Wrong casting on variable assigned array Element when array is empty

[JSinkler713]

Bug Report

🔎 Search Terms

🕗 Version & Regression Information

• This is a crash
• This changed between versions ______ and _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

let numbersArr = []


if (Math.random() > 0.5) {
  numbersArr.push(123)
  numbersArr.push(29)
  numbersArr.push(20)
}

// says its type number if you hover over but it could be undefined
let awesomeNumber = numbersArr[0];
console.log('typeof awesomeNumber', typeof awesomeNumber)

console.log(awesomeNumber)

// says number could be NaN
let coolNumber = 2 + awesomeNumber

Workbench Repro

### 🙁 Actual behavior

Typescript asserts the element is a number.

### 🙂 Expected behavior

The variable should be a number or undefined. Since the array may be empty and we are accessing the first element inside of it.
The provided is a simple one, but this actually happened with an array of objects. I was trying to access a property on one of the objects, and my app could crash, because sometimes that first element is not there and so it is undefined. I didn't do my optional chaining someObj?.objectProperty that I would normally do if warned my variable was undefined.

Thanks for looking

[RyanCavanaugh]

The option that controls this behavior is called noUncheckedIndexedAccess; you need to enable it if you want the type to be number | undefined

[fatcerberus]

Note that enabling noUncheckedIndexedAccess will also affect indexing in e.g. for loops, which might make the cure worse than the disease (hence why that option isn’t enabled by default)

[JSinkler713]

Thanks folks appreciate the quick responses.