Node.js - check if module is installed without actually requiring it

Question

I need to check whether "mocha" is installed, before running it. I came up with the following code:

try {
    var mocha = require("mocha");
} catch(e) {
    console.error(e.message);
    console.error("Mocha is probably not found. Try running `npm install mocha`.");
    process.exit(e.code);
}

I dont like the idea to catch an exception. Is there a better way?

I've answered but now I've noticed the "globally" word. By "globally" do you mean a module installed with `-g` option in `npm`? I.e. `npm install -g mocha`? Edit: AFAIK `require` will not find modules installed with `-g` option. — Jan Święcki, Mar 08 '13 at 21:45

score 85 · Accepted Answer · edited Sep 03 '17 at 13:35

85

You should use require.resolve() instead of require(). require will load the library if found, but require.resolve() will not, it will return the file name of the module.

See the documentation for require.resolve

try {
    console.log(require.resolve("mocha"));
} catch(e) {
    console.error("Mocha is not found");
    process.exit(e.code);
}

require.resolve() does throw error if module is not found so you have to handle it.

edited Sep 03 '17 at 13:35

answered Mar 08 '13 at 21:02

user568109

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`require.resolve` still throws a error - that is what I'm trying to avoid - catching an exception. – AndreyM Mar 08 '13 at 21:04
3

Accepting the answer since other solutions are not cleaner in any way. – AndreyM Mar 08 '13 at 22:32
yes, but will this also look for globally installed modules? I want to check if a module is installed locally. – Alexander Mills Apr 12 '16 at 05:44
@AlexMills A better way to do that is using [npm ls](https://docs.npmjs.com/cli/ls) where global is false by default. – user568109 Apr 12 '16 at 07:24

Jan Święcki · Answer 2 · 2013-03-08T21:58:23.963

module.paths stores array of search paths for require. Search paths are relative to the current module from where require is called. So:

var fs = require("fs");

// checks if module is available to load
var isModuleAvailableSync = function(moduleName)
{
    var ret = false; // return value, boolean
    var dirSeparator = require("path").sep

    // scan each module.paths. If there exists
    // node_modules/moduleName then
    // return true. Otherwise return false.
    module.paths.forEach(function(nodeModulesPath)
    {
        if(fs.existsSync(nodeModulesPath + dirSeparator + moduleName) === true)
        {
            ret = true;
            return false; // break forEach
        }
    });

    return ret;
}

And asynchronous version:

// asynchronous version, calls callback(true) on success
// or callback(false) on failure.
var isModuleAvailable = function(moduleName, callback)
{
    var counter = 0;
    var dirSeparator = require("path").sep

    module.paths.forEach(function(nodeModulesPath)
    {
        var path = nodeModulesPath + dirSeparator + moduleName;
        fs.exists(path, function(exists)
        {
            if(exists)
            {
                callback(true);
            }
            else
            {
                counter++;

                if(counter === module.paths.length)
                {
                    callback(false);
                }
            }
        });
    });
};

Usage:

if( isModuleAvailableSync("mocha") === true )
{
    console.log("yay!");
}

Or:

isModuleAvailable("colors", function(exists)
{
    if(exists)
    {
        console.log("yay!");
    }
    else
    {
        console.log("nay:(");
    }
});

Edit: Note:

module.paths is not in the API
Documentation states that you can add paths that will be scanned by require but I couldn't make it work (I'm on Windows XP).

@AndreyM, just curious to know what you didn't like about this solution and why you marked the accepted answer as one that uses a try/catch hack to determine if a module exists? Thank you. — jmort253, Aug 24 '14 at 06:30
I'm assuming because module.paths is not in the API, you didn't go with this. I can see this not being future proof if Node gets changed in some manner, whereas the try/catch strategy, despite being ugly, would be more reliable. Still, this is an awesome answer and solves this without try/catch! +1 :) — jmort253, Aug 24 '14 at 06:35

Node.js - check if module is installed without actually requiring it

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