CODEWITHSUNDEEP
javascript
Lukas
Lukas

Reputation: 10360

Is it possible to change the return type of a function called with new

I have to write a shim for another abandoned npm library.

This library will be called by others using the new keyword. This is what others do:

const Noble = require('noble/lib/noble');
var nobleInstance = new Noble(macBindings);
module.exports = nobleInstance;

Now I wrote a shim that changes what is being imported via require('noble/lib/noble');. To make things nice for myself I want to change what 

new Noble(macBindings);

is returning so whenever I call 

require('third-pary-library-using-new-noble');

I will actually get my own return value.

so for this I need to change what new Noble(macBindings); is returning. Reminder. I already managed to shim the Noble Function.

What I am wondering is if it is possible to change what is being returned when someone calls the function with the new keyword.

In the end, what I want to achieve is that when someone calls

var nobleInstance = new Noble(macBindings);

I want nobleInstance to be macBindings.

Upvotes: 1

Views: 52

Answers (1)

Filippo Possenti
Filippo Possenti

Reputation: 1410

You can't replace the instance returned when you call new Noble(...) as far as I know, but you can make your Noble have the same methods and functions of macBindings by iterating through the object, more or less as follows.

function Noble(arg) {
    for(var key in arg) {
        this[key] = arg[key];
    }
}

You might have to work around the above code a little bit (changing the scope or limiting the properties that get copied over) and it might not fit your case 100%, but it should be a decent starting point.

Broadly speaking what you want to do is to build your own version of Noble implementing the same methods and properties as the original version, possibly informing the user that Noble is deprecated and should be removed by a certain date, in order to promote refactorings meant to move away from the abandoned library.

Upvotes: 1

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