Question

我有以下代码，这是我之前的问题的答案：Looping through 2d Typescript array and making new array from value at index只要它有效。我以为我会重新发布，因为否则原来。帖子会变得太复杂。

我不明白为什么我似乎无法推送到数组，或者在循环外访问数组？

-- imports ...

routes :: ScottyM ()
routes = do 
    post "data/:id" $ do
        id <- param "id"
        -- HERE IS WHERE I GET CONFUSED
        -- This is what I want to do
        db <- open "store.db"
        exec db "INSERT INTO Store (id, value) VALUES (" <> id <> ", 'Test Value');" -- I know there is SQL Injection here I will learn about parameterized queries in haskell next
        close db
        -- END THE PART I AM CONFUSED BY
        text $ "created a record with " <> id <> " id."

main :: IO()
    scotty 3000 routes

我认为＆＃34; this.month_weekdays.push（wkday）;＆＃34;应该将字符串wkday推送到数组month_weekdays，以便对于数组中的每个值days_in_month它应循环工作日并将索引i％7处的字符串分配给month_weekdays的索引i

因此，对于days_in_month month_weekdays中的第一个值，应该如下所示：

MTWTFSSMTWTFSSMTWTFSSMTWTFSSMTW

（因为12月有31天，这也应该是month_weekdays的最终值）

Answer 1

我认为问题出现是因为您使用function() {}语法而不绑定this。因此，this.month_weekdays并未完全正确month_weekdays。有两种选择：

将您的功能更改为箭头语法或
在this

function() {}

对于第一个选项，您的代码将如下所示：

this.days_in_month.forEach((item) => {
  // console.log(item);
  item.forEach((value) => {
    // console.log(value);

    let thing: number = value;
      for(var i = 0; i < thing; i++) {
        let weekdays: string[] = [ 'M','T','W','T','F','S','S' ];
        let wkday: string = (weekdays[(i%7)]);
        this.month_weekdays.push(wkday);
      };
  });

}）;

您应该能够在循环外访问month_weekdays。会发生什么function() {}语法有自己的this，因此您必须绑定外部作用域中的this，以便function(){}引用正确的this 。另一方面，箭头语法的语法比function(){}短，并且不绑定自己的this，arguments，super或new.target;因此，不需要this的绑定。

对于第二个选项，您的代码将如下所示：

this.days_in_month.forEach(function(item) {
  // console.log(item);
  item.forEach(function(value) {
    // console.log(value);

    let thing: number = value;
      for(var i = 0; i < thing; i++) {
        let weekdays: string[] = [ 'M','T','W','T','F','S','S' ];
        let wkday: string = (weekdays[(i%7)]);
        this.month_weekdays.push(wkday);
      };
  }.bind(this));
}.bind(this));

Answer 2

在没有this的情况下工作：

const WEEK_DAYS: string[] = ['M', 'T', 'W', 'T', 'F', 'S', 'S'];
var days_in_month = [
  [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31],
  [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31],
  [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31],
  [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
];

let month_weekdays = [];
days_in_month.forEach(item => {
  item.forEach(value => {
    for (var i = 0; i < value; i++) {
      let wkday: string = WEEK_DAYS[(i % 7)];
      month_weekdays.push(wkday);
    };
  });
});

在Typescript中使用循环推送到数组

2 个答案: