How to write so that an object is not equal to undefined?

Question

typescript

const obj: {
    [ key: string ]: number
} = {
    "RUB": 83
}

const txt: {
    left: string;
} = {
    left: "RUB"
};

const qq = 1000 / obj[txt.left];

When dividing, it crashes, the object is undefined. How to properly handle such cases?

Made this workaround:

const ww = obj[txt.left] as number
const qq = 1000 / ww;