js error: Invalid left-hand side in assignment

The errors of js are as follows:

Uncaught ReferenceError: Invalid left-hand side in assignment

After writing js, when the page is loaded, the errors are as follows. I carefully checked and found that when comparing js, I wrote one less =;

if( mbQ == undefined || mbA = undefined) {
alert("The page preload failed, please click again and enter this page!");
return;

}

So be careful next time when writing js.