104 lines
2.4 KiB
Plaintext
104 lines
2.4 KiB
Plaintext
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---
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description: 'Disallow explicit type declarations for variables or parameters initialized to a number, string, or boolean.'
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---
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> 🛑 This file is source code, not the primary documentation location! 🛑
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>
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> See **https://typescript-eslint.io/rules/no-inferrable-types** for documentation.
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TypeScript is able to infer the types of parameters, properties, and variables from their default or initial values.
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There is no need to use an explicit `:` type annotation on one of those constructs initialized to a boolean, number, or string.
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Doing so adds unnecessary verbosity to code -making it harder to read- and in some cases can prevent TypeScript from inferring a more specific literal type (e.g. `10`) instead of the more general primitive type (e.g. `number`)
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## Examples
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<!--tabs-->
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### ❌ Incorrect
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```ts
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const a: bigint = 10n;
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const a: bigint = BigInt(10);
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const a: boolean = !0;
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const a: boolean = Boolean(null);
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const a: boolean = true;
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const a: null = null;
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const a: number = 10;
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const a: number = Infinity;
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const a: number = NaN;
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const a: number = Number('1');
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const a: RegExp = /a/;
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const a: RegExp = new RegExp('a');
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const a: string = `str`;
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const a: string = String(1);
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const a: symbol = Symbol('a');
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const a: undefined = undefined;
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const a: undefined = void someValue;
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class Foo {
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prop: number = 5;
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}
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function fn(a: number = 5, b: boolean = true) {}
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```
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### ✅ Correct
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```ts
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const a = 10n;
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const a = BigInt(10);
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const a = !0;
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const a = Boolean(null);
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const a = true;
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const a = null;
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const a = 10;
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const a = Infinity;
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const a = NaN;
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const a = Number('1');
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const a = /a/;
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const a = new RegExp('a');
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const a = `str`;
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const a = String(1);
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const a = Symbol('a');
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const a = undefined;
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const a = void someValue;
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class Foo {
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prop = 5;
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}
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function fn(a = 5, b = true) {}
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```
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<!--/tabs-->
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## Options
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### `ignoreParameters`
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When set to true, the following pattern is considered valid:
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```ts option='{ "ignoreParameters": true }' showPlaygroundButton
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function foo(a: number = 5, b: boolean = true) {
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// ...
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}
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```
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### `ignoreProperties`
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When set to true, the following pattern is considered valid:
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```ts option='{ "ignoreProperties": true }' showPlaygroundButton
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class Foo {
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prop: number = 5;
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}
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```
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## When Not To Use It
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If you strongly prefer to have explicit types regardless of whether they can be inferred, this rule may not be for you.
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## Further Reading
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- [TpeScript Inference](https://www.typescriptlang.org/docs/handbook/type-inference.html)
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