前端三连问，typescript的?? 和?: 和?.是什么意思？

function echo(x: number, y?: number) {
    return x + (y || 0);
}
getval(1); // 1
getval(1, null); 
// error, 'null' is not assignable to 'number | undefined'
interface IProListForm {
  enterpriseId: string | number;
  pageNum: number;
  pageSize: number;
  keyword?: string; // 可选属性
}

console.log(null || 5)   //5
console.log(null ?? 5)     //5

console.log(undefined || 5)      //5
console.log(undefined ?? 5)      //5

console.log(0 || 5)       //5
console.log(0 ?? 5)      //0

const a = {
      b: { c: 7 }
};
console.log(a?.b?.c);     //7
console.log(a && a.b && a.b.c);    //7