异步调用如何在更新同一个 state 存储变量时避免冲突？

Question

I have a variable called sum=0 then I have a loop that executes asynchronous function 2000 times, each one will return a value. I need to sum all the values returned from the 2000 calls into my variable sum.

How can I guarantee that 2 of the 2000 calls won't access the variable sum at the same time and miscalculate the sum?

For example:

Current sum : 50
Call X fetches value of : 8
Call Y fetches value of : 10

The 2 calls try to add the values they got to the value sum through sum = sum + returnedValue , in that operation they fetch the current value of sum (for both will be 50) and each of them tries to add their own returnedValue at the same time.

So now call X will try to set the value of sum to 58 and call Y will try to set it to 60 (and only one of them will succeed), while the actual value should be 68 .

How does this case get handled?

Answer 1

You're guaranteed that sum will only be updated by a single thread (never two simultaneously) because JavaScript guarantees that only one single thread can access the realm (loosely: the global environment and the stuff within it) in which your variable exists (directly, or indirectly in a function's environment within that global environment).

For Node.js in particular you can find the details here . For browsers, here .

For a long time, the JavaScript spec was silent on this and so in theory it was down to implementations, but popular implementations (those in browsers and in Node.js) used a single thread when running the JavaScript code within a realm, and doing that dramatically simplifies some kinds of things (such as your concurrent access situation), so the common near-universal practice was codified in the specification. (Which proved a very useful precursor to the subsequent addition of shared memory — memory shared between realms and thus, potentially, between threads.)