zs-extract/lib/extract.mjs

import vm from 'vm';
import url from 'url';
import fetch from 'node-fetch';
import { WINDOW } from "./data.mjs";

/**
 * The default request implementation.
 *
 * @param options Options object.
 * @param cb Callback function.
 */
function request(options, cb) {
  let response = {
    statusCode: 0,
    headers: {}
  };
  const {
    encoding
  } = options;
  (async () => {
    const res = await fetch(options.url, {
      method: options.method || 'GET',
      headers: {
        'User-Agent': '-',
        ...(options.headers || {})
      },
      compress: !!options.gzip
    });
    const {
      status,
      headers
    } = res;
    const headersRaw = headers.raw();
    const headersObject = {};

    for (const p of Object.keys(headersRaw)) {
      headersObject[p] = headersRaw[p].join(', ');
    }

    response = {
      statusCode: status,
      headers: headersObject
    };
    const data = await res.buffer();
    return encoding === null ? data : data.toString(encoding);
  })().then(data => {
    cb(null, response, data);
  }, err => {
    cb(err, response, null);
  });
}
/**
 * A request promise wrapper.
 *
 * @param req Request function.
 * @param options Request options.
 * @returns Request response and body.
 */


async function requestP(req, options) {
  const r = await new Promise((resolve, reject) => {
    req(options, (error, response, body) => {
      if (error) {
        reject(error);
        return;
      }

      resolve({
        response,
        body
      });
    });
  });
  return r;
}
/**
 * Create a VM sandbox safe from context leakage.
 *
 * @returns Methods to run code in the VM sandbox.
 */


function createSandbox() {
  // Create a context with which to run code in.
  // Creating the object with a null prototype is very important.
  // Prevents host variables from leaking into the sanbox.
  const ctxObj = Object.create(null);

  if (ctxObj.toString) {
    throw new Error('Failed to create object without prototype');
  }

  const ctx = vm.createContext(ctxObj);
  return {
    /**
     * Run code, no return.
     *
     * @param code Code string.
     * @param opts VM options.
     */
    run: (code, opts) => {
      try {
        vm.runInContext(code, ctx, opts);
      } catch (err) {// Do nothing.
      }
    },

    /**
     * Run code, return data.
     *
     * @param data The data to get.
     * @param opts VM options.
     * @returns Data object.
     */
    data: (data, opts) => {
      const body = Object.entries(data).map(a => `${JSON.stringify(a[0])}:${a[1]}`).join(',');
      const script = `(""+JSON.stringify({${body}}))`;
      let r = null;

      try {
        // Force return value string with concatenation, NOT casting.
        // This prevents any funny business from sandboxed code.
        r = JSON.parse( // eslint-disable-next-line
        '' + vm.runInContext(script, ctx, opts));
      } catch (err) {// Do nothing.
      }

      return r;
    }
  };
}
/**
 * Code to create window.
 *
 * @param body HTML body.
 * @returns JavaScript code.
 */


function codeWindow(body) {
  return `(${WINDOW})(this,${JSON.stringify(body)})`;
}
/**
 * Extract file info from a URL.
 *
 * @param uri The URI to extract info from.
 * @param req Optional custom request function or null.
 * @returns File info.
 */


export async function extract(uri, req = null) {
  const requester = req || request;
  const {
    response,
    body
  } = await requestP(requester, {
    url: uri,
    gzip: true
  });
  const {
    statusCode
  } = response;

  if (statusCode !== 200) {
    throw new Error(`Invalid status code: ${statusCode}`);
  }

  const bodyType = typeof body;

  if (bodyType !== 'string') {
    throw new Error(`Invalid body type: ${bodyType}`);
  }

  const sandbox = createSandbox();
  const timeout = 1000; // Setup environment.

  sandbox.run(codeWindow(body.toString()), {}); // Extract info from environment.

  const info = sandbox.data({
    scripts: '(function(i,r,l){' + 'while(++i<l.length){' + 'r.push(l[i].textContent)' + '}' + 'return r' + '})(-1,[],document.getElementsByTagName("script"))'
  }, {
    timeout
  });

  if (!info) {
    // Should not be possible.
    throw new Error('Internal error');
  } // Run the scripts that modify the download button.


  for (const script of info.scripts) {
    if (script.includes('dlbutton')) {
      sandbox.run(script, {
        timeout
      });
    }
  } // Extract info about environment.


  const result = sandbox.data({
    dlbutton: 'document.getElementById("dlbutton").href'
  }, {
    timeout
  }); // Check result.

  if (!result || !result.dlbutton) {
    throw new Error('Failed to extract info');
  } // Parse download link and file name.


  const u = new url.URL(result.dlbutton, uri);
  return {
    download: u.href,
    filename: decodeURI(u.pathname.split('/').pop() || '') || null
  };
}
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