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97 lines
2.6 KiB
Markdown
97 lines
2.6 KiB
Markdown
---
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date: 20201126
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id: 81de1606-f1e2-41e6-a386-103e110436cd
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title: noUncheckedIndexAccess
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---
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# Default
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`false`
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# Description
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TypeScript supports [index signatures](20201008092225-index_signatures).
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These signatures are a way to signal to the type system that users can
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access arbitrarily-named properties:
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``` typescript
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interface Options {
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path: string;
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permissions: number;
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// Extra properties are caught by this index signature.
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[propName: string]: string | number;
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}
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function checkOptions(opts: Options) {
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opts.path // string
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opts.permissions // number
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// These are all allowed too!
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// They have the type 'string | number'.
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opts.yadda.toString();
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opts["foo bar baz"].toString();
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opts[Math.random()].toString();
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}
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```
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When the `--noUncheckedIndexAccess` flag is used, every property access
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(like `foo.bar`) or indexed access (like `foo["bar"]`) that ends up
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resolving to an index signature is considered potentially undefined. Ie:
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`opts.yadda` will have the type `string | number | undefined` as opposed
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to just `string | number`. If you need to access that property, you’ll
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either have to check for its existence first or use a non-null assertion
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operator (the postfix `!` character):
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``` typescript
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// Checking if it's really there first.
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if (opts.yadda) {
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console.log(opts.yadda.toString());
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}
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// Basically saying "trust me I know what I'm doing"
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// with the '!' non-null assertion operator.
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opts.yadda!.toString();
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```
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One consequence of using `--noUncheckedIndexedAccess` is that indexing
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into an [array](20200929162129-arrays) is also more strictly checked,
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even in a bounds-checked loop:
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``` typescript
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function screamLines(strs: string[]) {
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// this will have issues
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for (let i = 0; i < strs.length; i++) {
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console.log(strs[i].toUpperCase());
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// ~~~~~~~
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// error! Object is possibly 'undefined'.
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}
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}
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```
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If you don’t need the indexes, you can iterate over individual elements
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by using a [for-of](20201030093304-javascript_for_of) loop or a forEach
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call:
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``` typescript
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function screamLines(strs: string[]) {
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// this works fine
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for (const str of strs) {
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console.log(str.toUpperCase());
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}
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// this works fine
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strs.forEach(str => {
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console.log(str.toUpperCase());
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});
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}
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```
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This flag can be handy for catching out-of-bounds errors, but it might
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be noisy for a lot of code, so it is not automatically enabled by the
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`--strict` flag. More information can be found in the PR[^1].
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# Footnotes
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[^1]: <https://github.com/microsoft/TypeScript/pull/39560>
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