Number values changing when toString() applied

问题

I am noticing something very strange when testing one of my functions that checks the validity of numbers, I first noticed it when testing the 'number.toString()[0]'. If I console logged (fLetter(019272)); the '0' is always showing '1'.
After checking online I couldn't find any issues associated with using the toString() method on numbers containing '0'.

function numberValidate(number) {

        var numCheck = [0, 5, 7, 8, 9];
        var fLetter = number.toString()[0];

        if (number.match(/^\d{6}$/)) {
            for (var i = 0; i < 5; i++) {
                if (fLetter == numCheck[i]) {
                    return false;
                } else {
                    return true;
                }
            } 
        } 

    }

I tried checking a few other test variables with and without applying the toString() method and got some results I couldn't understand. **Note - the toString() addition produced the same results (carried out in JSfiddle)

var numTest = 00000000;
var numTest1 = 20203020;
var numTest2 = 011000110100;
var numTest3 = 01928;
var numTest4 = 012345;

console.log(numTest);     //Returned 0
console.log(numTest1);    //Returned 20203020
console.log(numTest2);    //Returned 1207996480
console.log(numTest3);    //Returned 1928
console.log(numTest4);    //Returned Returned 5349

Could any one shed any light on why I am getting these values returned? Thanks

回答1:

Number is a 64bit floating point representation of a number. It does not hold the digits as a array of characters so the leading zero is not available. String holds an array of characters and thus the leading 0 is not lost

JavaScript allows for transparent type conversion if possible thus the String "0111" == to the Number 111. So if you write the code "0111" == 111 it will return true.

Javascript provides two equality operators == and the strict equality === the second requires that both side are the same type so the Number(111) === String("111") will return false as they are not the same type. The same applies for inequality operators != and !==

回答2:

010 is treated as an octal notation because of the leading 0, as well as 0x10 will be treated as an hexadecimal notation. The octal numeral system uses the digits 0 to 7. Thus, 019 gives 19 because 9 is not an octal digit. Here some decimal numbers and their octal notations:

1  1 | 5  5 | 9  11 | 13 15
2  2 | 6  6 | 10 12 | 14 16
3  3 | 7  7 | 11 13 | 15 17
4  4 | 8 10 | 12 14 | 16 20

As you can see, since we have only 8 digits available (0, 1, 2, 3, 4, 5, 6, 7), 8 becomes 10 and 16 (8+8) becomes 20 (10+10). To make things even more clear, let's see how to count with 10 digits (as usual) then with 8 digits.

Using 10 digits:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, no more digits
nevermind, add 1 to the tenths and restart from 0
10, 11, 12, 13, 14, 15, 16, 17, 18, 19, no more digits
nevermind, add 1 to the tenths and restart from 0
20, 21, etc...

Using 8 digits:

0, 1, 2, 3, 4, 5, 6, 7, no more digits
nevermind, add 1 to the "tenths" and restart from 0
10, 11, 12, 13, 14, 15, 16, 17, no more digits
nevermind, add 1 to the "tenths" and restart from 0
20, 21, etc...

The same applies to binary numbers:

   0,   1, no more digits
  10,  11, no more digits
 100, 101, no more digits
 110, 111, no more digits
1000, etc...

Let's compare with the decimal notation:

1    1 | 5  101 |  9 1001
2   10 | 6  110 | 10 1010
3   11 | 7  111
4  100 | 8 1000

Related stuffs: https://stackoverflow.com/a/32107058/1636522 :-)

来源：https://stackoverflow.com/questions/32155673/number-values-changing-when-tostring-applied

标签

javascript

tostring

number-formatting