// this example requires "noUncheckedIndexedAccess" to be enabled to make sense

function someNumberFunction(_: number) {}

const myArray: number[] = []
myArray[5] = 5

for(let i = 0; i < myArray.length; ++i) {
    const v = myArray[i]   // v is "number | undefined"...
    if (v != null) {       // ... so it needs to be checked for undefined
        someNumberFunction(v)
    }
}

for(const v of myArray) {
    // v is "number" according to TypeScript...
    // ... so we can erroneously call the function without checking for undefined
    someNumberFunction(v)
}