"." 的作用

The ./ in ./script.js indicates that the script file is located in the same directory as the current HTML file. This is known as a relative path.

On the other hand, the / in /script.js indicates that the script file is located at the root directory of the server. This is known as an absolute path.

Using a relative path allows you to include a script file that is located in the same directory as the HTML file, or in a subdirectory of that directory. This can be helpful if you want to keep your HTML, CSS, and JavaScript files organized in a logical structure.

Using an absolute path allows you to include a script file from any location on the server, regardless of where the HTML file is located. This can be useful if you want to reuse a script file in multiple HTML files, or if you want to include a script file that is not located in the same directory as the HTML file.

It's generally a good idea to use relative paths when possible, because they are more portable and easier to maintain. An absolute path will only work if the script file is actually located at the specified location on the server. If you move the HTML file to a different location, a relative path will still work as long as the script file is located in the same relative location. An absolute path, on the other hand, would need to be updated to reflect the new location of the HTML file.

# **f-string,timer**
## f-string
```f'{timer.stop():.2f} sec'```

```f'```前缀是一种特殊字符串字面量，称为“f-strings”（也称为“格式化字符串字面量”）。它允许在字符串内嵌入表达式，并使用大括号将表达式包装起来。这种字符串字面量通常用于格式化字符串。

例如，假设有一个变量name和一个变量age，分别存储用户的名字和年龄。你可以使用f-strings将这些值插入字符串中，如下所示：

```python
name = "Alice"
age = 30
greeting = f"Hello, {name}! You are {age} years old."
print(greeting)  
# Output: "Hello, Alice! You are 30 years old."

在这个例子中，f-string中的两个大括号内分别包含了name和age变量。这些表达式在运行时被求值，并将结果插入字符串中。

你还可以使用格式说明符来控制表达式的输出格式。例如，你可以在表达式前面加上一个冒号和格式说明符，如下所示：

value = 123.456
formatted_value = f"{value:.2f}"
print(formatted_value)  # Output: "123.46"

在这个例子中，.2f格式说明符指定了值应格式化为具有2个小数位的浮点数。这将输出浮点数的值，并将其保留到2个小数位。

timer

is a function that returns a value, and ```:.2f``` is a format specifier that specifies how the returned value should be formatted as a string. The ```:``` indicates the start of the format specifier, ```.2f``` specifies that the value should be formatted as a floating-point number with 2 decimal places, and ```sec``` is a string that is appended to the end of the formatted value.

 ```timer.stop()```是一个返回值的函数，而```:.2f```是一个格式说明符，指定返回值应如何格式化为字符串。```:```表示格式说明符的开头，```.2f```指定值应格式化为具有2个小数位的浮点数，而```sec```是一个字符串，该字符串追加到格式化值的末尾。

Here's an example of how you could use this line of code:

```python
import time

class Timer:
    def __init__(self):
        self.start_time = time.perf_counter()

    def stop(self):
        return time.perf_counter() - self.start_time

timer = Timer()
# Do some work here
elapsed_time = timer.stop()
print(f'{elapsed_time:.2f} sec')

This would print the elapsed time in seconds with 2 decimal places. For example, if the elapsed time was 1.2345 seconds, the output would be 1.23 sec.