【JSリファレンス】Map.prototype.keys()

const mapObj = new Map();

const obj1 = {name:"obj1"};
const obj2 = {name:"obj2"};

mapObj.set( obj1 , 100 );
mapObj.set( obj2 , 200 );

for( const key of mapObj.keys() ){
    console.log( `key.name:"${ key.name }"` );
}
// 結果
// key.name:"obj1"
// key.name:"obj2"

const mapObj = new Map( [
        [1,"A"],
        [2,"B"],
        [3,"C"]
    ]);

const keysArray = [...mapObj.keys()];
console.log( Array.isArray( keysArray ) ); // true
console.log( keysArray ); // [ 1, 2, 3 ]