formats.exposed/public/gif/constants.js

// @ts-check

/** @enum {string} */
const types = [
  "root",

  "header",
  "headerSignature",
  "headerVersion",

  "logicalScreenDescriptor",
  "logicalScreenWidth",
  "logicalScreenHeight",
  "logicalScreenDescriptorPackedFields",
  "logicalScreenBackgroundColorIndex",
  "logicalScreenPixelAspectRatio",

  "globalColorTable",
  "globalColorTableColor",

  "imageSection",
  "imageDescriptor",
  "imageSeparator",
  "imageDescriptorLeftPosition",
  "imageDescriptorTopPosition",
  "imageDescriptorWidth",
  "imageDescriptorHeight",
  "imageDescriptorPackedFields",

  "localColorTable",
  "localColorTableColor",

  "imageData",
  "imageDataLzwMinimumCodeSize",
  "imageDataSubBlock",
  "imageDataSubBlockSize",
  "imageDataSubBlockData",
  "imageDataTerminator",

  "extensionBlockIntroducer",
  "extensionBlockSize",

  "unknownExtensionBlock",
  "unknownExtensionBlockLabel",
  "unknownExtensionBlockData",

  "gifTerminator",
].reduce((result, id) => {
  result[id] = id;
  return result;
}, Object.create(null));

// TODO: Revert this and just export the thing
export const GifNodeType = new Proxy(types, {
  get(target, prop) {
    const result = target[prop];
    if (!result) {
      throw new Error(`cannot get ${String(prop)}`);
    }
    return result;
  },
});