"Head First Javascript" Learning note 0--to make a CHM reference manual material

Variable declaration: Var

Constant declaration: const

Data format conversions:

1. Conversion function

parseint (a): Converts the string A to an integer, where A is a string containing only numbers

Parsefloat (a): Converts the string A to a floating-point number, where a is a string containing only numbers

parseint () and parsefloat () will parse the string carefully before judging whether the string is a numeric value. The parseint () method first looks at the character at position 0, determines if it is a valid number, and if not, the method returns Nan, and no further action is taken. However, if the character is a valid number, the method will look at the character at position 1 and perform the same test. This process continues until a character is found that is not a valid number, at which point the parseint () converts the string before the character to a number.

The parseint () method also has a base mode: parseint ("A", 16); Convert A to 16 binary integers
Of course, for binary, octal, or even decimal (the default mode), you can call the parseint () method this way:
parseint ("10", 2); Returns 2
parseint ("10", 8); Returns 8
parseint ("10", 10); Returns 10
If the decimal number contains a leading 0, it is best to use cardinality 10 so that the octal value is not unexpectedly obtained. For example:
parseint ("010"); Returns 8
parseint ("010", 8); Returns 8
parseint ("010", 10); Returns 10

The Parsefloat () method is similar to the parseint () method, viewing each character starting at position 0 until the first non-valid character is found, and then converting the string before the character to a number. However, for this method, the first decimal point that appears is a valid character. If there are two decimal points, the second decimal point is considered invalid, and the Parsefloat () method converts the string before the decimal point to a number. This means that the string "22.34.5" will be parsed into 22.34.

2. Forced type conversion

Boolean (value)-converts the given value to a Boolean type;
Number (value)-converts the given value to a digit (which can be an integer or a floating point);
String (value)-converts the given value to a string.

3. Using JS variable weakly for type conversion

<script>
var str= ' 012.345 ';
var x = str-0;
x = x*1;
</script>

"Head First Javascript" Learning note 0--to make a CHM reference manual material