The parameter 'host: string | undefined; user: string | undefined' does not match the expected type 'string | ConnectionConfig' and cannot be assigned

Question

The parameter 'host: string | undefined; user: string | undefined' does not match the expected type 'string | ConnectionConfig' and cannot be assigned

My attempt to establish a connection to an AWS MySQL database looks like this:

const config = {
  host: process.env.RDS_HOSTNAME,
  user: process.env.RDS_USERNAME,
  password: process.env.RDS_PASSWORD,
  port: 3306,
  database: process.env.RDS_DB_NAME,
}

const db = mysql.createConnection(config) // the 'config' here is highlighted

However, I encounter the following error message:

Argument of type '{ host: string | undefined; user: string | undefined; password: string | undefined; port: number; database: string | undefined; }' is not assignable to parameter of type 'string | ConnectionConfig'.

Type '{ host: string | undefined; user: string | undefined; password: string | undefined; port: number; database: string | undefined; }' is not assignable to type 'ConnectionConfig'.

    Types of property 'host' are incompatible.
      Type 'string | undefined' is not assignable to type 'string'.
        Type 'undefined' is not assignable to type 'string'.ts(2345)

Previously, I encountered an issue with the port being sourced from .env. After switching to setting the port directly, this problem emerged.

The cause of the issue and its resolution elude me at this point.

typescript express mysql-connector

Answer 1

Answer №1

The issue arises from the declaration of process.env in @types/node, as shown below:

// process.d.ts
   ...
   interface ProcessEnv extends Dict<string> {}
   ...
   env: ProcessEnv

// global.d.ts
    interface Dict<T> {
        [key: string]: T | undefined;
    }
    ...

The problem lies in the fact that any lookup in env results in string | undefined, while the function createConnection requires at least a string for the host property.

To resolve this issue, you have several options to ensure that the compiler accepts the passed config:

If you are certain that all environment variables are correctly set, you can typecast your config like so:

type NoUndefinedField<T> = {
  [P in keyof T]: Exclude<T[P], undefined>;
};

createConnection(config as NoUndefinedField<typeof config>)

Update

In this solution, we utilize generics (<T>) to abstract over concrete types, mapped type [P in keyof T] to iterate through all properties (using keyof T) of type T, and the utility type Exclude<> to remove undesired types from each property of type T[K]. By removing all instances of undefined types from the properties of typeof config, we effectively cast the config value to this updated type.

Overall, by recalculating the precise structure of the config object type without undefined types, we maintain compatibility with the expected type of Config used by mysql.

This proactive approach helps identify any inconsistencies between the actual runtime values and the expected type definitions at compile time, ensuring more robust code correctness without relying solely on manual assertions or reactive troubleshooting.

Answer 2

The issue arises from the declaration of process.env in @types/node, as shown below:

// process.d.ts
   ...
   interface ProcessEnv extends Dict<string> {}
   ...
   env: ProcessEnv

// global.d.ts
    interface Dict<T> {
        [key: string]: T | undefined;
    }
    ...

The problem lies in the fact that any lookup in env results in string | undefined, while the function createConnection requires at least a string for the host property.

To resolve this issue, you have several options to ensure that the compiler accepts the passed config:

If you are certain that all environment variables are correctly set, you can typecast your config like so:

type NoUndefinedField<T> = {
  [P in keyof T]: Exclude<T[P], undefined>;
};

createConnection(config as NoUndefinedField<typeof config>)

Update

In this solution, we utilize generics (<T>) to abstract over concrete types, mapped type [P in keyof T] to iterate through all properties (using keyof T) of type T, and the utility type Exclude<> to remove undesired types from each property of type T[K]. By removing all instances of undefined types from the properties of typeof config, we effectively cast the config value to this updated type.

Overall, by recalculating the precise structure of the config object type without undefined types, we maintain compatibility with the expected type of Config used by mysql.

This proactive approach helps identify any inconsistencies between the actual runtime values and the expected type definitions at compile time, ensuring more robust code correctness without relying solely on manual assertions or reactive troubleshooting.

The parameter 'host: string | undefined; user: string | undefined' does not match the expected type 'string | ConnectionConfig' and cannot be assigned

Answer №1

Update

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