CODEWITHSUNDEEP
javascript
Mucahit Sancar Kahveci
Mucahit Sancar Kahveci

Reputation: 343

What happens when two javascript events try to modify same variable at the same time

There is a javascript code that has variable messageList keeping the messages list and this can be updated by a function updateMessageList() as below:

var messageList=[];                           //contains list of messages
function updateMessageList(id, lastUpdate);   //updates messageList item for given id

//at some point while running
mesasgeList=[
    {id:1, lastUpdate:1371650818000},
    {id:2, lastUpdate:1371650821000},
    .....
]

What happens if two different sources call updateMessageList() function at he same time for same messageList item. Let's say one update for message with id:1 comes from client (current user updating the message) and another from the server also for message with id:1 (another user updates the message). They will both then try to access and modify messageList[0].lastUpdate property at the same time.

In Source 1: When the client updates the message form submit event triggers the function handleReply()

function handleReply(event) {
    .....
    var id=event.currentTarget.find('input[name="id"]').val();
    //form has an hidden input for id of the message
    var d = new Date();
    updateMessageList(id, d.getTime());
    .....
}

In Source 2: JS makes a AJAX call to server to check if there are any updates, and than runsupdateMessages() function in return

//server sends following JSON data:{id:1, lastUpdate:1371650823269}
function updateMessages(data) {
    .....
    updateMessageList(data.id, data.lastUpdate);
    .....
}

The updateMessageList() functions will both try to change lastUpdate property of messageList[0].

So should we think these kind of situations while programming or does javascript can handle these?

Upvotes: 2

Views: 798

Answers (1)

000
000

Reputation: 27247

Javascript is single-threaded. There is no such thing as concurrency in javascript. One call will always be before or after the other. Maybe it can differ by a millisecond, but one call will exit its block before the other begins.

Upvotes: 8

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