Review the code snippet below to understand how errors are handled:
const foo = { someKey: 'foo' };
foo['NonExistentKey'].bar.baz // no errors
foo.NonExistentKey.bar.baz // NonExistentKey doesn't exist
Click here for TypeScript playground link
Can this be caught in any way?
When transitioning from JavaScript to TypeScript, it was decided that dynamic object access using [] should not result in an error and simply return any value.
However, this introduces a significant weakness in the type system, which can be addressed by using a compiler flag called noImplicitAny. Enabling this flag will cause errors to be raised as expected (but keep in mind that the compiler may also flag instances where it assumes any as an error).
If you explore the TypeScript playground and enable this option under settings, you will observe the anticipated error message.
If you want to easily catch errors of this type, consider turning on TypeScript's strict mode or enabling the noImplicitAny flag in your tsconfig.json configuration file.
Once one of these options is activated, you will receive an error message similar to the following:
wat.ts:2:1 - error TS7017: Element implicitly has an 'any' type because type '{ someKey: string; }' has no index signature.
foo['NonExistentKey'].bar.baz
In the absence of these flags, the variable foo is automatically typed as
const foo: { [index: string]: any, someKey: string }Personally, I find the behavior allowed by non-strict mode to be confusing and potentially error-prone. Therefore, I recommend setting strict: true in your tsconfig.json file for better code quality and safety.
You can refer to the full documentation on TypeScript compiler options here.
For more information about tsconfig.json configurations, visit this link.
Additional details on index signatures in TypeScript can be found here.
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