从 Object.entries 中的条件键推断值的类型

Question

Say I have this JavaScript tree object with this type and shape

type Tree = { [key: string]: number | Tree } & { SIZE: number }

// example
const tree: Tree = {
  a: {
    b: 1,
    SIZE: 1
  },
  c: 2,
  SIZE: 3
}

TypeScript manages without problem to infer that the type of tree[key] is number here

let total = 0
for (const [key, value] of Object.entries(tree)) {
  if (key === 'SIZE') {
    total += tree[key]
  }
}

But can't infer that the type of value is also number here?

let total = 0
for (const [key, value] of Object.entries(tree)) {
  if (key === 'SIZE') {
    total += value    // Operator '+=' cannot be applied to types 'number' and 'number | Tree'.(2365)
  }
}

Shouldn't tree[key] and value be equivalent inside the if-statement?

Have I defined the type of my tree wrong or is there any other simple way that I can make TypeScript infer the type? I'd rather not use type-casting.

Answer 1

In the second example, TypeScript is unable to infer that value is a number because you are not using any operations that would require it to be a number. Instead, TypeScript infers the type of value to be the union type number | Tree number | Tree , because that is the type of the values in the tree object.