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In createNodeArray, create a fresh array (2) #28441

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In createNodeArray, create a fresh array (2) #28441

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12 changes: 8 additions & 4 deletions src/compiler/factory.ts
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Expand Up @@ -21,11 +21,15 @@ namespace ts {
* Make `elements` into a `NodeArray<T>`. If `elements` is `undefined`, returns an empty `NodeArray<T>`.
*/
export function createNodeArray<T extends Node>(elements?: ReadonlyArray<T>, hasTrailingComma?: boolean): NodeArray<T> {
if (!elements || elements === emptyArray) {
elements = [];
if (elements) {
if (isNodeArray(elements)) {
return elements;
}
// elements is a ReadonlyArray, so create a fresh array before adding properties.
elements = elements.slice();
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Is there a scenario that breaks because of this? I think from below code looks like the intention of the code is to make elements array into node array?

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That is generally the purpose of this function, yes. elements was not always a ReadonlyArray, but now that it is it makes sense to make a fresh copy. We should verify the performance/memory characteristics of this change, however.

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Although, mutating the array this way does not actually change its "ReadonlyArray"-ness, just promotes it to a Node Array (which is also a ReadonlyArray).

}
else if (isNodeArray(elements)) {
return elements;
else {
elements = [];
}

const array = <NodeArray<T>>elements;
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