What is the return value of the expression in C?

Today, when I was learning JavaScript, I found a syntax like this (A = "haha ). length; in this way, the length of the Haha string can be output. However, when I use C #, the Branch also uses this expression to read the TXT text. I used to understand this usage well, but now I think this syntax is strange and awkward, so I have a reason to explain it. I will not talk much about the text to explain. Let's look at the following code.

See what results can be returned by an expression.

String S, S1;

S = S1 = "Haha"; // The Code assigns the Haha string to both the S and S1 variables.

S = (S1 = "Haha"); // There is no difference between this code and the above Code. I just added a bracket to the next value assignment statement and changed the priority, but the results are exactly the same,

The code above shows that,A value indicates that the value is returned.