Question

我必须首先将component属性设置为null，因为在页面加载时我希望输入为空。我需要检查字符串是否为空，但是当它为null时我不能这样做，实际上，从用户角度来看，一切仍然可行，但是会引发错误，因此我想我不应该这样做。那么如何检查带有可为空变量的空字符串？

export class ExpensesLogComponent implements OnInit {
  inputHelperTextVisible: boolean = false;
  expenses: number = null;

检查是否为空：

 onKey(event: any) {
    if (event.key === 'Enter') {
      console.log('enter pressed!');
    }
    // error is this.expenses = null
    if (this.expenses.toString() === '') {
      this.inputHelperTextVisible = false;
    } else {
      this.inputHelperTextVisible = true;
    }
  }

Answer 1

您是否尝试过删除toString？

    SELECT (SUM(DATEPART(HOUR, InTime) * 60) + SUM(DATEPART(MINUTE, InTime))) / (SELECT COUNT(InTime) FROM #Temp WHERE DATEPART(HOUR, InTime) <> 0) / 60 AS [InTime avg hours]
        , (SUM(DATEPART(HOUR, InTime) * 60) + SUM(DATEPART(MINUTE, InTime))) / (SELECT COUNT(InTime) FROM #Temp WHERE DATEPART(HOUR, InTime) <> 0) % 60 AS [InTime avg minutes]
        , (SUM(DATEPART(HOUR, OutTime) * 60) + SUM(DATEPART(MINUTE, OutTime))) / (SELECT COUNT(OutTime) FROM #Temp WHERE DATEPART(HOUR, OutTime) <> 0) / 60 AS [OutTime avg hours]
        , (SUM(DATEPART(HOUR, OutTime) * 60) + SUM(DATEPART(MINUTE, OutTime))) / (SELECT COUNT(OutTime) FROM #Temp WHERE DATEPART(HOUR, OutTime) <> 0) % 60 AS [OutTime avg minutes]
    FROM #Temp

如果使用==而不是===，除了检查费用是否为null之外，还将检查费用是否未定义。

Answer 2

您不必设置为空

export class ExpensesLogComponent implements OnInit {
  inputHelperTextVisible = false;
  expenses: number;

并检查是否为空

if (!this.expenses || this.expenses.toString() == '') { }

Answer 3

您有几种选择可以避免出错。但是我建议使用嵌套循环之一以避免打字稿出现任何问题。

    // it will work
    if(this.expenses != null) {
       if (this.expenses.toString() === '') {
          this.inputHelperTextVisible = false;
        } else {
          this.inputHelperTextVisible = true;
       }
    } else{
      this.inputHelperTextVisible = false;
    }

用null初始化数字变量并检查空值

3 个答案: