Tslint: Rule no-inferrable-types is not working

[sandrocsimas]

Bug Report

• __TSLint version__:
  json "tslint": "5.10.0", "tslint-config-airbnb": "5.9.2", "tslint-react": "3.6.0",
• __TypeScript version__:
  json "typescript": "2.9.1",
• __Running TSLint via__: CLI

TypeScript code being linted

I have the following configuration:
json { "rules": { "align": [true, "elements", "members", "parameters", "statements"], "interface-name" : [true, "never-prefix"], "max-line-length": {"options": {"limit": 999}}, "no-console": false, "no-inferrable-types": true, "no-string-literal": true, "object-curly-spacing": [true, "never"], "object-literal-sort-keys": false, "ordered-imports": false, "space-before-function-paren": false, "trailing-comma": [true, { "singleline": "never", "multiline": { "objects": "always", "arrays": "always", "functions": "never", "typeLiterals": "ignore" } }], "typedef": [true, "call-signature", "parameter"], "variable-name": [true, "ban-keywords", "check-format", "allow-leading-underscore"] }, "extends": [ "tslint:recommended", "tslint-eslint-rules", "tslint-config-airbnb", "tslint-react" ] }
And this Typescript code:
javascript private static readonly DEVICE_UID: string = 'device_uid'; private static readonly DEVICE_PLATFORM = 'browser';

Why I'm not getting error in the first declaration?

Actual behavior

No errors throwed.

Expected behavior

Rule no-inferrable-types should throw an error.

[aervin]

@sandrocsimas There is a bit of difference between those two member declarations. DEVICE_UID will have a type of string while DEVICE_PLATFORM will have the type 'browser'.

The rule makes a special exception for the readonly modifier (by ignoring declarations with that modifier). But since the static keyword is also there, I think there should be some sort of warning about unnecessary widening of the type. Maybe there's a use-case I'm not thinking of. @ajafff ?

[sandrocsimas]

@aervin, I didn't understood why DEVICE_PLATFORM will have the type 'browser'? Isn't this a string?
In my mind these three examples should result in a error:
javascript private static readonly DEVICE_UID: string = 'device_uid'; private static DEVICE_UID: string = 'device_uid'; private DEVICE_UID: string = 'device_uid';
In both cases you're defining the type string and setting a string value. This is redundant.
Why they have different behavior?

[aervin]

@sandrocsimas I agree it's a bit confusing. See this tslint issue and this TS issue for some background.

[sandrocsimas]

OK @aervin, I understood that is some strange behavior in Typescript. In my case, even without the type definition, I don't get an error (the opposite of what happen in the examples provided in the issues you've mentioned). This should be due to the static definition.

[aervin]

It seems like this:

// case #1
// DEVICE_UID can only be one thing: 'device_uid'
class A {
  private static readonly DEVICE_UID = 'device_uid';
}

should always be preferred over this:

// case #2
// same as above, but explicitly tells TS not to narrow the type of DEVICE_UID
class B {
  private static readonly DEVICE_UID: string = 'device_uid';
}

I agree that tslint should warn about the typedef in the case #2 example. However, I think it's preferable that the TS compiler point out case #2's problem. Because of the string typedef, a class member could do something silly like this:

switch(B.DEVICE_UID) {
  case 'random_string':
    // ... this will never be executed
}

EDIT: TS playground example

[giladgray]

@sandrocsimas @aervin is there an actual issue in the code here? seems like it was all a big misunderstanding.

[sandrocsimas]

@giladgray, I was expecting an error, but @aervin said this is the expected behavior since I'm using readonly. I think the issue can be closed, but it would be nice if you analyze the behavior when readonly is used with static.

[giladgray]

👍

[giladgray]

readonly and static mean different things and can be combined elegantly to make a singleton constant

[aervin]

@giladgray for the record, I think TS should warn about this issue, not tslint.

[giladgray]

agreed