then方法的實現裏，確實是返回了壹個新的Promise，妳說的double，其實是then的第壹個參數onFulfilled。this.then=function(onFulfilled,onRejected){varself=this;returnnewPromise(function(resolve,reject){returnself.done(function(result){if(typeofonFulfilled==='function'){try{returnresolve(onFulfilled(result));}catch(ex){returnreject(ex);}}else{returnresolve(result);}},function(error){if(typeofonRejected==='function'){try{returnresolve(onRejected(error));}catch(ex){returnreject(ex);}}else{returnreject(error);}});});}
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