Javascript 如何向分配给变量c的对象文本添加方法，以便满足类型检查？

如何满足下面代码段中的类型检查

interface Counter {
    getCount: () => number
    increment: (n: number) => void
}
let c: Counter = {}

像这样

let c: Counter = {
    getCount: () => 1,
    increment: (n: number) => {}
}