fix: getprecomputedinstance should always return an instance

[sameerank]

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labels: mergeable

Fixes: #issue

Motivation and Context

Some feedback in a previous PR that we should avoid overwriting the Eppo client instance.

We shall prefer creating a new instance of the client in initialization instead of resetting the existing one.

This is challenging because it breaks the expected behavior that getPrecomputedInstance always returns an instance of EppoPrecomputedClient.

js-client-sdk/src/index.ts

Lines 784 to 786 in f5d4eb4

	export function getPrecomputedInstance(): EppoPrecomputedClient {
	return EppoPrecomputedJSClient.instance;
	}

It would be undefined before any of the initialization functions (precomputedInit or offlinePrecomputedInit) are called. This is likely to happen in complex applications with race conditions between various rendering components. This means the code would need an extra protection that the singleton client is defined before calling client.getStringAssignment(..., defaultValue), and this is poor DX because they can't simply rely on the function falling back on returning the defaultValue if called before initialization and need to write in their own extra protection.

Description

This initializes the client with an instance with an empty precomputed config store.

So instead of doing

const eppoPrecomputedClient = getPrecomputedInstance();
const precomputedAssignment = eppoPrecomputedClient ? eppoPrecomputedClient.getStringAssignment(flagKey, defaultValue) : defaultValue;

you can just do

const eppoPrecomputedClient = getPrecomputedInstance();
const precomputedAssignment = eppoPrecomputedClient.getStringAssignment(flagKey, defaultValue);

How has this been tested?

[sameerank]

Another bit of feedback was about "shutting down" the client #117 (comment)

Since the client instance now will always be defined, I am switching to use the initialized check. This makes it so you can't replace the singleton by re-running precomputedInit: it'll just return the previously initialized instance. There is a private method visible below (shutdownEppoPrecomputedClient ) that would need to be called first before re-initializing, if we want to use it here to allow re-initialization/overwriting the singleton

[typotter]

just to confirm, this instantiation doesn't kick off any network requests or polling, right?

[sameerank]

Nope, you have to go through the precomputedInit method to start the poller