node.js map 的用法

map 遍歷數組每一個元素並調用回調，並返回一個包含所有結果的數組。
函數聲明如下：

    /**
      * Calls a defined callback function on each element of an array, and returns an array that contains the results.
      * @param callbackfn A function that accepts up to three arguments. The map method calls the callbackfn function one time for each element in the array.
      * @param thisArg An object to which the this keyword can refer in the callbackfn function. If thisArg is omitted, undefined is used as the this value.
      */
    map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[];

map函數遍歷操作和調用回調函數是同步，會阻塞整個線程直到遍歷完成。如果回調函數中有異步操作，不會等異步操作完成而是往下遍歷。
如下代碼，map 的回調函數是一個同步阻塞函數阻塞線程 10 毫秒，那麼遍歷 10個成員則會阻塞主線程 100 毫秒。在這 100 毫秒內事件線程內所有的異步回調都不會被調用。下面代碼中，當 map 阻塞線程時，setInterval 的回調不會被調用，直到 map 遍歷結束。

const TestMapBlock = () => {
    const blockTenMillisecond = () => {
        const before = Date.now();
        while (true) {
            if (Date.now() > before + 10) {
                console.log(`sync task finished`);
                break;
            }
        }
    };
    const ls = [];
    for (let i = 0; i < 10; i++) {
        ls.push(i);
    }
    setInterval(() => {
        console.log('-');
    }, 1);
    ls.map(blockTenMillisecond);
    console.log('map finished');
};
TestMapBlock()

/*output:
sync task finished
省略8個相同的
sync task finished
map finished //遍歷完之後才執行該語句
-
// 省略多個相同的
-
*/

如下代碼 map 的回調函數中包含異步操作，map 在調用回調函數時不會阻塞，繼續往下執行，異步操作的回調直到異步完成後纔會被調用。

const MapAsyncCallback = () => {
    const asyncOperation = (value: number) => {
        setTimeout(() => {
            console.log(`${value} async operation finished`);
        }, 10);
    };
    const ls = [];
    for (let i = 0; i < 10; i++) {
        ls.push(i);
    }
    ls.map(asyncOperation);
    setTimeout(() => {
        console.log('.');
    }, 1);
    console.log('map finished');
};
/* 
output:
map finished
0 async operation finished
1 async operation finished
2 async operation finished
3 async operation finished
4 async operation finished
5 async operation finished
6 async operation finished
7 async operation finished
8 async operation finished
9 async operation finished
*/