Promise的缺省处理函数以及处理函数的缺省返回值

以下实验平台为node v8引擎

处理函数的缺省返回值

实验1

new Promise(function (resolve, reject) {
    resolve(0)
}).then(function (params) {
    console.log('1:resolve',params)
},function (params) {
    console.log('1:reject',params)
}).then(function (params) {
    console.log('2:resolve',params)
},function (params) {
    console.log('2:reject',params)
})

以上代码打印为

1:resolve 0
2:resolve undefined

实验2

new Promise(function (resolve, reject) {
    reject(0)
}).then(function (params) {
    console.log('1:resolve',params)
},function (params) {
    console.log('1:reject',params)
}).then(function (params) {
    console.log('2:resolve',params)
},function (params) {
    console.log('2:reject',params)
})

以上代码打印为

1:reject 0
2:resolve undefined

结论

无论是resolve还是reject的处理函数，默认返回值都是Promise.resolve()

缺省处理函数

试验1

new Promise(function (resolve, reject) {
    reject(0)
}).then(function (params) {
    console.log('1:resolve',params)
}).then(function (params) {
    console.log('2:resolve',params)
},function (params) {
    console.log('2:reject',params)
})

以上代码打印为

2:reject 0

试验2

new Promise(function (resolve, reject) {
    resolve(0)
}).then(undefined,function (params) {
    console.log('1:reject',params)
}).then(function (params) {
    console.log('2:resolve',params)
},function (params) {
    console.log('2:reject',params)
})

以上代码打印为

2:resolve 0

结论

缺省resolve处理函数时，其处理函数相当于

function handleResolve(params) {
  return Promise.resolve(params)
}

缺省reject处理函数时，其处理函数相当于

function handleResolve(params) {
  return Promise.reject(params)
}

返回其他值

试验1

new Promise(function (resolve, reject) {
    resolve(0)
}).then(function (params) {
    console.log('1:resolve',params)
    return 1
},function (params) {
    console.log('1:reject',params)
    return 2
}).then(function (params) {
    console.log('2:resolve',params)
},function (params) {
    console.log('2:reject',params)
})

以上代码打印为

1:resolve 0
2:resolve 1

试验2

new Promise(function (resolve, reject) {
    reject(0)
}).then(function (params) {
    console.log('1:resolve',params)
    return 1
},function (params) {
    console.log('1:reject',params)
    return 2
}).then(function (params) {
    console.log('2:resolve',params)
},function (params) {
    console.log('2:reject',params)
})

以上代码打印为

1:resolve 0
2:resolve 2

结论

无论是resolve还是reject的处理函数，return value都相当于是return Promise.resolve(value)