920. As used in this chapter, unless the context otherwise indicates, "promise" includes promise, undertaking, contract, or agreement, whether written or oral, express or implied. 921. Every promise made after August 21, 1933, between any employee or prospective employee and his employer, prospective employer or any other person is contrary to public policy if either party thereto promises any of the following: (a) To join or to remain a member of a labor organization or to join or remain a member of an employer organization, (b) Not to join or not to remain a member of a labor organization or of an employer organization, (c) To withdraw from an employment relation in the event that he joins or remains a member of a labor organization or of an employer organization. Such promise shall not afford any basis for the granting of legal or equitable relief by any court against a party to such promise, or against any other persons who advise, urge, or induce, without fraud or violence or threat thereof, either party thereto to act in disregard of such promise. 922. Any person or agent or officer thereof who coerces or compels any person to enter into an agreement, written or verbal, not to join or become a member of any labor organization, as a condition of securing employment or continuing in the employment of any such person is guilty of a misdemeanor. 923. In the interpretation and application of this chapter, the public policy of this State is declared as follows: Negotiation of terms and conditions of labor should result from voluntary agreement between employer and employees. Governmental authority has permitted and encouraged employers to organize in the corporate and other forms of capital control. In dealing with such employers, the individual unorganized worker is helpless to exercise actual liberty of contract and to protect his freedom of labor, and thereby to obtain acceptable terms and conditions of employment. Therefore it is necessary that the individual workman have full freedom of association, self-organization, and designation of representatives of his own choosing, to negotiate the terms and conditions of his employment, and that he shall be free from the interference, restraint, or coercion of employers of labor, or their agents, in the designation of such representatives or in self-organization or in other concerted activities for the purpose of collective bargaining or other mutual aid or protection.