ERROR - [JSC_JS_MODULE_LOAD_WARNING] Failed to load module "util"

[mistersomebody]

Context

I would like to compile a node based project which dependencies include util and other node builtins.

However, the compiler won't load them despite using the --process_common_js_modules and --module_resolution NODE flags. Every other dependencies is loaded properly with --js.

My understanding was that, by using --externs and --output_wrapper (or --output_wrapper_file), the compiler would inject and use them as global variables within the final output. I've used @externs/nodejs to that effect.

Following here is a very basic failing setup:

Setup

Compiler

npm i -D google-closure-compiler
# + [email protected]

I also tried [email protected] from a year ago in case it would be related to the latest version.

Source

const util = require('util')

console.log('it works...')

module.exports = {}

Command

google-closure-compiler index.js --process_common_js_modules --compilation_level ADVANCED --module_resolution NODE

Error

index.js:1:13: ERROR - [JSC_JS_MODULE_LOAD_WARNING] Failed to load module "util"
  1| const util = require('util')
                  ^

1 error(s), 0 warning(s)

Question

Is my initial assumption correct? Or is it not possible to compile with node builtins. If yes, could you help out with the configuration?

[mprobst]

I believe this is working as expected; you need to pass every JS source with the --js flag, including sources from node modules.

[mistersomebody]

Ok, I see. It works indeed if I place mock packages of builtins in node_modules and pass them with --js.

Thank you.

[ctjlewis]

@mprobst, util is a Node builtin and does not exist in node_modules/. This does not work as expected, as these cannot be flagged as externs and apparently only work if you spoof them in node_modules/ and pass them in as normal JS (?).

This is entirely unintuitive, see https://github.com/google/closure-compiler/issues/3740.

[ctjlewis]

@mistersomebody, could you please upload your project directory to a repo so that I can replicate the hack? Your creativity is much appreciated.

[mistersomebody]

@ctjlewis

Here's an adaptation of what I ended up doing:

Sample

./index.js

const util = require('util')

console.log('It works:', util.isArray([]))

Mock

./node_modules/util/index.js

module.exports = util

Wrapper

./wrapper.txt

(util => {
  %output%  
})(require('util'))

Externs

Copy the util extern and remove module.exports = util at the end

./externs.js

/* LICENCE ... */

/**
 * @const
 */
var util = {};

/**
 * @param {string} format
 * @param {...*} var_args
 * @return {string}
 * @nosideeffects
 */
util.format;

/**
 * @param {string} string
 * @return {void}
 */
util.debug;

/**
 * @param {...*} var_args
 * @return {void}
 */
util.error;

/**
 * @param {...*} var_args
 * @return {void}
 */
util.puts;

/**
 * @param {...*} var_args
 * @return {void}
 */
util.print;

/**
 * @param {string} string
 * @return {void}
 */
util.log;

/**
 * @param {*} object
 * @param {{showHidden: (boolean|undefined),
 *          depth: (number|null|undefined),
 *          colors: (boolean|undefined),
 *          customInspect: (boolean|undefined)}=} options
 * @return {string}
 * @nosideeffects
 */
util.inspect;

/**
 * @param {*} object
 * @return {boolean}
 * @nosideeffects
 */
util.isArray;

/**
 * @param {*} object
 * @return {boolean}
 * @nosideeffects
 */
util.isRegExp;

/**
 * @param {*} object
 * @return {boolean}
 * @nosideeffects
 */
util.isDate;

/**
 * @param {*} object
 * @return {boolean}
 * @nosideeffects
 */
util.isError;

/**
 * @param {Function} constructor
 * @param {Function} superConstructor
 * @return {void}
 */
util.inherits;

Command

google-closure-compiler \
  --compilation_level ADVANCED \
  --js_output_file dist/index.js \
  --module_resolution NODE \
  --process_common_js_modules \
  --export_local_property_definitions \
  --generate_exports \
  --externs ./externs.js \
  --js node_modules/util/index.js \
  --output_wrapper_file ./wrapper.txt \
  index.js

Output

./dist/index.js

(util => {
  'use strict';console.log("It works",util.isArray([]));  
})(require('util'))

Cheers