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Only use 'es2015.iterable' to avoid accidental es2015 feature usage. #28951

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Dec 11, 2018
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Only use 'es2015.iterable' to avoid accidental es2015 feature usage. #28951

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cc477ee
Only use 'es2015.iterable' to avoid accidental es2015 feature usage.
DanielRosenwasser Dec 11, 2018
28c3663
Don't use 'fill' in 'fill'.
DanielRosenwasser Dec 11, 2018
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6 changes: 5 additions & 1 deletion src/compiler/core.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -2165,6 +2165,10 @@ namespace ts {
}

export function fill<T>(length: number, cb: (index: number) => T): T[] {
return new Array(length).fill(0).map((_, i) => cb(i));
const result = Array<T>(length);
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At least in V8 this causes the array to be considered "holey" and therefore it's not as optimized as packed arrays.
On the other hand repeatedly calling push needs to grow the backing store.

I'd say this is fine as is, as this function is only used in a single place in generateTypes.ts. So the holey array won't cause too much deoptimization where it's used. The alternative using push wouldn't be a problem either because the arrays are unlikely to contain more than a dozen elements.

for (let i = 0; i < length; i++) {
result[i] = cb(i);
}
return result;
}
}
2 changes: 1 addition & 1 deletion src/tsconfig-base.json
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@@ -1,7 +1,7 @@
{
"compilerOptions": {
"pretty": true,
"lib": ["es2015"],
"lib": ["es2015.iterable", "es5"],
"target": "es5",
"rootDir": ".",

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