# How many users does Parse

## Convert strings to numbers and check

### Only number, no integer, no floating point

Unlike most programming languages, Javascript only knows one type of number - no integer, short, long or floating point numbers. Numbers are simply number, with or without decimal places.

*parseInt*, *parseFloat*, *isFinite* and *encodeURI / decodeURI* are independent of objects and are therefore referred to as "top-level functions". Otherwise, most of the Javascript functionality is provided by objects and their methods.

In any case, be careful when entering forms: Javascript requires a point as a decimal separator!

### parseInt ()

Parses (parses) a string "x" and returns the first number (integer) it finds.

parseInt (x, [radix])If the first character is not a digit, space, or a leading minus sign, there *parseInt ()*NaN back.

*parseInt ()* supports a second optional parameter *radix*to specify the base of the parsed number (in the range 2-36).

- With the value "10" the parser interprets the number in the decimal system, while "16" interprets a number as a hexadecimal value.
- If the parameter does not exist, works
*parseInt ()*assume that numbers beginning with "0x"*radix 16*are, "0"*radix 8*and all other numbers*radix 10*.

*// returns 50*parseInt ("3-time master")

*// returns 3*parseInt ("5 lives")

*// returns 5*parseInt ("I have 3 computers")

*// returns NaN*parseInt ("17", 8)

*// returns 15*

As a precaution, base 10 should always be specified when evaluating form fields so that leading zeros do not lead to errors.

parseInt ('017')*// returns 15*parseInt ('17 ')

*// returns 17*

### parseFloat (x)

Parses a string x and returns the first floating point number - i.e. the first number with a decimal point. If the first character in the string is not a digit, white space or a leading minus sign, there is *parseFloat ()*NaN back.

*// returns 15.87*parseFloat ("-17.8 meters")

*// returns -17*parseFloat ("foo")

*// returns NaN*

*parseFloat ()* e.g. converts entries in form fields into numbers. Javascript always interprets the values of form fields as a string, even if the user has entered '17 .87 'in a field. When transforming floating point numbers from form fields, commas must be used before calling *parseInt ()* be converted into points since *parseInt ()* otherwise only converts the number to the decimal point.

### isNaN ()

isNaN () checks whether x has the value *NaN* (Not a Number) - an illegal number.

isNaN there *true* returns if x is not an ordinary number (NaN) and otherwise *false*. A typical example for NaN is »0 divided by 0«, but when checking entries in form fields, you must also check whether the entry can actually be converted into a number. IsNaN is required for the check, since the equality operators do not work for both an if (num == NaN) and if (num === Nan) query.

### isFinite (x)

Determines whether a number is finite. Gives *false* back if x + is infinity, -infinity or NaN (from JavaScript 1.3).

*// returns true*alert (1.5E + 317);

*// returns false*alert (17/3);

*// returns true*alert (0/0);

*// returns false*

### eval (s)

Javascript *eval* first checks whether s is a string, then develops the string and executes it if the string contains Javascript instructions. At the end, eval () outputs the value of the last instruction - i.e. eval () executes instructions that are available as a character string - e.g. from a user input. *eval ()* allows the dynamic construction of JavaScript statements and expressions.

*eval ()* expects a string. If the string represents a valid expression, results *eval ()* the expression. The argument is optional - if no argument is passed, eval () returns undefined. If the argument is not a string, returns *eval ()* returns the argument unchanged.

*eval ()*not called to execute an expression as in these examples. Javascript executes expressions automatically. Is useful

*eval ()*E.g. when expressions are taken from a form field as a string as user input.

*eval ()* is considered questionable, unclean to nasty: In the input field of the example, not only pious mathematical expressions are developed and harmless Javascript instructions such as *alert ('Hello world')*but every instruction. Javascript eval (s) is a popular basis for obfuscating hacks and Trojans, e.g. for redirects to advertising popups from an external Javascript.

### Number (obj)

Javascript Number (obj) converts the object in the argument to a number that represents the object. If the object cannot be converted into a number, there is *Number ()* NaN (not a number). The object must be a Javascript object. If no argument is passed, gives *Number ()* 0 back.

returns 1

### String (obj)

Javascript String (obj) creates a string from any object.

The argument *obj* must be a Javascript object. If no argument is passed, gives *String ()* returns an empty string.

gives *Elvira, Rosinante, 14, true* out.

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