Scope of dependencies when exporting a module in Node.js

Question

When exporting a module that has another dependency, is it best to include that dependency within the module export function or outside of it? I usually see the latter, but it seems like it would be best to keep it in the local scope.

For example:

var foo = require('foo');

module.exports = function(d) {
    return foo(d)/2;
}

vs.

module.exports = function(d) {
    var foo = require('foo');

    return foo(d)/2;
}

Sean Vieira · Accepted Answer · 2014-11-19 05:35:32Z

7

Only things exposed on module.exports and global can be accessed from other modules in node. Unlike in the browser, var creates a local reference. To quote from node's documentation:

In browsers, the top-level scope is the global scope. That means that in browsers if you're in the global scope var something will define a global variable. In Node this is different. The top-level scope is not the global scope; var something inside a Node module will be local to that module.

The difference between the two versions is therefore minimal - the first incurs a lookup in the local scope, while the other hits require.cache every time the function is invoked. From what I have seen of node code, the former (var someVar = require('something');) seems to be preferred.

answered Nov 19, 2014 at 5:35

Sean Vieira

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2 Comments

jczaplew Over a year ago

Thanks for the great answer! My understanding is that the lookup in the local scope happens only once - when the function is defined. Is that true? If so, the former method makes the most sense. Otherwise, do you know how expensive a lookup in the local scope is compared to a hit to require.cache?

Sean Vieira Over a year ago

The scope lookup is likely to be the less intensive one - the lookup is dynamic (IIRC) but is almost certainly easier for the JITC to optimize than the lookup for require + the function call + the lookup in the require.cache.

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