Use Promise to wait until polled condition is satisfied

Question

I need to create a JavaScript Promise that will not resolve until a specific condition is true. Let's say I have a 3rd party library, and I need to wait until a certain data condition exists within that library.

The scenario I am interested in is one where there is no way to know when this condition is satisfied other than by simply polling.

I can create a promise that waits on it - and this code works, but is there a better or more concise approach to this problem?

function ensureFooIsSet() {     return new Promise(function (resolve, reject) {         waitForFoo(resolve);     }); }  function waitForFoo(resolve) {     if (!lib.foo) {         setTimeout(waitForFoo.bind(this, resolve), 30);     } else {         resolve();     } } 

Usage:

ensureFooIsSet().then(function(){     ... }); 

I would normally implement a max poll time, but didn't want that to cloud the issue here.

Answer 1

A small variation would be to use a named IIFE so that your code is a little more concise and avoids polluting the external scope:

function ensureFooIsSet() {     return new Promise(function (resolve, reject) {         (function waitForFoo(){             if (lib.foo) return resolve();             setTimeout(waitForFoo, 30);         })();     }); }