How does this work??
a = b can be seen as both a statement and an expression.
The result of the expression is b.
In other words,
a = b = c;
which can be written as
a = (b = c);
is equivalent to
b = c;
a = b;
Thus your code is equivalent to:
this.slug = this.sliceHashFromHref(href);
document.location.hash = this.slug;