After shifting an array This condition will always evaluate to false

I’m writing some code for work, and am having an issue with typescript not realizing that I’ve changed the array.

function example() {
  const arr = [1,2,3,4,5,6,7];
  switch (arr[0]) {
    case 1:
      arr.shift();
      if (arr[0] === 2) {// Typescript complains that this will always evaluate to false
        console.log(2);
      }
      break;
  }
}

I know that I can get around this by casting arr[0] as a number, but is there any other way to inform typescript that I’ve modified the array?

Sorry if this is a duplicate, I’ve googled a bit before posting here and couldn’t find any relevant discussion.

>Solution :

Seams like you found a bug !

The assumption the compiler make is true without the the shift because arr[0] is narrowed to 1 thanks to the switch case.

But since shift() changes the array in place, the compiler is making a wrong assumption here.

function example() {
  const arr = [1,2,3,4,5,6,7];
  switch (arr[0]) {
    case 1:
      arr.shift();
      const two = arr[0]; // 
        //  ^? '1'
      console.log(two); // 2
      break;
  }
}

Playground

Yup, it’s a known bug (pointed by this one closed as duplicate)