TypeScript conditional type resolution and optional properties

What I'm try to obtain:

(I use an example to be more quick and clear)

// a function func which works with all following tests
function func<...>(opts: { inOpt?: boolean }): ... {}

// test suite 1
const t1 = func({ inOpt: false });
t1.outOpt = 0; // OK if lecit
t1.outOpt = "0"; // OK if error
t1.outOpt = undefined; // OPTIONAL OK if error

// test suite 2
const t2 = func({ inOpt: true });
t2.outOpt = 0; // OK if error
t2.outOpt = "0"; // OK if lecit
t2.outOpt = undefined; // OPTIONAL OK if error

// test suite 3
const t3 = func({});
t3.outOpt = 0; // OK if lecit
t3.outOpt = "0"; // OK if error
t3.outOpt = undefined; // OPTIONAL OK if error

What I've tried:

I did several attempts, the function declarations most close to what I need are:

// this makes test suites 1 and 2 to work
function func1<T extends boolean>(opts: { inOpt?: T }): { outOpt?: T extends true ? string : number } {
    return {};
// this makes test suit 3 to work
function func5<T extends boolean, R extends { inOpt?: T }>(opts: R): { outOpt?: R extends { inOpt: T } ? (T extends true ? string : number) : number } {
    return {};

I would have expected that simply func1 to work: if T is not specified it doesn't extends true.

What I need is quite close to the example explained in the official TypeScript documentation, but the ? to make the inOpt optional. What I need more to the official TypeScript documentation example is that when inOpt is not specified it is treated as if it was false, but after several attempts I feel I'm getting stuck.

Here is the Playgroung I used for my tests/attempts.


Looking at the first two comments I received I try to explain by words what I need.

I need a function which takes as argument an object with an optional inOpt attribute of type boolean and returns an object with an optional outOpt attribute of type depending on inOpt value. The requirements is that the outOpt attribute must be:

  • of type number when inOpt is false
  • of type string when inOpt is true
  • of type number when inOpt is not provided (treated as if it was false)

Read more here: https://stackoverflow.com/questions/64945425/typescript-conditional-type-resolution-and-optional-properties

Content Attribution

This content was originally published by Daniele Ricci at Recent Questions - Stack Overflow, and is syndicated here via their RSS feed. You can read the original post over there.

%d bloggers like this: