Promise_A_plus_practice
Promise A+ practice
step1
第一步先看看a+规范
Promise States
A promise must be in one of three states: pending, fulfilled, or rejected.
When pending, a promise:
1.may transition to either the fulfilled or rejected state.
When fulfilled, a promise:
1.must not transition to any other state.
2.must have a value, which must not change.
When rejected, a promise:
1.must not transition to any other state.
2.must have a reason, which must not change.
Here, “must not change” means immutable identity (i.e. ===), but does not imply deep immutability
大概意思是有三个状态 pending, fulfilled, rejected
pending状态可以转换成fulfilled和rejected状态 转换为fulfilled，value必须有值，并且不能修改 转换成rejected状态 reason必须有值