GeezerVison

function move(direction: "up" | "down") {
    // ...
}

function do1(command: string) {
    if (command === "up" || command === "down") {
        move(command);  // Narrowed to type "up" | "down"
    }
}

function do2(command: string) {
    switch (command) {
        case "up":
        case "down":
            move(command);  // Narrowed to type "up" | "down"
            break;
    }
}

function f1(x: number, y: 1 | 2) {
    if (x === 0 || x === y) {
        x;  // Narrowed to type 0 | 1 | 2
    }
}

function f2(x: number | "foo" | "bar", y: 1 | 2 | string) {
    if (x === y) {
        x;  // Narrowed to type "foo" | "bar" | 1 | 2
        y;  // Narrowed to type "foo" | "bar" | 1 | 2
    }
}