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From: Anton Shepelev <anton.txt@gmail.com>
Newsgroups: comp.lang.javascript
Subject: input/output parameters
Date: Sat, 13 Mar 2021 15:32:32 +0300
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Hello, all

I started learning JavaScript but yesterday, but am already
stuck at implementing input/output parameters. As I
understand, they are not part of the language, yet I find
them very convenient for normal procedural programming.  For
example, they allow returning a boolean flag and using
functions inside `if` tests, similar to the very convenient
TryParse() and TryGetValue() functions in C#:

   TryParse:    https://docs.microsoft.com/en-us/dotnet/api/system.int32.tryparse
   TryGetValue: https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.dictionary-2.trygetvalue

Unfortunately for me, JavaScript seems to compensate the
lack of in/out parameters with much more complicated
functional and object-oriented mechanisms. I therefore have
tried to implement them myself, but have not come up with a
satisfactory solution. The examples below demonstrate a
function for integer division that stores the result in
variable and returns a boolean value indicating whether the
division was exact:

// for bi-direction in/out parameters, return a code that
// creates an anonymous object storing the the value *and*
// setter clousure:

function out_1(vn)
{   return "x => " + vn + " = x";  }

function int_div_1(a, b ,c)
{   var /*integer */ r; // ratio
    var /*boolean */ x; // result is exact

    r = Math.floor(a / b);
    x = r * b === a;
    c( r );
    return x;
}

function out_2( vn, val, ret )
{   return `${vn} = ${val}, ${ret}`;  }

function int_div_2(a, b, vn)
{   var /*integer */ r; // ratio
    var /*boolean */ x; // result is exact

    r = Math.floor(a / b);
    x = r * b === a;
    return out_2(vn, r, x);
}

{   let /*integer */ ratio;
    let /*boolean */ exact;

    exact = int_div_1(7, 3, eval(out_1("ratio")));
    console.log(`7/3: ratio = ${ratio}; exact = ${exact}.`);

    exact = int_div_1(6, 3, eval(out_1("r")));
    console.log(`6/3: ratio = ${ratio}; exact = ${exact}.`);

    exact = eval(int_div_2(7, 3, "ratio"));
    console.log(`7/3: ratio = ${ratio}; exact = ${exact}.`);

    exact = eval(int_div_2(6, 3, "ratio"));
    console.log(`6/3: ratio = ${ratio}; exact = ${exact}.`);
}

Can you think of better way to emulate output parameters,
and preferable also bi-directional input/output parameters?

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