The attribute 'forEach' is not recognized on the data type 'string | string[]'

I'm experiencing an issue with the following code snippet:

  @Where(
    ['owner', 'Manager', 'Email', 'productEmail', 'Region'],
    (keys: string[], values: unknown) => {
      const query = {};

      keys.forEach((k: string) => {            
    }
  )

After enabling strict rules for type checks, I encountered this error:

Argument of type '(keys: string[], values: unknown) => {}' is not assignable to parameter of type '(key: string | string[], values: unknown) > unknown'.
  Types of parameters 'keys' and 'key' are incompatible.
    Type 'string | string[]' is not assignable to type 'string[]'.
      Type 'string' is not assignable to type 'string[]'.ts

Changing the keys type to 

(keys: string | string[], values: unknown) >
resulted in another error:

Property 'forEach' does not exist on type 'string | string[]'.
  Property 'forEach' does not exist on type 'string'.ts

If anyone has any ideas on how to resolve these issues, please share them.

typescripttype-conversion

Answer №1

The problem arises because Typescript is cautioning against mistakenly using forEach on keys when its type is string. This is to prevent runtime errors where forEach property does not exist for the given type, leading to failure. Is the @Where decorator something you created yourself or is it from a third-party library?

If you wish to keep the type as string | string[] and still use forEach, you will need to ensure that keys is an array before attempting to call the method. Here's how:

if (Array.isArray(keys)) {
    keys.forEach(...);
}

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