Declaring libraries as constants

Question

In NodeJS world we require modules using require function:

var foo = require ("foo");

In JavaScript (also in NodeJS) we have const keyword that creates a constant:

const

Creates a constant that can be global or local to the function in which it is declared. Constants follow the same scope rules as variables.

Example:

$ node
> const a = 10
undefined
> a
10
> a = 7
7
> a
10

My question is: would it be good to require libraries as constans?

Example:

const foo = require ("foo")
    , http = require ("http")
    ;

/* do something with foo and http */

Are there any bad/good effects using const instead of var when requiring libraries?

Answer 1

It turns out that it became a common practice to use const over var for dependencies. At least in the Node.js source code this is happening:

• http.js

  

So, I guess it's a good practice. I started to use it as well in my modules.

Answer 2

NodeJS hasn't any optimisations for library that requires as const - require is a simple non native function, which nothing is known about the type of the variable to which is assigned. There is source code of require:

Module.prototype.require = function(path) {
  assert(util.isString(path), 'path must be a string');
  assert(path, 'missing path');
  return Module._load(path, this);
};

Module._load = function(request, parent, isMain) {
  if (parent) {
    debug('Module._load REQUEST  ' + (request) + ' parent: ' + parent.id);
  }

  var filename = Module._resolveFilename(request, parent);

  var cachedModule = Module._cache[filename];
  if (cachedModule) {
    return cachedModule.exports;
  }

  if (NativeModule.exists(filename)) {
    // REPL is a special case, because it needs the real require.
    if (filename == 'repl') {
      var replModule = new Module('repl');
      replModule._compile(NativeModule.getSource('repl'), 'repl.js');
      NativeModule._cache.repl = replModule;
      return replModule.exports;
    }

    debug('load native module ' + request);
    return NativeModule.require(filename);
  }

  var module = new Module(filename, parent);

  if (isMain) {
    process.mainModule = module;
    module.id = '.';
  }

  Module._cache[filename] = module;

  var hadException = true;

  try {
    module.load(filename);
    hadException = false;
  } finally {
    if (hadException) {
      delete Module._cache[filename];
    }
  }

  return module.exports;
};

For more time library is an object (I think you don't have library like this module.exports = 10).

You can change all fields of object also if it is declared as const (If you want to get realy const object use Object.freeze(someObject)).

For conclusion: the effect is the same as for a common variable. Link to V8 Variable Declaration Function used in NodeJS