Before You File a Proposal Please Confirm You Have Done The Following...

• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

My proposal is suitable for this project

• My proposal specifically checks TypeScript syntax, or it proposes a check that requires type information to be accurate.
• My proposal is not a "formatting rule"; meaning it does not just enforce how code is formatted (whitespace, brace placement, etc).
• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Description

Code should not use an as type assertion when a : type annotation would do. In some cases (see examples later) they act the same, but as assertions are generally something that should be avoided whenever possible. as removes some type safety from TypeScript. Not ideal.

Fail Cases

const errorMessages = [] as string[];

Pass Cases

const errorMessages: string[] = [];

Additional Info

This should also trigger for arrays that have contents in them.

- const errorMessages = ['a'] as string[];
+ const errorMessages: string[] = ['a'];

Alas, knowing whether the assertion actually was doing anything important is blocked on my favorite TypeScript issue microsoft/TypeScript#9879. But since folks really shouldn't be using an assertion for a variable with an initial value, I'm ok with this edge case bug being in the rule:

// The + is now an error... 😈 deal with it!
- const errorMessages2 = [Math.random() ? 'a' : 'b'] as 'a'[];
+ const errorMessages3: 'a'[] = [Math.random() ? 'a' : 'b'];