Typescript (previously) can't narrow using const variables

Prior to https://github.com/microsoft/TypeScript/pull/44730, it was not possible to narrow conditional expressions that are referenced in a const. They had to be inserted directly into the if statement, which added some duplication to the code. As of Typescript 4.4, the control flow analysis has been updated so this is possible:

function f1(x: unknown) {
    const isString = typeof x === 'string';
    if (isString) {
        x.length;  // Ok
    }
}

[[20210705102809-ts-narrowing]]

[[ts]]