Or HaChaim on Genesis 47:30/אור החיים על בראשית מ״ז:ל׳

1 ויאמר אנכי אעשה כדבריך. He said: "l will do in accordance with your instructions." Why did Joseph have to announce that he was going to do all this? All he had to do was to place his hand where his father had asked and make the promise and that would be his reply to his father's request. This is what Eliezer did when Abraham made him swear an oath concerning his selecting a wife for Isaac (Genesis 24,9).

2Actually, the Torah mentions Joseph's declaration as proof that he said he did not need to swear an oath, that his promise was sufficient.

3Alternatively, he meant that he was legally obligated to comply with his father's request either by dint of being his father's son or by dint of his father's request being that of a man about to die (compare Choshen Mishpat 252,2). Joseph added the word כדברך to demonstrate that he would fulfil the request quite independent of any oath he would swear.