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Chris Tate
3195f98401
A const initialized by an array literal and spread once can still run that spread more than once: in a loop body, loop test or update, a function, or a class field initializer. Each run inserts the same elements in JavaScript, so copying them into the destination's layout would break identity. Those spreads keep the element-layout refusal.
20 lines
1.1 KiB
TypeScript
20 lines
1.1 KiB
TypeScript
// Spreading a fresh array literal whose elements are typed by their own
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// literal (an omitted member reads `undefined`) into an array of the
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// declared element type converts the elements; nothing else can reach
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// them.
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type Result = { message: string; file: { path: string } | undefined; noisy: boolean };
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const results: Result[] = [{ message: "first", file: { path: "/a" }, noisy: false }];
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const spam = results.length > 0 ? [{ message: "spam", file: undefined, noisy: true }] : [];
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const more: Result[] = [{ message: "m", file: undefined, noisy: false }];
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results.unshift(...spam, ...more);
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const tail = results.length > 5 ? [] : [{ message: "tail", file: undefined, noisy: false }];
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results.push(...tail);
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console.log(results.map((r) => `${r.message}:${r.file?.path ?? "-"}:${r.noisy}`).join(" "));
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console.log(JSON.stringify(results[1]));
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// A const declared in the loop body is a new array on every iteration.
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for (let i = 0; i < 2; i++) {
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const step = [{ message: `step${i}`, file: undefined, noisy: false }];
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results.push(...step);
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}
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console.log(results.map((r) => r.message).join(" "), results[results.length - 1] === results[results.length - 2]);
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