DeepSource

When writing asynchronous code, it is possible to create subtle race condition bugs. Consider the following example:

let totalLength = 0;

async function addLengthOfSinglePage(pageNum) {
  totalLength += await getPageLength(pageNum);
}

Promise.all([addLengthOfSinglePage(1), addLengthOfSinglePage(2)]).then(() => {
  console.log('The combined length of both pages is', totalLength);
});

This code looks like it will sum the results of calling getPageLength(1) and getPageLength(2), but in reality the final value of totalLength will only be the length of one of the two pages. The bug is in the statement totalLength += await getPageLength(pageNum);. This statement first reads an initial value of totalLength, then calls getPageLength(pageNum) and waits for that Promise to fulfill. Finally, it sets the value of totalLength to the sum of await getPageLength(pageNum) and the initial value of totalLength. If the totalLength variable is updated in a separate function call during the time that the getPageLength(pageNum) Promise is pending, that update will be lost because the new value is overwritten without being read. One way to fix this issue would be to ensure that totalLength is read at the same time as it's updated, like this:

async function addLengthOfSinglePage(pageNum) {
  const lengthOfThisPage = await getPageLength(pageNum);

  totalLength += lengthOfThisPage;
}

Another solution would be to avoid using a mutable variable reference at all:

Promise.all([getPageLength(1), getPageLength(2)]).then(pageLengths => {
  const totalLength = pageLengths.reduce((accumulator, length) => accumulator + length, 0);

  console.log('The combined length of both pages is', totalLength);
});

Bad Practice

let result;
async function foo() {
  result += await somethingElse;
  result = result + await somethingElse;
  result = result + doSomething(await somethingElse);
}

function* bar() {
  result += yield;
  result = result + (yield somethingElse);
  result = result + doSomething(yield somethingElse);
}

let result;
async function foo() {
  result = await somethingElse + result;
  const tmp = await somethingElse;
  result += tmp;

  const localVariable = 0;
  localVariable += await somethingElse;
}

function* bar() {
  result += yield;
  result = (yield somethingElse) + result;
  result = doSomething(yield somethingElse, result);
}

JavaScript

Race condition in compound assignment JS-0040

Bad Practice

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