declare function isString(v: any): v is string;
declare function isNumber(v: any): v is number;
declare let value: number;

// this condition will probably always be false, otherwise you have some really messed up code
if (isString(value)) {
    value.charAt(0);
}

// typeguard is useless because `value` is already `number` or a subtype thereof
if (isNumber(value)) {
    value.toFixed(0);
}