SetBuffer.js

class SetBuffer {
    constructor(idFn) {
        this.items = new Map()
        this.idFn = idFn
    }

    isEmpty() {
        return this.items.size === 0
    }

    take() {
        if (this.isEmpty()) return

        const [key, value] = this.items.entries().next().value
        this.items.delete(key)
        return value
    }

    put(action) {
        const { items, idFn } = this
        const id = idFn(action)
        if (items.has(id)) {
            return
        }

        items.set(id, action)
    }
}

function* request(payload) {
    /* ... */
}

export default function* () {
    const requestChannel = yield actionChannel(
        actions.FETCH,
        new SetBuffer(action => action.payload.foo),
    )

    while (true) { // eslint-disable-line
        const { payload } = yield take(requestChannel)
        const state = yield select(selector, payload)

        if (!state) {
            yield request(payload)
        }
    }
}