Minimal Example Code Reproducing the Issue

Oct 13, 2025 by wbrum.org 72 views

Value Returning by await ctx.scene.enter(nextScene.id) Must be the Same that Returned by nextScene enter Middleware

Introduction

When working with scenes in Telegraf.js, it's essential to understand how the enter method works and how it interacts with middleware. In this article, we'll explore a common issue where the value returned by await ctx.scene.enter(nextScene.id) is not the same as the value returned by the next scene's enter middleware.

Context

Telegraf.js Version: 4.16.3
Node.js Version: v23.9.0
Operating System: Mac OS 15.1

Minimal Example Code Reproducing the Issue

Let's start with a minimal example code that reproduces the issue. We have two scenes: FirstScene and SecondScene.

First Scene

export default class FirstScene<C extends BotContext> extends Scenes.BaseScene<BotContext> {
    constructor(id: string, options?: SceneOptions<C>) {
        super(id, options)

        this.enter(async (ctx) => {
            const result = await ctx.scene.enter(secondScene.id, { a: 1 })
            console.log("RESULT " + result)
            //....
        })
    }
}

Second Scene

export default class SecondScene<C extends BotContext> extends Scenes.BaseScene<BotContext> {
    constructor(id: string, options?: SceneOptions<C>) {
        super(id, options)
        this.enter(async (ctx) => {
            return "SOME_VALUE"
        })
    }
}

The Issue

The problem arises when we use the enter method to transition from one scene to another. In the FirstScene, we call await ctx.scene.enter(secondScene.id, { a: 1 }), which should return the value returned by the enter middleware of the SecondScene. However, when we debug this, we see that there are two middlewares: the first one is created by the compose method in scenes/base.ts, and the second one is our custom middleware.

Current Behavior

The current behavior is that we added just one enter(...) middleware, but we don't get the returned value because there are two middlewares: the second one is our custom middleware, and the first one is created by the compose method.

Expected Behavior

The expected behavior is that we should have just one middleware added by enter(...) and we should be able to get the returned value from the previous scene's await call.

Understanding the Issue

To understand the issue, let's take a closer look at the composer.js file, which contains the compose method. This method is responsible for creating the middleware chain. When we call enter(...) multiple times, the compose method creates a new middleware chain by concatenating the existing middleware chain with the new middleware.

The Problem with compose

The problem with the compose method is that it creates a new middleware chain by concatenating the existing middleware chain with the new middleware. This means that when we call enter(...) multiple times, we end up with multiple middlewares in the chain, instead of just one.

Solution

To solve this issue, we need to modify the enter method in scenes/base.ts to create a new middleware chain only when the enterHandler is not defined. This way, we ensure that we have only one middleware in the chain.

Here's the modified code:

enter(...fns: Array<Middleware<C>>) {
    if (!this.enterHandler) {
        this.enterHandler = compose(fns)
    } else {
        this.enterHandler = compose([this.enterHandler, ...fns])
    }
    return this
}

Conclusion

In conclusion, the issue with await ctx.scene.enter(nextScene.id) not returning the same value as the next scene's enter middleware is due to the way the compose method creates the middleware chain. By modifying the enter method to create a new middleware chain only when the enterHandler is not defined, we can ensure that we have only one middleware in the chain and get the expected behavior.
Value Returning by await ctx.scene.enter(nextScene.id) Must be the Same that Returned by nextScene enter Middleware

Q&A

Q: What is the issue with await ctx.scene.enter(nextScene.id) not returning the same value as the next scene's enter middleware?

A: The issue is due to the way the compose method creates the middleware chain. When we call enter(...) multiple times, the compose method creates a new middleware chain by concatenating the existing middleware chain with the new middleware. This means that we end up with multiple middlewares in the chain, instead of just one.

Q: How can I fix this issue?

A: To fix this issue, you need to modify the enter method in scenes/base.ts to create a new middleware chain only when the enterHandler is not defined. This way, you ensure that you have only one middleware in the chain.

Q: What is the modified code for the enter method?

A: The modified code for the enter method is as follows:

enter(...fns: Array<Middleware<C>>) {
    if (!this.enterHandler) {
        this.enterHandler = compose(fns)
    } else {
        this.enterHandler = compose([this.enterHandler, ...fns])
    }
    return this
}

Q: Why do I need to modify the enter method?

A: You need to modify the enter method because the current implementation creates a new middleware chain every time you call enter(...) multiple times. This leads to multiple middlewares in the chain, which is not the expected behavior.

Q: What is the expected behavior?

A: The expected behavior is that you should have only one middleware in the chain, and you should be able to get the returned value from the previous scene's await call.

Q: How can I ensure that I have only one middleware in the chain?

A: You can ensure that you have only one middleware in the chain by modifying the enter method to create a new middleware chain only when the enterHandler is not defined.

Q: What are the benefits of modifying the enter method?

A: The benefits of modifying the enter method are that you ensure that you have only one middleware in the chain, and you can get the expected behavior.

Q: Are there any other solutions to this issue?

A: Yes, there are other solutions to this issue. One solution is to use a different approach to create the middleware chain, such as using a separate method to create the middleware chain.

Q: What are the pros and cons of using a separate method to create the middleware chain?

A: The pros of using a separate method to create the middleware chain are that you can have more control over the creation of the middleware chain, and you can ensure that you have only one middleware in the chain. The cons are that you need to create a new method, which can add complexity to your code.

Conclusion

In conclusion, the issue with await ctx.scene.enter(nextScene.id) not returning the same value as the next scene's enter middleware is due to the way the compose method creates the middleware chain. By modifying the enter method to create a new middleware chain only when the enterHandler is not defined, you can ensure that you have only one middleware in the chain and get the expected behavior.