Node.js process.mainModule Property

The process.mainModule property is an inbuilt application programming interface of the processing module which is used to get the main module. This is an alternative way to get require.main but unlike require.main, process.mainModule dynamically changes in runtime. Generally, we can assume those two modules are the same. 

Syntax:

process.mainModule

Return Value: This property returns an object that contains the reference of the main module. 

Below examples illustrate the use of process.mainModule property in Node.js: 

Note: This property has been deprecated.

Example 1:

javascript

// Node.js program to demonstrate the
// process.mainModule Property

// Include process module
const process = require('process');

// Printing process.mainModule property value
console.log(process.mainModule);

Output:

Module {
  id: '.',
  exports: {},
  parent: null,
  filename: 'C:\\nodejs\\g\\process\\mainmodule_1.js',
  loaded: false,
  children: [],
  paths:
   [ 'C:\\nodejs\\g\\process\\node_modules',
     'C:\\nodejs\\g\\node_modules',
     'C:\\nodejs\\node_modules',
     'C:\\node_modules' 
   ]
}

Example 2:

javascript

// Node.js program to demonstrate the
// process.mainModule Property

// Include process module
const process = require('process');

// Printing process.mainModule property value
var mainModule = process.mainModule;
for(mod in mainModule) {
    console.log(mod + ":" + mainModule[mod]);
}

Output:

id:.
exports:[object Object]
parent:null
filename:/home/cg/root/6720369/main.js
loaded:false
children:
paths:/home/cg/root/6720369/node_modules, /home/cg/root/node_modules,
 /home/cg/node_modules, /home/node_modules, /node_modules
load:function (filename) {
  debug('load %j for module %j', filename, this.id);

  assert(!this.loaded);
  this.filename = filename;
  this.paths = Module._nodeModulePaths(path.dirname(filename));

  var extension = path.extname(filename) || '.js';
  if (!Module._extensions[extension]) extension = '.js';
  Module._extensions[extension](this, filename);
  this.loaded = true;
}
require:function (path) {
  assert(path, 'missing path');
  assert(typeof path === 'string', 'path must be a string');
  return Module._load(path, this, /* isMain */ false);
}
_compile:function (content, filename) {
  // Remove shebang
  var contLen = content.length;
  if (contLen >= 2) {
    if (content.charCodeAt(0) === 35/*#*/ &&
        content.charCodeAt(1) === 33/*!*/) {
      if (contLen === 2) {
        // Exact match
        content = '';
      } else {
        // Find end of shebang line and slice it off
        var i = 2;
        for (; i < contLen; ++i) {
          var code = content.charCodeAt(i);
          if (code === 10/*\n*/ || code === 13/*\r*/)
            break;
        }
        if (i === contLen)
          content = '';
        else {
          // Note that this actually includes the newline character(s) in the
          // new output. This duplicates the behavior of the regular expression
          // that was previously used to replace the shebang line
          content = content.slice(i);
        }
      }
    }
  }

  // create wrapper function
  var wrapper = Module.wrap(content);

  var compiledWrapper = vm.runInThisContext(wrapper, {
    filename: filename,
    lineOffset: 0,
    displayErrors: true
  });

  if (process._debugWaitConnect && process._eval == null) {
    if (!resolvedArgv) {
      // we enter the repl if we're not given a filename argument.
      if (process.argv[1]) {
        resolvedArgv = Module._resolveFilename(process.argv[1], null);
      } else {
        resolvedArgv = 'repl';
      }
    }

    // Set breakpoint on module start
    if (filename === resolvedArgv) {
      delete process._debugWaitConnect;
      const Debug = vm.runInDebugContext('Debug');
      Debug.setBreakPoint(compiledWrapper, 0, 0);
    }
  }
  var dirname = path.dirname(filename);
  var require = internalModule.makeRequireFunction.call(this);
  var args = [this.exports, require, this, filename, dirname];
  var depth = internalModule.requireDepth;
  if (depth === 0) stat.cache = new Map();
  var result = compiledWrapper.apply(this.exports, args);
  if (depth === 0) stat.cache = null;
  return result;
}

Note: The above program will compile and run by using the node filename.js command. 

Reference: https://nodejs.org/api/process.html#process_process_mainmodule