); // This preserves the existing frame settings
// Alternatively you can do
//   let frame = Image.generateFrame({ path: 'different.webp' });
//   img.frames[3] = frame;
// Which will completely replace the frame
// Save a new copy
await img.save({ path: 'newimg.webp' });
// Or alternatively, img.save() to save over the existing one

TYPE_LOSSY
TYPE_LOSSLESS
TYPE_EXTENDED
Constants for what type of image is loaded.

encodeResults: enum of values that set[Image/Frame]Data returns.

Image: The main class.

The width of the loaded image.

The height of the loaded image.

The type of image from the TYPE_* constants table.

A boolean flag for easily checking if the image is an animation.

A boolean flag for easily checking if the image has transparency in any way.

Returns the array of frames, if any, or undefined.
Note that while the frames themselves are read/write, you shouldn't modify them.

The number of frames in the image's animation, or 0 if it's not an animation.

Direct access to the raw animation data (see below in the Layout for internal Image data section).

A Buffer containing the raw ICCP data stored in the image, or undefined if there isn't any.

A Buffer containing the raw EXIF data stored in the image, or undefined if there isn't any.

A Buffer containing the raw XMP data stored in the image, or undefined if there isn't any.

Calls Image.initLib(). This member function is no longer particularly useful and is kept for convenience.

If source is a string, it tries to load that as a path to a WebP image.
If source is a buffer, it tries to load the contents of the buffer as a WebP image.

Sets the image up for being an animation.

Dump the individual, unprocessed WebP frames to a directory.

• path: The directory to dump the frames to, if desired.
• buffers: Return the frames as an array of Buffers instead of dumping to a path.
• prefix: What to prefix the frame names with. Default is the file name of the original image (without .webp). Format is <prefix>_<frame number>.webp.
• frame: What frame to dump. Defaults to -1, which has it dump all available frames. Overrides start/end.
• start: The first frame to dump. Defaults to the first frame.
• end: The last frame to dump. Defaults to the last frame.

Replaces a frame in the animation with another image from source. All other frame settings are preserved.

• frameIndex: Which frame to replace. Frame indexes are 0-based.
• source: If this is a string, the frame is loaded from disk. If this is a Buffer, the frame is loaded from there.

Save the image to path. Options are described below in the Options for saving section.
If path is null, this will save the image to a Buffer and return it.

Get the raw RGBA pixel data for the image.
Returns a Buffer in the format [ r, g, b, a, r, g, b, a, ... ]. Values are range 0 - 255.
Use this for non-animations.
On error, this returns a Buffer full of 0s.

Encode buf as a new WebP using the provided settings and replace the image pixel data with it.
This preserves EXIF/ICCP/XMP if present.
Use this for non-animations.

• buffer: A Buffer object with the raw pixels in RGBA order.
  Options:
• width/height
  If either are > 0, override the existing width and/or height with this value.
  Use this if the pixel data in buf has different dimensions than the original image.
• preset: What image preset to use, if any.
  Range is 0 - 5
  Default is 0 (DEFAULT).
  An enum of constants can be found under WebP.presets
• quality: What quality to set.
  Range is 0 - 100.
  Default is 75.
• exact: Preserve data in transparent pixels.
  Defaults to false, which means transparent pixels may be modified to help with compression.
• lossless: Save the data as a lossy/lossless image.
  Range is 0 - 9.
  Default is 0 (lossy).
  Higher values will result in smaller files, but requires more processing time.
• method: Compression method to use.
  Range is 0 - 6.
  Default is 4.
  Higher values will result in smaller files, but requires more processing time.
• advanced: Access to more advanced encoding settings offered by libwebp
  
• • imageHint: Hint for what type of image it is (only used for lossless encoding for now, according to libwebp spec).
    Range is 0 - 3.
    Default is 0 (DEFAULT).
    An enum of constants can be found under WebP.hints
• • targetSize: Specifies the desired target size in bytes.
    Default is 0 (no target).
    Takes precedence over the method parameter.
• • targetPSNR: Specifies the minimum distortion to try to achieve.
    Default is 0 (no target).
    Takes precedence over the targetSize parameter.
• • segments: Maximum number of segments to use.
    Range is 1 - 4.
    Default is 4.
• • snsStrength: Spacial Noise Shaping.
    Range is 0 - 100.
    Default is 50.
• • filterStrength
    Range is 0 - 100.
    Default is 0 (off).
• • filterSharpness
    Range is 0 - 7, with 7 being the least sharp.
    Default is 0 (off).
• • filterType
    Range is 0 - 1.
    Default is 1.
    0 is simple; 1 is strong.
    Only used if filterStrength > 0 or autoFilter > 0.
• • autoFilter: Auto-adjust the filter's strength.
    Range is 0 - 1.
    Default is 0 (off).
• • alphaCompression: Algorithm for encoding the alpha plane.
    Range is 0 - 1.
    Default is 1 (Lossless).
    0 is off; 1 is lossless.
• • alphaFiltering: Predictive filtering method for alpha place.
    Range is 0 - 2.
    Default is 1 (Fast).
    0 is none; 1 is fast; 2 is best
• • alphaQuality
    Range is 0 - 100.
    Default is 100.
• • pass: Number of entropy-analysis passes.
    Range is 1 - 10.
    Default is 1.
• • showCompressed: Export the compressed picture back.
    Range is 0 - 1.
    Default is 0 (don't).
    In-loop filtering is not applied.
• • preprocessing: Preprocessing filter.
    Range is 0 - 2.
    Default is 0 (None).
    0 is none; 1 is segment-smooth; 2 is pseudo-random dithering.
• • partitions: log2(number of token partitions).
    Range is 0 - 3.
    Default is 0.
    Higher values result in harder progressive decoding.
• • partitionLimit: Quality degredation allowed to fit the 512k limit on prediction modes coding.
    Range is 0 - 100.
    Default is 0.
• • emulateJpegSize: Compression parameters are remapped to better mat the expected output size from JPEG compression.
    Range is 0 - 1.
    Default is 0 (Off).
    Generally, the output size will be smaller but the degredation will be lower.
• • threadLevel: Try to use multi-threaded encoding.
    Default is 0 (Off).
    NOTE: Currently the WebAssembly is NOT compiled with support for threads, so this option does nothing.
    NodeJS doesn't support threads in WebAssembly without an experimental flag, and my testing with it didn't appear to use threads regardless.
• • lowMemory: Reduce memory usage but increase CPU use.
    Range is 0 - 1.
    Default is 0 (Off).
• • nearLossless: Near lossless encoding.
    Range is 0 - 100.
    Default is 100 (off).
    0 is max loss, 100 is off.
• • useDeltaPalette: Reserved for future lossless feature.
    Range is 0 - 0.
    Default is 0 (Off).
    Setting this will do nothing, as it's forced back to 0.
• • useSharpYUV: Use sharp (and slow) RGB->YUV conversion.
    Range is 0 - 1.
    Default is 0 (Off).
• • qMin: Minimum permissible quality factor.
    Range is 0 - 100.
    Default is 0.
• • qMax: Maximum permissible quality factor.
    Range is 0 - 100.
    Default is 100.

If lossless is set above 0, then setting quality or method is discouraged as they will override settings in the lossless preset.
Return value can be checked against the values in WebP.encodeResults.

Get the raw RGBA pixel data for a specific frame.
Use this for animations.

• frameIndex: Which frame to get. Frame indexes are 0-based.
  Otherwise identical to .getImageData()

Encode buffer as a new WebP using the provided settings and replace an existing frame's pixel data with it.
Use this for animations.

• frameIndex: Which frame to get. Frame indexes are 0-based.
  Otherwise identical to .setImageData().

Initialize the internal library used for [get/set]ImageData and [get/set]FrameData described above.
There is no need to call this unless you plan to use one of those 4 functions.

Use the contents of webp and return a new Image using them.
Mainly useful for passing Image into a Web Worker or NodeJS Worker and converting the passed object back into an Image instance.
Such an approach can be used to greatly speed up saving of animations or multiple images as libwebp is not multi-threaded beyond saving the alpha layer of lossy images.

Save the options using Image.getEmptyImage().
Works the same as .save() otherwise.
Can be used to create an animation from scratch by passing frames in options.
  Example: Image.save('animation.webp', { frames: ... }) for saving to file   OR   Example: Image.save(null, { frames: ... }) for saving to Buffer

Returns a basic, lossy 1x1 black image with no alpha or metadata.
Useful if you need to create a WebP from scratch, such as when converting from PNG.
.setImageData() would be used to change the canvas size/contents.
Set ext to true to force the image to be an extended type, if desired. This is mainly for use internally.

Generates enough of an anmf structure to be placed in .frames.
Note that, at the moment, only static images are supported in this function.

• path/buffer/img Only one of these can be present. path will load image data from file. buffer will load from the buffer. img will use an existing Image instance.
• x/y/delay/blend/dispose Explicitly set these properties. See the Options for saving section for what these do.

• exif/iccp/xmp
  Save or override EXIF/ICCP/XMP chunks.
  Pass true to save the existing ones, or pass a Buffer to replace them.
  Note that there is no verification whatsoever that the data passed is valid.

• width/height: Width/height of the image.
  Range 0 - 16777216.
  The product of width*height must NOT exceed (2 ** 32) - 1.
  Passing 0 to either flags it for being set automatically.
• bgColor: The background color of the animation.
  Format is [ r, g, b, a ].
  Defaults to [ 255, 255, 255, 255 ].
• loops: Number of times the animation loops.
  Range is 0 - 65535, with 0 being an infinite loop.
  Default is 0.
• x/y/delay/blend/dispose: Changes the default frame x/y position where a frame omits it (see below).
• • x/y defaults to 0.
• • delay defaults to 100.
• • blend defaults to true.
• • dispose defaults to false.
• • frames: An array of objects defining each frame of the animation with the following properties.
• • • x/y: x, y offset to place the frame within the animation.
      Range 0 - 16777215.
      Default is 0,0 (defined above).
• • • delay: Length of this frame in miliseconds.
      Range 0 - 16777215.
      Default is 100 (defined above).
      According to the documentation, delays <= 10ms are WebP implementation defined, and many tools/browsers/etc assign their own minimum-allowed delay.
• • • blend: Boolean flag for whether or not to use alpha blending when drawing the frame.
      Default is true (defined above).
• • • dispose: Boolean flag to control frame disposal method.
      true causes the background color to be drawn under the frame.
      false draws the new frame directly.
      Default is false (defined above).

[get/set]ImageData and [get/set]FrameData are powered by Google's official libwebp library obtained from the GitHub mirror.
Commit 89c5b91 was the latest at the time of compilation.
This library was compiled with Emscripten with the command emcc -O3 -s WASM=1 -s MODULARIZE -s EXPORTED_RUNTIME_METHODS='[cwrap]' -s ALLOW_MEMORY_GROWTH=1 -s EXPORT_NAME=LibWebP -DHAVE_CONFIG_H -I libwebp binding.cpp libwebp/src/{dec,dsp,demux,enc,mux,utils}/*.c libwebp/sharpyuv/*.c --bind -o libwebp.js.
binding.cpp is a shim I wrote to bridge the needed parts together and can be found in the libwebp/ directory. libwebp.js, found in the root, is the Javascript interface to it.

At present, the only options for encoding are setting the lossless preset, quality, method, and exact flag.
If access to other options is desired (see upstream libwebp/src/webp/encode.h, struct WebPConfig for settings), leave a feature request and I'll add it.
The upstream command line tool cwebp can be used to play with the features and see what you find useful.

{
  path, // The path loaded.
  loaded, // Boolean flag for if this object has an image loaded.
  data: { // The loaded data.
    type, // The type of image from the constants table.
    vp8: { // The lossy format image. Only if .type is TYPE_LOSSY or TYPE_EXTENDED.
      raw, // The raw, compressed image data from the VP8 chunk.
      width, height // The width/height, extracted from the VP8 image data.
    },
    vp8l: { // The lossless format image. Only if .type is TYPE_LOSSLESS or TYPE_EXTENDED.
      raw, // The raw, compressed image data from the VP8L chunk.
      alpha, // A flag for if this image has alpha data, extracted from the VP8L image data.
      width, height // The width/height, extracted from the VP8L image data.
    },
    extended: { // Only if .type is TYPE_EXTENDED.
      raw, // The raw data for the VP8X chunk.
      hasICCP, // Flag for if there's an ICC profile chunk defined.
      hasAlpha, // Flag for if any image/frame defined has alpha data.
      hasEXIF, // Flag for if there's an EXIF chunk defined.
      hasXMP, // Flag for if there's an XMP chunk defined.
      hasAnim, // Flag for if this image has an animation defined.
      width, height // Width/height of the image.
    },
    anim: {
      raw, // A Buffer containing the raw data for the ANIM chunk. Mainly for internal use.
      bgColor, // The background color in [ r, g, b, a ] format.
      loops, // The loop count.
      frames: [ // Array of frames
        { // The frame object definition
          raw, // The raw data for the ANMF chunk. Mainly for internal use.
          type, // The type of image this frame is, from the constants table.
          x, y, // The frame's x, y position.
          width, height, // The frame's width and height.
          delay, // The duration of the frame.
          blend, dispose, // The frame's blend/dispose flags.
          // Additionally, one or more of the following.
          vp8, // The raw, compressed WebP data for a lossy image. If present, there will be no `vp8l`.
          vp8l, // The raw, compressed WebP data for a lossless image. If present, there will be no `vp8` or `alph`.
          alph // The raw, compressed WebP data for an alpha map. Might be present if the image is lossy.
        },
        ...
      ]
    },
    alph: {
      raw // The raw alpha map chunk. Only likely to be here if .vp8 is also defined and .type is TYPE_EXTENDED.
    },
    iccp: {
      raw // The raw ICCP chunk, if defined.
    },
    exif: {
      raw // The raw EXIF chunk, if defined.
    },
    xmp: {
      raw // The raw XMP chunk, if defined.
    }
  }
}

Image.muxAnim and .muxAnim were merged into Image.save and .save respectively.

• Replace Image.muxAnim({ path, frames, width, height, bgColor, loops, delay, x, y, blend, dispose, exif, iccp, xmp })
• With Image.save(path, undefined, { frames, width, height, bgColor, loops, delay, x, y, blend, dispose, exif, iccp, xmp })
  
  
• Replace .muxAnim({ path, width, height, bgColor, loops, delay, x, y, blend, dispose, exif, iccp, xmp })
• With .save(path, { width, height, bgColor, loops, delay, x, y, blend, dispose, exif, iccp, xmp })

.anim.backgroundColor renamed to .anim.bgColor for brevity and consisteny.
.anim.loopCount renamed to .anim.loop for consistency.
.anim.frameCount and .frameCount were removed. Should use .anim.frames.length and .frames.length respectively instead.
.demuxAnim() was renamed to .demux()

Image.generateFrame()'s duration input renamed to delay

File and buffer codepaths have been merged.

• Replace .loadBuffer(buffer)
• With .load(buffer)
• Replace Image.loadBuffer(buffer)
• With Image.load(buffer)
  
  
• Replace .saveBuffer(settings)
• With .save(null, settings)
• Replace Image.saveBuffer(settings)
• With Image.save(null, settings)
• Note that it's specifically null here. This is because the default behavior of .save() is still saving to the path it was loaded from.
  
  
• Replace .demuxToBuffers({ setting, setting, ... })
• With .demux({ buffers: true, setting, setting, ... })
• Replace .demux(path, settings)
• With .demux({ path, setting, setting, ... })
  
  
• Replace .replaceFrameBuffer(frame, buffer)
• With .replaceFrame(frame, buffer)