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Learn how to convert a string to a number in JavaScript, using parseFloat(), parseInt(), and the unary plus (+) operator.
Resources
Example parseInt() and parseFloat() Return Values
| Parse Method | Return Value |
|---|---|
| parseInt('11') | 11 |
| parseInt('135px') | 135 |
| parseInt('202.99') | 202 |
| parseInt('Plan 9') | NaN |
| parseFloat('32.50') | 32.50 |
| parseFloat('-273.15°') | -273.15 |
| parseFloat('Absolute zero is -273.15') | NaN |
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Earlier you learned about the unexpected
behavior caused by treating strings like
0:00
numeric values.
0:05
And that if you're dealing with input
that is supposed to be a number,
0:06
it's best to convert strings that contain
numeric characters into real numbers or
0:10
numeric data types.
0:14
JavaScript provides a few ways
to convert strings to numbers.
0:16
One way is with the parseInt method,
which converts a string to an integer.
0:20
An integer is a whole number without a
decimal point, like 1, 5, or negative 233.
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You provide the parseInt method a string
and it returns an integer value.
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This is a good way to solve
the problem with my program.
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So first, I'll test it in the console
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with typeof parseInt, HTMLBadges and
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typeof parseInt CSSBadges.
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Notice how this time,
typeof HTMLBadges returns number and
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typeof CSS badges also returns number.
1:03
Good.
1:05
So now I'll change the value of
the total badges variable to
1:06
parseInt HTMLBadges,
plus parseInt CSSBadges.
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Let's see how this works now.
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I'll enter 10 HTML badges,
and 5 CSS badges.
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Great, and now I get 15 total badges.
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There's a similar method for
numbers that have decimal points.
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It's called parseFloat.
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It also gets a string value and
returns a floating point value.
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Let me comment out these three lines so
that they don't run in the browser.
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Now I'll declare a new
variable named pi and
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log it to the console.
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The quote marks around 3.14 means that
the value stored in pi is a string.
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For example, adding the type of
operator in front of pi returns string.
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This time,
I'll use the parseFloat method and
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provide it the string value stored in pi.
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Checking the console,
notice how type of pi returns number.
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That means it's a number now.
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As you can see, the typeof operator is
really handy and developers use it all
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the time to check a variable's type and
ensure that it's the correct data type.
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You can even use the parseInt and
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parseFloat methods on numbers that include
letters and non numeric characters.
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For example, I'll pass parseFloat
the string 1.89 light years away.
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If your string starts with a number
followed by other characters,
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parseFloat returns the number only.
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Notice how it returns just the number part
at the very beginning of the string, 1.89.
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However, if the string begins with
letters, you get a strange value.
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This time, I'll use parseInt,
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and pass it the string, that is so 2008.
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And the console, we see NaN.
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NaN stands for not a number.
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And it's JavaScript's way of saying
that it can't find a number,
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at least at the beginning of the string.
3:25
NaN usually represents an error,
for example,
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a result of an incorrect
mathematical operation.
3:30
You'll get a variety of different
return values from the parseInt and
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parseFloat methods.
3:37
For example, if you provide the parseInt
method a string that's a decimal
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number like 202.99,
you only get the 202 part back,
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because parseInt only returns
a whole number or an integer.
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You can see more examples of parseInt and
3:54
parseFloat return values in
the teacher's notes with this video.
3:57
There are other ways to convert
a string to a number in JavaScript.
4:01
For example, in a previous course,
you learned to use the unary plus
4:04
operator as a quick way of
converting a string to a number.
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You place a plus symbol just
before the variable name or
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string that you wanna convert.
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Back in my JavaScript file,
I'll uncomment these three statements.
4:16
And change the value of the totalBadges
variable to plus HTMLBadges,
4:20
followed by the addition operator,
then, plus CSSBadges.
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And console.log, totalBadges.
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The unary plus operator attempts to
convert the value to a number if it isn't
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already.
4:46
Back in the browser,
when I enter 10 HTML badges and
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15 CSS badges,
the console prints 25 total badges.
4:52
Keep in mind that the unary plus
operator works with both integers and
4:57
floating point numbers.
5:01
Let's try placing the plus just before
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the pi variable with typeof plus pi.
5:09
The console lets me know
that pi is now a number.
5:14
Because the syntax is so brief, the unary
plus is becoming one of the fastest and
5:18
preferred ways of converting
a string into a number.
5:23
However, many developers often prefer
code readability over brevity.
5:26
So it's still really common
to use the parseInt and
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parseFloat methods, because they
clearly explain their purpose.
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The approach you use is up to you.
5:38
Also, parseInt, parseFloat, and the unary
plus operator can help you when making
5:40
making comparisons and
working with conditional statements.
5:45
For example,
within console.log, I'll write
5:48
the comparison pi is
strictly equal to 3.14.
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And I'll display a prompt
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dialogue that asks, what is pi?
6:04
Remember, if you compare a number
value with a string using the triple
6:09
equal sign or strict equality operator,
it will always evaluate to false,
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because you're comparing
a number with a string.
6:20
In this case, converting the string
stored in pi to a number returns true.
6:28
Since you'll work with strings frequently,
especially when collecting input from
6:36
a user, you will need to be careful when
dealing with strings that contain numbers.
6:40
Remember, to do math using input values,
always convert the string to an integer or
6:45
float using the methods you just learned.
6:49
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