Feature request: allow hook for overriding serialize function in jest-snapshot

[anilanar]

I have a custom component implementation based on incremental-dom and I wish to do snapshot testing for it.

I've checked the source code of jest and from what I've figured, it is as easy as replacing serialize function in jest-snapshot/src/SnapshotFile.js, which converts a given data to a string against which comparison/matching is done. In React's case, serialize takes a jsonized react component and it returns it after piping it through pretty-format.

I will fork jest for now; but it would save me maintaining the fork if this feature was implemented.

[anilanar]

Just tested serialize(data: any): string { return data; } for comparing plain strings just to see if it works. It works great.

[cpojer]

You can overwrite toJSON on your top level object to provide a custom return value.

Try:

const object = …;
object.toJSON = () => 'my string';
expect(object).toMatchSnapshot();

[anilanar]

@cpojer Mind if I make a PR for specifying packages that would export objects that conform to pretty-format plugin interface?

// package.json
"jest": {
    // ReactElementPlugin, ReactTestComponentPlugin are natural members of this array
    "snapshot-serializers": ["my-serializer-package"]
}

// my-serializer-package
var MyCustomClass = require('my-package');
module.exports = {
    test: function(val) {
        return val instanceof MyCustomClass;
    }
    print: function(val) {
        // custom implementation
    }
};

Usage

test(() => {
    var foo = new MyCustomClass();
    expect(foo).toMatchSnapshot();
});

[cpojer]

yeah I'd be happy to accept that!

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