It doesn't have a real meaning or function, it's just a playful assertion at the end of a sentence, like "よ" and "さ” are the same thing, except "さ” is more colloquial. And "の” at the end of a question is used for rhetorical questions or like the speaker challenging the listener to keep up. So yeah, "のさ” isn't a grammatical function, just adds attitude to the sentence.

The particle 「のさ」 gives the reader 2 information. 1. The speaker is a boy. (probably 10 to 25 years old) 2. He thinks he himself is cool. -> The speaker is probably a manga/anime hero, or a pop singer. These lines are of manga/anime, or of lyrics of a song. 　　　　　 ※ 「の」: turning verb/adjective to noun 「さ」：particle to draw the listner's attention. ****** > why sometimes it's used the particle の before さ (especially after verbs). The speaker is： 僕は王様になるのさ。 a boy (10 to 25 years) 僕、王様になるの。 a boy (a child 捜し物を探しに行くの／行くって、どこへ行くの？： 　　　　　　　　　　probably a girl/or a young woman. さあ、行くわよ！ 　　　　　　　　　　probably a girl or a young woman, who is boysh.