only allow value from certain object as type

June 27, 2023

I want to use MyBar as an enum replacement. So in my function testme I want that the parameter value can only be 1|2. How can I achieve this?

const MyBar = {
  Benchpress : 1,
  Squats : 2
};

testMe(MyBar.Squats); // Error Argument number is not  assignable to parameter `Benchpress| Squats`

// I would like to use it like this:
testMe(MyBar.Squats);
testMe(2);
// And this should be an error 
testMe(3);


function testMe(value:  keyof typeof  MyBar)
{
  console.log(value);
}

>Solution :

Use the custom ValueOf Type with a combination of as const

const MyBar = {
  Benchpress : 1,
  Squats : 2
} as const;

type ValueOf = typeof MyBar[keyof typeof MyBar];

testMe(MyBar.Squats); // works
testMe(2); // works
testMe(3); // doesn't work


function testMe(value: ValueOf) {
  console.log(value);
}