How to wait until a predicate condition becomes true in JavaScript?

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Javascript is single threaded, hence the page blocking behaviour. You can use the deferred/promise approach suggested by others. The most basic way would be to use window.setTimeout. E.g.

function checkFlag() {
    if(flag === false) {
       window.setTimeout(checkFlag, 100); /* this checks the flag every 100 milliseconds*/
    } else {
      /* do something*/
    }
}
checkFlag();

Here is a good tutorial with further explanation: Tutorial

EDIT

As others pointed out, the best way would be to re-structure your code to use callbacks. However, this answer should give you an idea how you can ‘simulate’ an asynchronous behaviour with window.setTimeout.

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