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let promise = Promise.resolve();

promise.then(() => alert(" !"));

alert(" "); //  alert  

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unhandledrejection : ?

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let promise = Promise.reject(new Error("  !"));
promise.catch(err => alert(''));

//  :  
window.addEventListener('unhandledrejection', event => alert(event.reason));

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let promise = Promise.reject(new Error("  !"));

//   !
window.addEventListener('unhandledrejection', event => alert(event.reason));

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let promise = Promise.reject(new Error("  !"));
setTimeout(() => promise.catch(err => alert('')), 1000);

// Error:   !
window.addEventListener('unhandledrejection', event => alert(event.reason));

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