While variable is not defined - wait

The following will keep looking for someVariable until it is found. It checks every 0.25 seconds.

function waitForElement(){    if(typeof someVariable !== "undefined"){        //variable exists, do what you want    }    else{        setTimeout(waitForElement, 250);    }}

async, await implementation, improvement over @Toprak's answer

(async() => {    console.log("waiting for variable");    while(!window.hasOwnProperty("myVar")) // define the condition as you like        await new Promise(resolve => setTimeout(resolve, 1000));    console.log("variable is defined");})();console.log("above code doesn't block main function stack");

After revisiting the OP's question. There is actually a better way to implement what was intended: "variable set callback". Although the below code only works if the desired variable is encapsulated by an object (or window) instead of declared by let or var (I left the first answer because I was just doing improvement over existing answers without actually reading the original question):

let obj = encapsulatedObject || window;Object.defineProperty(obj, "myVar", {    configurable: true,    set(v){        Object.defineProperty(obj, "myVar", {            configurable: true, enumerable: true, writable: true, value: v });        console.log("window.myVar is defined");    }});    

see Object.definePropertyor use es6 proxy (which is probably overkill)

If you are looking for more:

/** * combining the two as suggested by @Emmanuel Mahuni, * and showing an alternative to handle defineProperty setter and getter */let obj = {} || window;(async() => {  let _foo = await new Promise(res => {    Object.defineProperty(obj, "foo", { set: res });  });  console.log("obj.foo is defined with value:", _foo);})();/*IMPORTANT: note that obj.foo is still undefinedthe reason is out of scope of this question/answertake a research of Object.defineProperty to see more*/// TEST CODEconsole.log("test start");setTimeout(async () => {  console.log("about to assign obj.foo");  obj.foo = "Hello World!";  // try uncomment the following line and compare the output  // await new Promise(res => setTimeout(res));  console.log("finished assigning obj.foo");  console.log("value of obj.foo:", obj.foo); // undefined  // console: obj.foo is defined with value: Hello World!}, 2000);