JavaScript design patterns: Injecting a dependency that is not yet created

Question

I have a CommonJS module:

// main-module
module.exports = function () {
  var foo,
      someModule = require('other-module')(foo);

  // A value is given to foo after other-module has been initialised
  foo = "bar";
}

As you can see, this requires other-module:

// other-module.js
module.exports = function (foo) {
  function example() {
    console.log(foo);
    // > "bar"
  }
}

I would like the example function inside of other-module to be aware of the foo variable inside of main-module, even though is it established after the module is required.

When other-module runs, foo will not be undefined. However, the point is that by time my example function runs, foo will have been given a value of bar.

The pattern above obviously does not work. What design pattern do I need to implement?

Matt Ball · Accepted Answer · 2013-02-25 23:56:04Z

2

I'm not super-familiar with CommonJS, so this might not be the idiomatic way to do it, but using a function instead of a variable should work:

// main-module
module.exports = function () {
  var foo,
      someModule = require('other-module')(function() { return foo; });

  foo = "bar";
}

// other-module.js
module.exports = function (fooFn) {
  function example() {
    console.log(fooFn());
  }
}

answered Feb 25, 2013 at 23:56

Matt Ball

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Comments

Matt Zeunert · Accepted Answer · 2013-02-26 00:19:50Z

0

The foo value (a string) will be passed by value, so it's undefined inside other-module. You could use an options object that is passed by reference:

var options = {},
someModule = require('other-module')(options);

options.foo = "bar";

edited Feb 26, 2013 at 0:19

answered Feb 25, 2013 at 23:56

Matt Zeunert

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2 Comments

Matt Ball Over a year ago

@OliverJosephAsh change options = {"foo": "bar"}; to options.foo = 'bar'; and it will.

Matt Zeunert Over a year ago

Thanks @MattBall, you're right. If we assign a new value to options we have the same problem as when we passed the string by value - so we need to keep the options reference the same.

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