Recursively merge two dictionaries together
npm install merge-dictionariesA wrapper around the Lodash 3 .merge() function that addresses some issues with arrays and object references. Intended for merging configuration files together.
``
npm install merge-dictionaries
`
var mergeDictionaries = require('merge-dictionaries');
mergeDictionaries(dictA, dictB);
The .merge() function works great in most cases, but the default behavior has two problems:
First, array values in the second argument are merged weirdly with values in the first argument. Examples:
`
// Two arrays are "merged" together by replacing values in the first array with values from
// the second, by index:
var dictA = { foo: ['owl', 'snake', 'fish'] };
var dictB = { foo: ['cat', 'dog']};
_.merge(dictA, dictB);
// Results in:
// { foo: ['cat', 'dog', 'fish'] }
// Merging an array into a string causes the string to be busted up into an array of characters,
// which is then merged on-top-of as above:
var dictA = { foo: 'abcde' };
var dictB = { foo: ['cat', 'dog']};
_.merge(dictA, dictB);
// Results in:
// { foo: ['cat', 'dog', 'c', 'd', 'e'] }
`
Second, dictionaries in the second argument that do not have corresponding dictionary values in the first argument (or whose corresponding values are {}) are copied over by _value_ instead of by _reference_:
`
var dictA = { foo: 'bar' };
var dictB = { nested: { owl: 'hoot' } };
var owl = dictB.nested; // <-- owl is { owl: 'hoot' }
var merged = _.merge(dictA, dictB);
// Results in:
// { foo: 'bar', nested: { owl: 'hoot' } }
console.log(owl === merged.nested);
// Results in:
// false
`
This might not seem like a big issue, but it can be a real problem when merging dictionaries that contain references to objects created by another module. For example imagine:
`
var configA = { someConfigValue: 'some default value' };
var configB = { someConfigValue: 'a custom value', someModule: require('my-module') };
var mergedObj = _.merge(configA, configB);
where my-module looks like:
`
module.exports = ( function() {
// Declare the public data dictionary exposed by this module.
var publicData = {};
return {
// Expose the public data to the outside world.
somePublicData: publicData
// Declare a function for initializing the module.
init: function() {
publicData.foo = 'bar';
}
}
} )()
If you call mergedObj.someModule.init() later, you might expect mergedObj.someModule.somePublicData to be set to {foo: 'bar'}, but it’ll still just be an empty dictionary, because a _different somePublicData dictionary_ was copied into the merged object.
The solution is very simple, because the _.merge() function can take a third argument that allows you to customize the merge behavior. We can use this to tell _.merge() to _only_ do its regular thing when the left-hand value is a non-empty plain dictionary. In all other cases, a is replaced by b.
> Keep in mind that this means that if a _looks_ like a dictionary, but was created by a custom constructor (i.e. it is not a “plain” dictionary, it will be replaced by b! For example:`
>
>
> var myClass = function() {this.foo = 'bar'};
> var obj1 = { abc: new myClass() };
> // Result:
> // { abc: { foo: 'bar' } }
>
> var obj2 = { abc: { owl: 'hoot' } };
> var merged = mergeDictionaries(obj1, obj2);
>
> // Result:
> // { abc: { owl: 'hoot' } }
>
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Like the Sails framework, this package is free and open-source under the MIT License.