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The Deno Standard Library
import * as mod from "";

sprintf and printf for printing formatted strings to stdout.

This implementation is inspired by POSIX and Golang but does not port implementation code.

sprintf converts and formats a variable number of arguments as is specified by a format string. In it's basic form, a format string may just be a literal. In case arguments are meant to be formatted, a directive is contained in the format string, preceded by a '%' character:


E.g. the verb s indicates the directive should be replaced by the string representation of the argument in the corresponding position of the argument list. E.g.:

Hello %s!

applied to the arguments "World" yields "Hello World!".

The meaning of the format string is modelled after POSIX format strings as well as well as Golang format strings. Both contain elements specific to the respective programming language that don't apply to JavaScript, so they can not be fully supported. Furthermore we implement some functionality that is specific to JS.


The following verbs are supported:

Verb Meaning
% print a literal percent
t evaluate arg as boolean, print true or false
b eval as number, print binary
c eval as number, print character corresponding to the codePoint
o eval as number, print octal
x X print as hex (ff FF), treat string as list of bytes
e E print number in scientific/exponent format 1.123123e+01
f F print number as float with decimal point and no exponent
g G use %e %E or %f %F depending on size of argument
s interpolate string
T type of arg, as returned by typeof
v value of argument in 'default' format (see below)
j argument as formatted by JSON.stringify
i argument as formatted by Deno.inspect
I argument as formatted by Deno.inspect in compact format

Width and Precision

Verbs may be modified by providing them with width and precision, either or both may be omitted:

%9f    width 9, default precision
%.9f   default width, precision 9
%8.9f  width 8, precision 9
%8.f   width 9, precision 0

In general, 'width' describes the minimum length of the output, while 'precision' limits the output.

verb precision
t n/a
b c o n/a
x X n/a for number, strings are truncated to p bytes(!)
e E f F number of places after decimal, default 6
g G set maximum number of digits
s truncate input
T truncate
v truncate, or depth if used with # see "'default' format", below
j n/a

Numerical values for width and precision can be substituted for the * char, in which case the values are obtained from the next args, e.g.:

sprintf("%*.*f", 9, 8, 456.0)

is equivalent to:

sprintf("%9.8f", 456.0)


The effects of the verb may be further influenced by using flags to modify the directive:

Flag Verb Meaning
+ numeric always print sign
- all pad to the right (left justify)
# alternate format
# b o x X prefix with 0b 0 0x
# g G don't remove trailing zeros
# v use output of inspect instead of toString
' ' space character
' ' x X leave spaces between bytes when printing string
' ' d insert space for missing + sign character
0 all pad with zero, - takes precedence, sign is appended in front of padding
< all format elements of the passed array according to the directive (extension)

'default' format

The default format used by %v is the result of calling toString() on the relevant argument. If the # flags is used, the result of calling inspect() is interpolated. In this case, the precision, if set is passed to inspect() as the 'depth' config parameter.

Positional arguments

Arguments do not need to be consumed in the order they are provided and may be consumed more than once. E.g.:

sprintf("%[2]s %[1]s", "World", "Hello")

returns "Hello World". The presence of a positional indicator resets the arg counter allowing args to be reused:

sprintf("dec[%d]=%d hex[%[1]d]=%x oct[%[1]d]=%#o %s", 1, 255, "Third")

returns dec[1]=255 hex[1]=0xff oct[1]=0377 Third

Width and precision my also use positionals:

"%[2]*.[1]*d", 1, 2

This follows the golang conventions and not POSIX.


The following errors are handled:

Incorrect verb:

S("%h", "") %!(BAD VERB 'h')

Too few arguments:

S("%d") %!(MISSING 'd')"


Converts and format a variable number of args as is specified by format. printf writes the formatted string to standard output.

Converts and format a variable number of args as is specified by format. sprintf returns the formatted string.