function isNumber(x: any): x is number { return typeof x === "number"; } function isString(x: any): x is string { return typeof x === "string"; } function padLeft(value: string, padding: string | number) { if (isNumber(padding)) { return Array(padding + 1).join(" ") + value; } if (isString(padding)) { return padding + value; } throw new Error(`Expected string or number, got '${padding}'.`); }