建構子函數當函數是參數

JavaScript

Word count: 244Reading time: 1 min

 2023/04/27 

在研究JS的Promise時，對這種寫法感到陌生

const promise1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('foo');
  }, 300);
});

不太能理解，傳參是function，裡頭的參數又有兩個function，然後裡面再去執行參數(as function)

後來看Promise源碼，比較能理解了

const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';
function Promise(executor) {
    let self = this;
    self.status = PENDING;
    self.onFulfilled = [];//成功的回调
    self.onRejected = []; //失败的回调
    //PromiseA+ 2.1
    function resolve(value) {
        if (self.status === PENDING) {
            self.status = FULFILLED;
            self.value = value;
            self.onFulfilled.forEach(fn => fn());//PromiseA+ 2.2.6.1
        }
    }

    function reject(reason) {
        if (self.status === PENDING) {
            self.status = REJECTED;
            self.reason = reason;
            self.onRejected.forEach(fn => fn());//PromiseA+ 2.2.6.2
        }
    }

    try {
        executor(resolve, reject);
    } catch (e) {
        reject(e);
    }
}

用簡單的範例來看的話

function prom(executor) {
    let self = this;
    function resolve(value) {
        console.log(value)
    };
    function reject(value) {
        console.log(value)
    };
    executor(resolve, reject);
}

let a = new prom((a, b) => {
    a('123');
    b('abc');
})

// output:
// 123
// abc

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建構子函數 當函數是參數

建構子函數當函數是參數