Promise Foundation (2)

Promise.resolve()

The promise.resolve () method returns a Promise object. The returned Promise object depends on the value passed in as the following:

• parametervalueisPromise object, the input parameter is returned directlyPromise object;
• parametervalueisthenable(object with then method), then returnsPromise objectDepends on the execution of the THEN method;
• parametervalueIf it is any other value, this value is returned directly as the resultfulfilledThe state of thePromise object;

The input parameter is a Promise object

Let’s take a look at a few 🌰 :

const p1 = Promise.resolve(1);

const p2 = Promise.resolve(p1);

p1 === p2; // true

p2; // Promise {<fulfilled>: 1}
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const p1 = Promise.reject(1);

const p2 = Promise.resolve(p1);

p1 === p2; // true

p2; // Promise {<rejected>: 1}
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Whether the Promise object is a pity state or the Rejected state will not be affected. When the input parameter is a Promise object, the Promise object will be directly returned as the return value

The is thenable refs

Let’s start with a few 🌰

const thenable = {
  then: () => {
  }
};

const p = Promise.resolve(thenable);

p; // Promise {<pending>}
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const thenable = { then: (resolve, reject) => { resolve('fulfilled! '); }}; const p = Promise.resolve(thenable); p; // Promise {<fulfilled>: "fulfilled!" }Copy the code

const thenable = { then: (resolve, reject) => { reject('rejected'); }}; const p = Promise.resolve(thenable); p; // Promise {<rejected>: "rejected"}Copy the code

const thenable = {
  then: (resolve, reject) => {
    throw new Error('123')
  }
};

const p = Promise.resolve(thenable);

p; // Promise {<rejected>: Error: 123}
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In fact, we can simply understand that when the input parameter is Thenable, the returned Promise object is the result of the new Promise() instantiation by passing the then function in thenable as an argument.

If you think about it more deeply, the above statement is not very strict.

• Promise. Resolve specification
• Further Promise

Promise.reject()

Resolve, promise.reject is simpler than promise.resolve, promise.reject. The state of the Promise object must be Rejected, and the result value is the value of the input parameter.

To name a few 🌰 :

const p1 = Promise.resolve(1);

const p2 = Promise.reject(p1);

p1 === p2; // false

p2; // Promise {<rejected>: Promise}
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const thenable = {
  then: () => {}
};

const p = Promise.reject(thenable);

p; // Promise {<rejected>: {then: () => {}}}
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const p = Promise.reject(1);

p; // Promise {<rejected>: 1}
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conclusion

There is no epilogue to this issue.