As far I know primitive values (e.g. numbers, string, …) are passed by value.

mystr:string="abc";

chgstr (x:string)
{
    x = "xxx";
}

console.log (this.mystr);       // abc
this.chgstr (this.mystr);
console.log (this.mystr);       // abc -> no change!

Complex objects (e.g. objects, arrays) are passed by reference.

myobj:any={abc:123};
    
chgobj (x:any)
{
    x["abc"]="000";
}

console.log (this.myobj);       // Object {abc: 123}
this.chgobj (this.myobj);
console.log (this.myobj);       // Object {abc: 000} -> changed

Is it good style to expect a parameter to be a complex type and use the pass-by-reference (e.g. my sample of chgobj) or would it be better to explicitly return the changes object?

chgobj (x:any):any
{
    x["abc"]="000";
    return x;   // return!
}

myobj = this.chgobj (this.myobj);    // use the return-value