Frontend Weekly 2023-11-24

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import obj from "./something.json" with { type: "json" };
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import * as foo from "./foo.js" with { type: "fluffy bunny" };
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const obj = await import("./something.json", {
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  with: { type: "json" },
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});

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function f(x: unknown) {
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  switch (true) {
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    case typeof x === "string":
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      // 'x' is a 'string' here
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      console.log(x.toUpperCase());
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    // falls through...
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    case Array.isArray(x):
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      // 'x' is a 'string | any[]' here.
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      console.log(x.length);
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    // falls through...
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    default:
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    // 'x' is 'unknown' here.
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    // ...
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  }
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}

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interface A {
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  a: string;
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}
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interface B {
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  b: string;
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}
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type MyType = A | B;
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function isA(x: MyType): x is A {
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  return "a" in x;
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}
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// <=5.2
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function someFn(x: MyType) {
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  if (isA(x)) {
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    console.log(x.a); // works!
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  }
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}
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// <=5.2
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function someFn(x: MyType) {
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  if (isA(x) === true) {
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    console.log(x.a); // not works
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  }
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}
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// 5.3
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function someFn(x: MyType) {
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  if (isA(x) === true) {
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    console.log(x.a); // works!
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  }
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}