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Converting to a number

Calling

toString(10)

on a number is the same as calling

toString()

; they both return the deci-

mal equivalent of the number.

Converting to a number

ECMAScript provides two methods for converting non-number primitives into numbers:

parseInt()

and

parseFloat()

. As you may have guessed, the former converts a value into an integer whereas the

latter converts a value into a floating-point number. These methods only work properly when called on

strings; all other types return

NaN

Both

parseInt()

and

parseFloat()

look at a string carefully before deciding what its numeric value

should be. The

parseInt()

method starts with the character in position 0 and determines if this is a

valid number; if it isn’t, the method returns

NaN

and doesn’t continue. If, however, the number is valid,

the method goes on to the character in position 1 and does the same test. This process continues until

a character isn’t a valid number, at which point

parseInt()

takes the string (up to that point) and

converts it into a number. For example, if you want to convert the string

“1234blue”

to an integer,

parseInt()

would return a value of

1234

because it stops processing one it reaches the character

Any number literal contained in a string is also converted correctly, so the string

“0xA”

is properly con-

verted into the number 10. However, the string

“22.5”

will be converted to

, because the decimal

point is an invalid character for an integer. Some examples:

var iNum1 = parseInt(“1234blue”); //returns 1234

var iNum2 = parseInt(“0xA”); //returns 10

var iNum3 = parseInt(“22.5”); //returns 22

var iNum4 = parseInt(“blue”); //returns NaN

The

parseInt()

method also has a radix mode, allowing you to convert strings in binary, octal, hexa-

decimal, or any other base into an integer. The radix is specified as a second argument to

parseInt()

so a call to parse a hexadecimal value looks like this:

var iNum1 = parseInt(“AF”, 16); //returns 175

Of course, this can also be done for binary, octal, and even decimal (which is the default mode):

var iNum1 = parseInt(“10”, 2); //returns 2

var iNum2 = parseInt(“10”, 8); //returns 8

var iNum2 = parseInt(“10”, 10); //returns 10

If decimal numbers contain a leading zero, it’s always best to specify the radix as 10 so that you won’t

accidentally end up with an octal value. For example:

var iNum1 = parseInt(“010”); //returns 8

var iNum2 = parseInt(“010”, 8); //returns 8

var iNum3 = parseInt(“010”, 10); //returns 10

In this code, both lines are parsing the string

“010”

into a number. The first line thinks that the string is

an octal value and parses it the same way as the second line (which specifies the radix as 8). The last line

specifies a radix of 10, so

iNum3

ends up equal to 10.

ECMAScript Basics

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