Console Learner — An interactive JavaScript course.

1.1 Primitive Types

The JavaScript language defines these primitive data types:

undefined – Used when a value is missing. Stored in the undefined constant.
null – Used when a value is blank or empty. Stored in the null constant.
boolean – Used for logical values. Stored in the true & false constants.
number – Used for most numeric values. Stored as floating-point 64-bit IEEE 754.
bigint – Used for large integer values. Added in ES11 (2020) and not covered here.
string – Used for text values. Stored as UTF-16 encoded (16 bit Unicode) values.
symbol – Used for guaranteed unique values. Added in ES6 (2015) and not covered here.
object – Used for everything else. More details on the next page.

The undefined constant used to be a writable in legacy JavaScript (prior to ES5). Therefore, it is common for code to avoid it.

This is similar to a double in some other languages. Numbers being floating-point is a frequent cause for bugs. So never EVER rely on built-in numbers when performing monetary or precise calculations. Use a well-tested library instead.

>>>

Object.doesNotExist

undefined

>>>

null

null

>>>

true

true

>>>

4711.0

>>>

1.5E-5

0.000015

>>>

42 / 9

4.666666666666667

>>>

0.1 + 0.2

0.30000000000000004

Ouch! As numbers are always floating-point, rounding-errors like this lurks behind every corner.

>>>

0 / 0

NaN

The special NaN (Not A Number) value is also frequently a cause for problems.

>>>

"a 'string'"

"a 'string'"

>>>

'another "string"'

"another \"string\""

There are two syntaxes for string literals (using " or ') and both have exactly the same meaning.

>>>

"\061, \x32, \u0033, \u{34}"

"1, 2, 3, 4"

Octal and hexadecimal character escapes are available.

>>>

Need more magic? Check out the other developer tools on this site.