In Sartre's work entitled Existentialism Is a Humanism (1946), Sartre backed away from so radical a subjectivism by suggesting a version of Kant’s idea that moral judgments be applied universally. He does not reconcile this view with conflicting statements elsewhere in his writings, and it is doubtful whether it represents his final ethical position.
I think the book of his wife can much help to undrestand Sartre's philosophy about moral roles and reason.
In The Ethics of Ambiguity authored by Simone de Beauvoir
De Beauvoir, characterized existentialism as "austere, sad, but not evasive."
She said, existentialism does not offer to reader the consolations of an abstract evasion : existentialism proposees no evasion. On the contrary, its ethics is experienced in truth of life and it then appears as the only proposition of salvation which one can address to men.
Hare’s espousal of the view that moral judgments are prescriptions led reviewers of his first book to classify him with the emotivists as one who did not believe in the possibility of using reason to arrive at ethical conclusions. That this was a mistake became apparent with the publication of his second book, Freedom and Reason (1963). The aim of this work was to show that the moral freedom guaranteed by prescriptivism is, notwithstanding its element of choice, compatible with a substantial amount of reasoning about moral judgments. Such reasoning is possible, Hare wrote, because moral judgments must be “universalizable.” This notion owed something to the ancient Golden Rule and even more to Kant’s first formulation of the categorical imperative. In Hare’s treatment, however, these ideas were refined so as to eliminate their obvious defects. Moreover, for Hare universalizability was not a substantive moral principle but a logical feature of moral terms. This means that anyone who uses words such as right and ought is logically committed to universalizability.