TypeScript: use a nested, constant object definition with a type for inferred type definition-CodePudding

I have a static object definition that I would like to ensure conforms to a type but also be able to use the constant definition for inferred types. For example:

const TREE_BRANCH_TYPES = ['Type1', 'Type2'] as const;

type TreeBranchType = typeof TREE_BRANCH_TYPES[number];
type TreeRoot = Record<TreeBranchType, TreeBranch>;
type TreeBranch = { leaves: readonly TreeLeaf[] };
type TreeLeaf = { name: string };

const tree: TreeRoot = {
    Type1: {
        leaves: [
            { name: 'Type1-Leaf1' },
            { name: 'Type1-Leaf2' },
        ]
    },
    Type2: {
        leaves: [
            { name: 'Type2-Leaf1' },
            { name: 'Type2-Leaf2' },
        ]
    }
} as const;

type TreeLeafName = typeof tree[TreeBranchType]['leaves'][number]['name'];

function findLeaf(type: TreeBranchType, name: TreeLeafName): TreeLeaf | undefined {
    return tree[type].leaves.find(leaf => leaf.name === name);
}

Given the above, the compiler will require tree to conform to the TreeRoot definition, but TreeLeafName is interpreted by the compiler as type TreeLeafName = string.

However, if I remove the type from tree (i.e. const tree = { ... };), then TreeLeafName is recognized by the compiler as type TreeLeafName = "Type1-Leaf1" | "Type1-Leaf2" | "Type2-Leaf1" | "Type2-Leaf2".

Is there a way to get the best of both worlds? Type safety on the tree constant definition and type safety of the inferred TreeLeafName type?

CodePudding user response：

The easiest way to do this with TS4.9 and above is to use the satisfies operator to check that a value is assignable to a type without widening/upcasting it to that type:

const tree = {
    Type1: {
        leaves: [
            { name: 'Type1-Leaf1' },
            { name: 'Type1-Leaf2' },
        ]
    },
    Type2: {
        leaves: [
            { name: 'Type2-Leaf1' },
            { name: 'Type2-Leaf2' },
        ]
    }
} as const satisfies TreeRoot;

type TreeLeafName = typeof tree[TreeBranchType]['leaves'][number]['name'];
// type TreeLeafName = "Type1-Leaf1" | "Type1-Leaf2" | "Type2-Leaf1" | "Type2-Leaf2"

Playground link to code