What does this syntax mean?

There is a code like this
return this.len += (this.tail = this.tail.next = new f((t >>>= 0) < 128 ? 1 : t < 16384 ? 2 : t < 2097152 ? 3 : t < 268435456 ? 4 : 5,t)).len, this
This is the body of the function, how to understand "return number, this"?
Thank you in advance for your help, if the answer is obvious please send it to the dock.
Is it some kind of syntactic sugar?JavaScript Anonymous, Mar 28, 2020 
Apparently, it always returns this, but what is in brackets is the socalled side effect. Third party action.
TykAnonymous 
Very similar to code compressed via terser.js or something similar.
If you expand it a little, it will look like this:let a;
t >>>= 0;
if(t < 128) a = 1;
else if(t < 16384) a = 2;
else if(t < 2097152) a = 3;
else if(t < 268435456) a = 4;
else a = 5;
this.tail.next = new f(a, t);
this.tail = this.tail.next;
this.len += this.tail.len;
return this;
Well, the comma operator Vadim has already given a linkAnonymous
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