js promise怎么返回值

js promise怎么返回值

then方法的实现里，确实是返回了一个新的Promise，你说的double，其实是then的第一个参数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);}});});}