wait-until

This simple Node.js module provides a way to check for a condition every so often, then run a callback after the condition is met or the allotted time expires.

This is good if you need to wait for specific state changes in code that you don't have direct control over, like a headless web browser.

Usage

var waitUntil = require('wait-until');

waitUntil(interval, times, function condition() {
    return (someCondition ? true : false);
}, function done(result) {
    // result is true on success or false if the condition was never met
});

The condition function will be called up to times times, starting after interval milliseconds. Once it returns true (or a truthy value), the done callback will be called with result equal to true (or whatever value the condition function returned).

If the condition is never met within the specified timeframe, done will be called with result equal to false (or whatever falsy value the condition function returned the last time it was called).

Fluent interface

Don't want to remember the order of arguments? Then use the fluent interface:

var waitUntil = require('wait-until');

waitUntil()
    .interval(500)
    .times(10)
    .condition(function() {
        return (someCondition ? true : false);
    })
    .done(function(result) {
        // do stuff
    });

Async conditions

If the condition function accepts an argument, then it is assumed to be a callback function which will be called with the result of the test:

var waitUntil = require('wait-until');

waitUntil()
    .interval(500)
    .times(10)
    .condition(function(cb) {
        process.nextTick(function() {
            cb(someCondition ? true : false);
        });
    })
    .done(function(result) {
        // do stuff
    });

Other notes

You can specify .times(Infinity) if you like, but it's probably better to set and enforce a reasonable timeout.