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const thenable = {
  then: function(onFulfilled) {
    setTimeout(() => onFulfilled(42), 10);
  }
};

Promise.resolve().
  then(() => thenable).
  then(v => {
    v; 
  });

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// Thenable是包含`then()`方法的对象。
// 以下Thenable可以像Promise一样，
// 在10ms后fulfilled，返回42
const thenable = {
  then: function(onFulfilled) {
    setTimeout(() => onFulfilled(42), 10);
  }
};

const v = await thenable;
v; // 42

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const thenable = {
  then: function(onFulfilled) {
    setTimeout(() => onFulfilled(42), 10);
  }
};

const p = Promise.resolve(thenable);
p instanceof Promise; 

const v = await p;
v;