js Promise finally
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悬赏园豆：10
[待解决问题]
Promise的finally定义，为什么要加 isFunction(callback)) 这个判断，两个return的写法，有什么区别？
finally(callback) {
let promise = this;
let constructor = promise.constructor;
if (isFunction(callback)) {
return promise.then(value => constructor.resolve(callback()).then(() => value),
reason => constructor.resolve(callback()).then(() => { throw reason; }));
}
return promise.then(callback, callback);}
