Lesson
Monday
## Common Data Type Errors

## Using parseInt() to Avoid Data Type Errors

## Additional Information

## Summary

The most common error beginning JavaScript students experience is attempting to call functions meant for numbers on strings. This happens especially frequently because **all values returned from a prompt are saved as strings**. Even if the user enters a number into a prompt without quotation marks! Unfortunately, this is simply how JavaScript works.

If we wanted to collect a value from a user with `prompt()`

, and perform arithmetic with this value, we would need to convert it into a number with JavaScript's `parseInt()`

function. If not, we would receive some very unexpected output.

Here's what can happen if you *don't* parse integers correctly. In the example below, we use `prompt()`

to ask the user how old they are. The user enters 35. We then attempt to perform some arithmetic on `age`

:

```
> const age = prompt("Enter your age: ");
> age + age;
"3535"
```

Remember, **all values returned from a prompt are strings!** So, even though the user entered a *number*, `age`

is actually equal to the *string* `"35"`

. Since the `+`

operator can be used to concatenate strings `age + age`

actually returns`"3535"`

, instead of `70`

.

To make a string a number, we parse it using the `parseInt()`

function like this:

```
> const number = parseInt("5");
> number;
5
```

So, to change our `age`

from the example above into an integer, we do the following:

```
> const inputtedAge = prompt("Enter your age: ");
> const age = parseInt(inputtedAge);
> age + age;
70
```

Alternatively, the code does the exact same thing on a single line. Here we immediately pass the string returned by the `.prompt()`

function as the argument to the `parseInt()`

function:

```
> const age = parseInt(prompt("Enter your age: "));
> age + age;
70
```

Note: The *Int* part of `parseInt()`

is short for integer, which means it's a whole number. If we wanted to convert something into a number with a decimal, we'd use `parseFloat()`

:

```
> const pi = "3.14";
> parseFloat(pi);
3.14
```

**Floating point** numbers are simply numbers with decimals. If you're parsing a whole number, use `parseInt()`

, if you're parsing a number containing a decimal point, use `parseFloat()`

.

All values returned from a prompt are saved as strings.

To collect a value from a user with

`prompt()`

, and perform arithmetic with this value, we would need to convert it into a number with JavaScript's`parseInt()`

function. If not, we would receive some very unexpected output.The

*Int*part of`parseInt()`

is short for integer, which means it's a whole number. If we wanted to convert something into a number with a decimal, we'd use`.parseFloat()`

-Floating point numbers are numbers with decimals. We'll explore these more later!

Lesson 22 of 27

Last updated more than 3 months ago.