Discussion on: A "Gotcha" of JavaScript's Pass-by-Reference

[Adam Nathaniel Davis]

I've been noticing that a lot of people who seem entrenched in the idea that "JS has no pass-by-reference" seem to give me examples from C/C++. So I'm honestly wondering if this is just an artifact of people trained in a particular paradigm then (naturally) clinging to that paradigm even when they move outside their original area??

I was curious about how C++ would define pass-by-reference. The IBM Knowledge Center is one of the first pages that comes up: ibm.com/support/knowledgecenter/SS...

It has some very... interesting detail. It starts off with this:

Pass-by-reference means to pass the reference of an argument in the calling function to the corresponding formal parameter of the called function. The called function can modify the value of the argument by using its reference passed in.

Hmm...

const callingFunction = () => {
  const importantNumber = 3.14;
  const spanishNumbers = { one: 'uno', two: 'dos', three: 'tres' };
  calledFunction(importantNumber, spanishNumbers);
  console.log('importantNumber', importantNumber); // 42
  console.log('spanishNumbers', spanishNumbers); // { one: 'einz', two: 'dos', three: 'tres' }
}

const calledFunction = (somePrimitive, someObject) => {
  somePrimitive = 42;
  someObject.one = 'einz';
}

callingFunction();

In this JS example, the calling function passes two arguments as parameters to the called function. The called function modifies the value of those arguments. But the change is only reflected on one of those arguments - the object. Why?? Because the object is passed by reference.

The IBM Knowledge Center definition goes on to state that:

The difference between pass-by-reference and pass-by-value is that modifications made to arguments passed in by reference in the called function have effect in the calling function, whereas modifications made to arguments passed in by value in the called function can not affect the calling function. Use pass-by-reference if you want to modify the argument value in the calling function. Otherwise, use pass-by-value to pass arguments.

So look at exactly what happened in my simple JS example. A modification made to an argument passed in by reference in the called function (someObject) has effect in the calling function. Whereas a modification made to an argument passed in by value in the called function (somePrimitive) does not affect the calling function.

There it is, defined by IBM with regard to C++. Even by that definition, JS is passing the object by reference.

[Michael Gaddis]

If C++ prevents reassignment of a reference to another reference through strong typing enforcement, (been decades since I programmed in C++ so those memories are lost, but a brief survey of the web would indicate that references are static once created in C++.) Maybe that difference is what is confusing to C++ folks about JS reference passing and maybe would get at the heart of why they are stuck on the idea that JS doesn't pass by reference. The difference is in strong versus loose typing not passing by reference. Does that make any sense?

If you CAN destroy a reference and reassign it to another object in C++ then an example that shows it's the same would be equally enlightening.

[Adam Nathaniel Davis]

Yes, this does make sense. I do think that the loose/inferred/runtime typing is what twists everyone's heads in knots.