Create an interface property value that behaves like Array.join(), but only accepts values from the keyof keyword

I am currently working with two interfaces:

interface A {
  foo: string;
  bar: string;
  baz: string;
}

and:

interface B {
  field: keyof A;
}

With interface B, I can set the field as 'foo' like this:

const b: B = {
  field: 'foo'
}

Now, I am trying to figure out a way to change the type of field in interface B so that I can set values that are combinations of any two keys joined with a dot, such as:

const c: B = {
  field: 'foo.bar'
}

I have been experimenting with different approaches but haven't found a viable solution yet. Any help or suggestions would be greatly appreciated. Thank you!

typescript

Answer №1

If you want to leverage the power of template literal types, you’re in for a treat.

These types are an extension of string literal types, allowing for expansion into multiple strings through unions.

With a syntax reminiscent of template literal strings in JavaScript, they are specifically designed for type contexts. When combined with specific literal types, a template literal creates a new string literal type by merging the contents together.

[...]

When a union is applied in the interpolated position, the resulting type encompasses every potential string literal from each union member.

[...]

For every interpolated position within the template literal, the unions are intertwined.

interface A {
  foo: string;
  bar: string;
  baz: string;
}

interface B {
  field: `${keyof A}.${keyof A}`;
}

const c: B = {
  field: 'foo.bar'
}

Check it out on Playground

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