Since I met the out keyword in C#, I’m trying to find an equivalent approach in JS.

So, please take a look at the code and leave your opinions of existing cases or suggestions of new ways to implement this.

The working version is inside this sandbox:

1 null on fail

const number_1 = tryParseNullOnFail(numberAsString);

if (null !== number_1) {
  page.log(`#1 Converted '${numberAsString}' to ${number_1}`);
} else {
  page.log(`Failed to convert '${numberAsString}'`);
}

2 kvp response

const { isNumber: isNumber_2, number: number_2 } = tryParseKvp(numberAsString);

if (isNumber_2) {
  page.log(`#2 Converted '${numberAsString}' to ${number_2}`);
} else {
  page.log(`Failed to convert '${numberAsString}'`);
}

3 by reference

const number_3 = [];

if (tryParseByReference(numberAsString, number_3)) {
  page.log(`#3 Converted '${numberAsString}' to ${number_3[0]}`);
} else {
  page.log(`Failed to convert '${numberAsString}'`);
}

4 condition expression

if (null != (number_4 = tryParseCondExp(numberAsString))) {
  page.log(`#4 Converted '${numberAsString}' to ${number_1}`);
} else {
  page.log(`Failed to convert '${numberAsString}'`);
}

5 IIFE short cercuit

const number_5 =
  tryParseNullOnFail(numberAsString) ??
  (() => {
    page.log(`Failed to convert '${numberAsString}'`);
  })();

page.log(`#5 Converted '${numberAsString}' to ${number_5}`);

6 high order function

function withTryParse(str, onSuccess, onFail) {
  const number = Number(str);
  isNumber(number) ? onSuccess(number) : onFail(str);
}

const onSuccess = (number_6) => {
  page.log(`#6 Converted '${numberAsString}' to ${number_6}`);
};

const onFail = (str) => {
  page.log(`Failed to convert '${str}'`);
};

withTryParse(numberAsString, onSuccess, onFail);

The mad professor

for (
  var done = false, number_ = tryParseMad(numberAsString);
  !done && null !== number_;
  done = true
) {
  page.log(`#* Converted '${numberAsString}' to ${number_}`);
}