Frontend Weekly 2024-06-27

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function isNumber(value: number | string): value is number {
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  return typeof value === "number";
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}

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// TypeScript 5.4
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const values = [2, null, 3].filter((x) => x !== null);
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// => (number | null)[]
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const values = [2, null, 3].filter((x): x is number => x !== null);
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// => number[]
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// TypeScript 5.5
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const values = [2, null, 3].filter((x) => x !== null);
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// => number[]

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function isDog(x: Animal): x is Dog {
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  return x instanceof Cat;
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}

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function f1(obj: Record<string, unknown>, key: string) {
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  if (typeof obj[key] === "string") {
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    // TypeScript 5.4: obj[key]がunknownとして推論されエラーが出ていた。
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    // TypeScript 5.5: obj[key]がstringに絞り込まれる。
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    obj[key].toUpperCase();
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  }
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}

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function f1(obj: Record<string, unknown>, key: string) {
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  const x = obj[key];
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  if (typeof x === "string") {
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    x.toUpperCase();
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  }
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}

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/** @import { Linter } from "eslint" */
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/** @type { Linter.FlatConfig[] } */
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const config = [];
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export default config;

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/** @type { import("eslint").Linter.FlatConfig[] } */
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const config = [];
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export default config;

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{
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    "compilerOptions": {
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        "typeRoots": [
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            "${configDir}/node_modules/@types"
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            "${configDir}/custom-types"
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        ],
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        "outDir": "${configDir}/dist"
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    }
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}