Why typescript complains that XXX is assignable to the constraint of type T but T could be instantiated with a different subtype of constraint X

0 votes

My code is this:

export interface TreeItem {
  id: string;
  children: this[];
  collapsed?: boolean;
}

const createTreeItem = <T extends TreeItem>(): T => {
  return {
    id: 'root',
    children: []
  }
}

But I get an error on the return type of createTreeItem which is the following:

TS2322: Type '{ id: string; children: never[]; }' is not assignable to type 'T'.   '{ id: string; children: never[]; }' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'TreeItem'.

I have absolutely no idea what does this mean.

Jul 18, 2022 in TypeSript by Logan
• 2,140 points
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