Javascript string substring/substr/slice

The two parameters of substring cannot be non-positive integers at the same time: when the two parameters are both positive integers, the return value of the string substring will be a string between the two parameters (including the values at the smaller positions of the two parameters, but it does not include the values of the two parameters at the Greater position, and the return value length is the difference between the two). When a parameter is not a positive integer, that is, 0 or negative, the Return Value of the string substring is the string before the position of the positive integer parameter value. The second parameter of substr must be a positive integer because it represents the number of returned strings. The first parameter can accept positive and negative values: when it is a non-negative integer, the value is calculated based on the parameter values in the forward order of the string. When the value is a negative integer, the value is calculated based on the parameter values in the reverse order of the string. (Note: both adopt positive values.) The two slice parameters are more flexible and can be negative or positive or negative values at the same time. If they are negative, this parameter specifies the position starting from the end of the string. That is to say,-1 refers to the last character of the string,-2 refers to the second to last character, and so on. The comparison method is the same as that of the substring method when the two parameters are both positive integers, regardless of the value they represent the position.