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Refine no-unnecessary-type-assertion rule to not flag necessary assertions #3120

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Aug 17, 2017
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Refine no-unnecessary-type-assertion rule to not flag necessary assertions #3120

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3730704
Refine no-unnecessary-type-assertion rule to not flag necessary asser…
calebegg Aug 11, 2017
04ac2d4
Fix lint errors.
calebegg Aug 11, 2017
f17e6bf
Use isObjectType to avoid cast
calebegg Aug 14, 2017
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10 changes: 10 additions & 0 deletions src/rules/noUnnecessaryTypeAssertionRule.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,7 @@
* limitations under the License.
*/

import { isObjectFlagSet, isObjectType, isTypeFlagSet } from "tsutils";
import * as ts from "typescript";
import * as Lint from "../index";

Expand Down Expand Up @@ -65,6 +66,15 @@ class Walker extends Lint.AbstractWalker<void> {
return;
}

if (node.kind !== ts.SyntaxKind.NonNullExpression &&
(isTypeFlagSet(castType, ts.TypeFlags.Literal) ||
isObjectType(castType) &&
isObjectFlagSet(castType, ts.ObjectFlags.Tuple))) {
// It's not always safe to remove a cast to a literal type or tuple
// type, as those types are sometimes widened without the cast.
return;
}

const uncastType = this.checker.getTypeAtLocation(node.expression);
if (uncastType === castType) {
this.addFailureAtNode(node, Rule.FAILURE_STRING, node.kind === ts.SyntaxKind.TypeAssertionExpression
Expand Down
10 changes: 10 additions & 0 deletions test/rules/no-unnecessary-type-assertion/test.ts.fix
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Expand Up @@ -69,3 +69,13 @@ function func(aOrB: TypeA|TypeB) {
let z = aOrB;
}
}

// Expecting no warning for these assertions as they are not unnecessary.

type Bar = 'bar';
const data = {
x: 'foo' as 'foo',
y: 'bar' as Bar,
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I don't get why you need this behavior. A type annotation would also work

const data = {x: 'foo', y: Bar} = {x: 'foo', y: bar}

Same goes for the arrow function below: just add the return type:

[1, 2, 3, 4, 5].map((x): [number, string] => [x, 'A' + x]);

I guess it's useful if the assertion is only a little part of a bigger object where you don't want to write the whole type?

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It's a relatively common pattern in our very large codebase where we're trying to turn on this rule. I don't think there's anything so wrong with the pattern that it would make sense to ban it. It keeps you from having to repeat the keys, at least.

Skimming through my list of breakages, I also see:

const FOO = {
  bar: 'a' as 'a',
  baz: 'stuff',
  // ... many more properties without 'as'

i.e., not all properties are cast like this. For those I would hate to have to make it so much more verbose.

I also see instances of:

for (const foo of ['bar' as 'bar', 'baz' as 'baz']) { /* ... */ }

Also, a search turned up a match in tslint itself:
https://github.com/palantir/tslint/blob/master/src/linter.ts#L110

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That makes sense, thank you for the detailed explanation

}

[1, 2, 3, 4, 5].map(x => [x, 'A' + x] as [number, string]);
10 changes: 10 additions & 0 deletions test/rules/no-unnecessary-type-assertion/test.ts.lint
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Expand Up @@ -86,3 +86,13 @@ function func(aOrB: TypeA|TypeB) {
~~~~~~~~~~~ [This assertion is unnecessary since it does not change the type of the expression.]
}
}

// Expecting no warning for these assertions as they are not unnecessary.

type Bar = 'bar';
const data = {
x: 'foo' as 'foo',
y: 'bar' as Bar,
}

[1, 2, 3, 4, 5].map(x => [x, 'A' + x] as [number, string]);