Frontend Quick Tips #16 How to Escape From Boolean Trap With State Machines

Bernard Klatka

Sep 3, 2021 • 3 min read
Paying for the service

No one likes those big articles - that’s why we’re creating Quick Tips - short tips to change your developer's life from the moment you read them.

Those may be some patterns explained in JS code on real-life examples or some techniques for better code.

Problem

In a typical scenario when an application needs to connect to external resources/services (e.g. by API) to gather some data, our component’s state (or state container) has a bunch of boolean flags with a lot of conditional checking, where the initial state is:

{

isPending: true,

isFulfilled: false,

isRejected: false

}

and the next state (e.g. when request is successfully fulfilled):

{

isPending: false,

isFulfilled: true,

isRejected: false

}

Solution

Use a state machine

Quoting Wikipedia, a state machine (or FSM) is:

"It is an abstract machine that can be in exactly one of a finite number of states at any given time. The FSM can change from one state to another in response to some inputs; the change from one state to another is called a transition".

I have prepared a super simple and minimal state machine, you can get it here:

https://gist.github.com/serrg/e794b30b8dc3ebee29b096770f075b1c

Now your component’s state (or state container) could be like:

{

state: {}

}

Where state is your state machine with declared transitions and status:

const machine: Machine = createMachine<State>({

init: "idle",

transitions: [

{ name: "start", from: "idle", to: "pending" },

{ name: "success", from: "pending", to: "fulfilled" },

{ name: "failure", from: "pending", to: "rejected" }

],

actions: {

start: () => {

console.log("action started");

},

success: () => {

console.log("action successed");

},

failure: () => {

console.log("action failed");

}

}

});




machine.state; // state: idle

machine.action("start"); // action started, state: pending

machine.action("success"); // action successed, state: fulfilled

machine.action("failure"); // action failed, state: rejected

By using a machine.state property rather than a boolean flag indicator we enable our users to know exactly what the state is at any given point in time.

Our flow now looks like:

diagram

There are more complex and more powerful state machines, you can find libraries for any framework or language here: https://github.com/leonardomso/awesome-fsm. When it comes to frontend, the most popular library is XState.

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Bernard Klatka

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