Tslint: strict-type-predicates: False positive with index types
Bug Report
- __TSLint version__: 5.4.3 5cf7100ad3c7b2fd3be62d94b7b4708460590a97
- __TypeScript version__: 2.4.0
- __Running TSLint via__: CLI
Related to: https://github.com/Microsoft/TypeScript/issues/13778
TypeScript code being linted
declare function get<T>(): T;
// index types
{
get<{ [key: string]: string }>()['foo'] === undefined
get<{ [key: string]: string }>()['foo'] === null
get<{ [key: string]: string }>()['foo'] == undefined
get<{ [key: string]: string }>()['foo'] == null
get<{ [index: number]: string }>()[0] === undefined
get<{ [index: number]: string }>()[0] === null
get<{ [index: number]: string }>()[0] == undefined
get<{ [index: number]: string }>()[0] == null
}
with tslint.json configuration:
(Using test/rules/strict-type-predicates/strict-null-checks/tsconfig.json)
Actual / Expected behavior
(A is Actual, E is Expected, B is Both)
{
get<{ [key: string]: string }>()['foo'] === undefined
A: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Expression is always false.]
get<{ [key: string]: string }>()['foo'] === null
B: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Expression is always false.]
get<{ [key: string]: string }>()['foo'] == undefined
A: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Expression is always false.]
E: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Use '=== undefined' instead.]
get<{ [key: string]: string }>()['foo'] == null
A: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Expression is always false.]
E: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Use '=== undefined' instead.]
get<{ [index: number]: string }>()[0] === undefined
A: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Expression is always false.]
get<{ [index: number]: string }>()[0] === null
B: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Expression is always false.]
get<{ [index: number]: string }>()[0] == undefined
A: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Expression is always false.]
E: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Use '=== undefined' instead.]
get<{ [index: number]: string }>()[0] == null
A: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Expression is always false.]
E: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [Use '=== undefined' instead.]
}
(Running example is here: https://github.com/ypresto/tslint/tree/strict-type-predicates-index-type )
External
ypresto
👍4
All 3 comments
More simple example:
const foo: { [key: string]: string } = {}
if (foo.bar === undefined) {
// blah
}
will produce
ERROR: /Users/yuya/repo/github.com-private/codetakt/pf-user-admin/client/test.ts[2, 5]: Expression is always false.
but no error is expected.
ypresto
on 22 Jun 2017
👍5
tslint is not the right place to add this kind of special handling. This would require a change in typescript's type inference. https://github.com/Microsoft/TypeScript/issues/13778
If you always check for undefined before using the value, you could also define your index signature to include undefined;
const foo: { [key: string]: string | undefined } = {}
ajafff
on 27 Jun 2017
+1 to ajafff - closing as this is a design point of TypeScript.
JoshuaKGoldberg
on 4 Nov 2018
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Most helpful comment
More simple example:
will produce
but no error is expected.