The printj npm package is a library for formatting strings in JavaScript, similar to the printf function in C. It allows users to format strings with placeholders and provides various specifier characters for different data types.
String Formatting
This feature allows users to format strings using placeholders. The '%s' is used for string substitution, and '%d' is for integer substitution. The example code outputs 'Alice is 30 years old.'
const printj = require('printj');
console.log(printj.sprintf('%s is %d years old.', 'Alice', 30));Localization and Internationalization
printj supports localization and internationalization, allowing numbers to be formatted according to different locale conventions. The example code sets the localization to French and formats the number with a comma as the decimal separator.
const printj = require('printj');
printj.setLocalization(printj.localization.FR);
console.log(printj.sprintf('%.2f', 1234567.89));Parsing Formatted Strings
The package also provides functionality to parse formatted strings back into individual components. The sscanf function is used to parse the input string according to the format provided. The example code parses the string and returns an array with the extracted values.
const printj = require('printj');
const result = printj.sscanf('Alice is 30 years old.', '%s is %d years old.');
console.log(result);sprintf-js is a similar package that provides string formatting with a syntax similar to the one used in C's sprintf function. It offers similar features to printj, such as placeholders for different data types and variable-width formatting.
The util package is a core Node.js module that includes a format method, util.format(), which can be used for string formatting with placeholders. It is not as feature-rich as printj in terms of localization and parsing formatted strings.
Extended sprintf implementation (for the browser and nodejs). Emphasis on
compliance, performance and IE6+ support.
PRINTJ.sprintf("Hello %s!", "World");
A self-contained specification of the printf format string is included below in this README, as well as a summary of the support against various printf implementations
With npm:
$ npm install printj
In the browser:
<script src="printj.js"></script>
The browser exposes a variable PRINTJ
When installed globally, npm installs a script printj that renders the format
string with the given arguments. Running the script with -h displays help.
The script will manipulate module.exports if available (e.g. in a CommonJS
require context). This is not always desirable. To prevent the behavior,
define DO_NOT_EXPORT_PRINTJ
In all cases, the relevant function takes a format and arguments to be rendered.
The return value is a JS string.
PRINTJ.sprintf(format, ...args) assumes the arguments are passed directly
PRINTJ.vsprintf(format, argv) assumes the arguments are passed in an array
For example:
> // var PRINTJ = require('printj'); // uncomment this line if in node
> var sprintf = PRINTJ.sprintf, vsprintf = PRINTJ.vsprintf;
> sprintf("Hello %s", "SheetJS") // 'Hello SheetJS'
> sprintf("%d + %d = %d", 2,3,2+3) // '2 + 3 = 5'
> vsprintf("%d + %d = %d", [2,3,5]) // '2 + 3 = 5'
> sprintf("%1$02hhx %1$u %1$i %1$o", -69) // 'bb 4294967227 -69 37777777673'
The command line script takes a format and arguments:
usage: printj [options] <format> [args...]
Options:
-h, --help output usage information
-d, --dump print debug information about format string
Arguments are treated as strings unless prefaced by a type indicator:
n:<integer> call parseInt (ex. n:3 -> 3)
f:<float> call parseFloat (ex. f:3.1 -> 3.1)
b:<boolean> false when lowercase value is "FALSE" or "0", else true
s:<string> interpret as string (ex. s:n:3 -> "n:3")
j:<JSON> interpret as an object using JSON.parse
e:<JS> evaluate argument (ex. e:1+1 -> 2, e:"1"+1 -> "11")
samples:
$ printj '|%02hhx%d|' n:50 e:0x7B # |32123|
$ printj '|%2$d + %3$d is %1$d|' e:1+2 n:1 n:2 # |1 + 2 is 3|
$ printj '|%s is %s|' s:1+2 e:1+2 # |1+2 is 3|
$ printj '|%c %c|' s:69 n:69 # |6 E|
make test will run the nodejs-based test.
make stress will run a larger test encompassing every possible conversion. It
requires access to a C compiler.
Please consult the attached LICENSE file for details. All rights not explicitly granted by the Apache 2.0 license are reserved by the Original Author.
The printf family of functions attempt to generate and output a string of
characters from a series of arguments, following a user-supplied "format string"
specification. The format string contains normal characters that are written to
the output string as well as specifiers that describe which parameter to insert
and how to render the parameter. This specification describes how a conformant
implementation should process the format string and generate an output string.
Any discrepancies between this document and the reference implementation are
considered bugs in the implementation.
Every function in the printf family follows the same logic to generate strings
but have different interfaces reflecting different input and output behaviors.
Some functions have wide variants that use wide wchar_t * strings rather than
normal C char *. The following variants are required by the POSIX spec:
| function | max length | output destination | vintage | wide ver |
|---|---|---|---|---|
printf | unbounded | standard output | K&R | wprintf |
fprintf | unbounded | stream (FILE *) | K&R | fwprintf |
sprintf | unbounded | string (char *) | K&R | swprintf |
snprintf | parameter | string (char *) | C99 | |
dprintf | unbounded | POSIX file descriptor | POSIX |
Each function has a dual function, whose name begins with v, that accepts the
parameters as a va_list rather than formal parameters. In all cases, they
return the number of characters written or a negative value to indicate error:
int sprintf(char *ostr, const char *fmt, ...);
int vsprintf(char *ostr, const char *fmt, va_list arg_list);
int swprintf(wchar_t *ostr, const wchar_t *fmt, ...);
int vswprintf(wchar_t *ostr, const wchar_t *fmt, va_list arg_list);
C "strings" are really just arrays of numbers. An external code page (such as
ASCII) maps those numbers to characters. K&R defines two types of strings:
basic character set strings (char *) and extended character set strings
(wchar_t *). In contrast, JS has a true string value type.
Unlike in C, JS strings do not treat the null character as an end-of-string marker. As a result, characters beyond the first null character will be used.
The JS equivalent of a C extended string would be an array of the individual
character codes. The C basic string equivalent would involve specifying a code
page and mapping back. The codepage JS library supports common codepages.
While capturing the essence of C strings, using arrays of character codes is not
idiomatic JS. Few developers leverage this and the downsides far exceed the
benefits of a more direct translation. The effect can be feigned, as shown in
the js2c code sample at the end of the document.
In the absence of a standard output or even a standard concept of a stream, the
non-string outputs are irrelevant. Similarly there is no JS analogue of wide
characters. While useful, lack of direct memory management obviates snprintf.
This implementation exports the remaining functions, sprintf and vsprintf.
Instead of replicating the original C signature and errno, functions directly
return the output string and throw Errors:
function sprintf(fmt:string, ...args):string;
function vsprintf(fmt:string, args:Array<any>):string;
The C functions return the number of characters written to the string, which is
directly accessible in JS via the length property. A direct replica of the
various string functions are included at the end of the document.
Note: The regular expressions follow perl /x style. Whitespace characters
outside of character classes are ignored. # is a comment character and every
character until the end of the line is ignored. To convert to a standard regex:
regex_string.replace(/#.*$/gm,"").replace(/^\s*/gm,"").replace(/\s*\n/gm,"");
Based on K&R, conversions originally followed the format:
%- (POSIX refers to this as the "flags")* (POSIX "width")* (POSIX "precision")h or l to indicate size of data (POSIX "length")This is captured by the regular expression:
/%(?:
([-])? # flags (only minus sign)
(\d+|\*)? # width
(?:\.(\d+|\*))? # period + precision
([hl])? # length
([dioxXucsfeEgGp%]) # conversion specifier
)/x
Various implementations of printf have added different functionality.
ANSI standards up through C99:
"+" " " "0" "#""L" "hh" "ll" "j" "z" "t""F" "a" "A" "n"The POSIX specification of printf added:
"'""C" "S"BSD implementations added:
"q""D" "U" "O"glibc (GNU) added:
"Z""m"Windows C Runtime (CRT) added:
"I" "I32" "I64" "w"glibc and CRT both added Z. glibc uses Z for the length size_t. CRT uses
Z as a conversion for length-prefixed strings. This implementation takes the
former approach, handling Z in the same way as z.
BSD and IBM C library both added D. BSD uses D as a conversion, namely as
an alias of ld. IBM uses D for the length for _Decimal64, a decimal
floating point type, in accordance with ISO/IEC TR 24732. This implementation
takes the former approach.
This implementation also adds new conversions:
"b" and "B" for binary (base-2) integer renderings"y" and "Y" for true/false and yes/no Boolean conversions"J" for JSON"T" and "V" for JS typeof and valueOf inspectionCombining the various extensions yields the following regular expression:
/%(?:
%| # literal %% (flags etc prohibited)
(\d+\$)? # positional parameter
([-+ 0\x23\x27]*)? # flags
(\d*|\*(?:\d+\$)?) # width
(?:\.(\d+|\*(?:\d+\$)?))? # precision
(hh?|ll?|[LzjtqZIw])? # length
([diouxXfFeEgGaAcCsSpnDUOmbByYJVT]) # conversion specifier
)/x
This implementation explicitly does not support certain non-standard extensions:
v with h/l/ll):I32 and I64| C | Type | Summary |
|---|---|---|
a | floating | base-2 exp form w/ hex mantissa and dec exponent, lowercase |
A | floating | base-2 exp form w/ hex mantissa and dec exponent, uppercase |
b | extended | cast to C unsigned int, standard form binary |
B | extended | cast to C unsigned long, standard form binary |
c | text | print latin-1 char from number OR first char of string |
C | text | print UCS-2 char from number OR first char of string |
d | integral | cast to C int, standard form decimal |
D | integral | cast to C long, standard form decimal |
e | floating | base-10 exp form w/dec mantissa and dec exponent, lowercase |
E | floating | base-10 exp form w/dec mantissa and dec exponent, uppercase |
f | floating | base-10 decimal form, lowercase extended values |
F | floating | base-10 decimal form, uppercase extended values |
g | floating | print using e or f conversion based on value/precision |
G | floating | print using E or F conversion based on value/precision |
i | integral | cast to C int, standard form decimal (alias of d) |
J | extended | prints objects using JSON or util.inspect |
m | misc | prints info about Error objects (JS equivalent of errno) |
n | misc | do not print! stores number of chars written to arg .len |
o | integral | cast to C unsigned int, standard form octal |
O | integral | cast to C unsigned long, standard form octal |
p | misc | print "l" field of object (fake pointer) |
s | text | print string argument |
S | text | print string argument (alias of "s") |
T | extended | print type information (typeof or Object toString) |
u | integral | cast to C unsigned int, standard form decimal |
U | integral | cast to C unsigned long, standard form decimal |
V | extended | print primitive value (valueOf) |
x | integral | cast to C unsigned int, standard form hex, lowercase |
X | integral | cast to C unsigned long, standard form hex, uppercase |
y | extended | prints true/false or yes/no based on Boolean value |
Y | extended | prints TRUE/FALSE or YES/NO based on Boolean value |
% | misc | print the literal % character |
The default behavior is to consume arguments in order:
printf("Count to 3: %d %d %d", 1, 2, 3); // Count to 3: 1 2 3
POSIX printf permits explicit argument selection, bypassing the standard
behavior of using the arguments in order. To select the n-th argument, use
n$ immediately after the % token to select an argument for the conversion:
printf("%d %d %d", 1, 2, 3); // 1 2 3 (implicit order 1, 2, 3 )
printf("%1$s %2$s %3$s", "a", "b", "c"); // a b c (explicit order 1, 2, 3 )
printf("%1$s %3$s %2$s", "a", "b", "c"); // a c b (explicit order 1, 3, 2 )
The POSIX standard asserts that mixing positional and non-positional conversions is undefined behavior. This implementation handles mixing by tracking the index for non-positional conversions:
printf("%s %4$s %s %5$s %s", "a", "b", "c", "d", "e"); // a d b e c
The POSIX standard requires that if an argument is used in the format, every preceding argument must be used. This implementation relaxes that requirement:
printf("%3$s", "a", "b", "c"); // c (technically invalid since "a"/"b" unused)
The width and precision specifiers may include the dynamic specifier * which
instructs the engine to read the next argument (assumed to be an integer). Just
as with the positional parameter, idx$ immediately after the * token selects
the numeric argument.
For example:
printf("|%5s|", "sheetjs"); // |sheetjs| (width = 5)
printf("|%*s|", 5, "sheetjs"); // |sheetjs| (width first argument)
printf("|%2$*1$s|", 5, "sheetjs", 10); // |sheetjs| (width is argument #1)
printf("|%10s|", "sheetjs"); // | sheetjs| (width = 10)
printf("|%2$*3$s|", 5, "sheetjs", 10); // | sheetjs| (width is argument #3)
Arguments are generally consumed in order as presented in the format string:
printf("|%s|", val);
printf("|%*s|", width, val);
printf("|%.*s|", prec, val);
printf("|%*.*s|", width, prec, val);
printf("|%0*.*d|", 4, 2, 1); // | 01| width=4 prec=2 value=1
Positional arguments can be applied to width and precision:
printf("|%*.*d|", width, prec, val);
printf("|%2$0*3$.*1$d|", prec, val, width);
printf("|%0*.*d|", 4, 2, 1); // | 01| width=4 prec=2 value=1 flags='0'
printf("|%1$0*3$.*2$d|", 1, 2, 4); // | 01| width=4 prec=2 value=1 flags='0'
A negative width is interpreted as the - flag with a positive width:
printf("|%*.*d|", 4, 2, 1); // | 01| width=4 prec=2 value=1 flags=''
printf("|%-*.*d|", 4, 2, 1); // |01 | width=4 prec=2 value=1 flags='-'
printf("|%*.*d|", -4, 2, 1); // |01 | width=4 prec=2 value=1 flags='-'
printf("|%-*.*d|", -4, 2, 1); // |01 | width=4 prec=2 value=1 flags='-'
A negative precision is discarded:
printf("|%*s|\n", 4, "sheetjs"); // |sheetjs| width=4
printf("|%*.*s|\n", 4, 3, "sheetjs"); // | she| width=4 prec=3
printf("|%*.*s|\n", 4, 2, "sheetjs"); // | sh| width=4 prec=2
printf("|%*.*s|\n", 4, 1, "sheetjs"); // | s| width=4 prec=1
printf("|%*.*s|\n", 4, 0, "sheetjs"); // | | width=4 prec=0
printf("|%*.*s|\n", 4, -1, "sheetjs"); // |sheetjs| width=4 (prec ignored)
JS has one numeric type Number which represents an IEEE754 double-precision
(64-bit) floating point number. C has a multitude of numeric types, including
floating point as well as integer types. The sizes of those data types are
implementation-dependent. A "C data model" specifies the sizes of the core C
data types.
POSIX printf specification references 8 integer types in integer conversions:
| C data type | fmt | unsigned type | fmt | signed type | fmt |
|---|---|---|---|---|---|
char | unsigned char | hhu | signed char | hhd | |
short | hd | unsigned short | hu | ||
int | d | unsigned int | u | ||
long | ld | unsigned long | lu | ||
long long | lld | unsigned long long | llu | ||
size_t | zu | ssize_t | zd | ||
intmax_t | jd | uintmax_t | ju | ||
ptrdiff_t | td |
C99 does not officially define a signed size_t or unsigned ptrdiff_t type.
POSIX does define ssize_t but no equivalent uptrdiff_t.
BSD additionally recognizes the types quad_t and u_quad_t, which this
implementation treats as long long int and unsigned long long int.
Two integer types are used in character and string conversions:
| type | fmt |
|---|---|
wchar_t | ls |
wint_t | lc |
Both wide types wchar_t and wint_t can be signed or unsigned according to
C99. Both types are used only in character and string conversions. Based on
K&R "printable characters are always positive", the types are assumed unsigned.
K&R recognizes 3 floating point types. C99 later tied it to IEC 60559:
| C data type | precision | total bits | exponent | mantissa | fmt |
|---|---|---|---|---|---|
float | single | 32 | 8 | 23 | |
double | double | 64 | 11 | 52 | f |
long double | extended | 80 | 15 | 64 | Lf |
Numerous "C data models", specifying the bit/byte sizes of the various types,
have been and continue to be used. For example, OSX and other modern 64-bit
UNIX flavors use the "LP64" C data model. 64-bit Windows currently uses the
"LLP64" model. 32-bit systems generally use the "ILP32" model. The 8-bit byte
sizes for the various types under the various models are defined in ctypes.json
in the Models object as per the following table:
| type | ctypes.json | LP64 | ILP32 | LLP64 |
|---|---|---|---|---|
char | char | 1 | 1 | 1 |
short | short | 2 | 2 | 2 |
int | int | 4 | 4 | 4 |
long | long | 8 | 4 | 4 |
long long | longlong | 8 | 8 | 8 |
wchar_t | wchar_t | 4 | 4 | 2 |
wint_t | wint_t | 4 | 4 | 2 |
size_t | size_t | 8 | 4 | 8 |
intmax_t | intmax_t | 8 | 8 | 8 |
ptrdiff_t | ptrdiff_t | 8 | 4 | 8 |
By default the source assumes the LP64 data model. Other data models are
supported in the source tree, controlled by the JSFLAGS variable in the build
process. Set the JS_MODEL variable to the desired index as specified in the
ModelNames array in bits/ctype.json:
$ <bits/ctypes.json jq -r '.ModelNames|.[]' # LP64 ILP32 LLP64
$ JSFLAGS=-DJS_MODEL=0 make # LP64
$ JSFLAGS=-DJS_MODEL=1 make # ILP32
$ JSFLAGS=-DJS_MODEL=2 make # LLP64
To create a custom model, add the spec to bits/ctypes.json by appending the
model name to the end of the ModelNames array and adding an entry to the
Models object. The current models are defined as follows:
{
"ModelNames":["LP64", "ILP32", "LLP64"],
"Models": {
"LP64": { "char":1, "short":2, "int":4, "long":8, "longlong":8, "wint_t":4, "wchar_t":4, "size_t":8, "intmax_t":8, "ptrdiff_t":8 },
"ILP32": { "char":1, "short":2, "int":4, "long":4, "longlong":8, "wint_t":4, "wchar_t":4, "size_t":4, "intmax_t":8, "ptrdiff_t":4 },
"LLP64": { "char":1, "short":2, "int":4, "long":4, "longlong":8, "wint_t":2, "wchar_t":2, "size_t":8, "intmax_t":8, "ptrdiff_t":8 }
}
}
This section covers the conversions diouxXDUO. The base-2 conversions bB
are an extension and are discussed at the end, but the same basic rules apply.
JS has one Number type (representing an IEEE754 8-byte floating point number) that is capable of representing a 32-bit integer. It cannot represent the full range of 64-bit integers exactly. Care is taken to avoid operations that may inadvertently result in a conversion to a smaller integral type.
JS Bitwise operations convert numbers to 32-bit integers before performing
operations. With the exception of the unsigned right shift operator >>>, all
operations act on signed integers. For example:
Math.pow(2,31) | 0; // -2147483648 == -Math.pow(2,31)
(Math.pow(2,32)-2) ^ 0; // -2
-1 >>> 0 // 4294967295 == Math.pow(2,32) - 1
JS Number can exactly represent every integer in the range -2^53 .. 2^53. For
lengths exceeding 32 bits, Math.round is appropriate.
| bits | unsigned | signed |
|---|---|---|
| 8 | V & 0xFF | V &= 0xFF; if(V > 0x7F) V-= 0x100 |
| 16 | V & 0xFFFF | V &= 0xFFFF; if(V > 0x7FFF) V-= 0x10000 |
| 32 | V >>> 0 | `V |
| 64 | Math.abs(Math.round(V)) | Math.round(V) |
When a length specifier implies a certain size (such as hh for a single-byte
integer), the number will be converted before rendering strings. For example:
printf("%1$02hhx %1$02hx %1$02lx %1$02llx\n", 256); // 00 100 100 100
printf("%1$02hhx %1$02hx %1$02lx %1$02llx\n", 4096); // 00 1000 1000 1000
printf("%1$02hhx %1$02hx %1$02lx %1$02llx\n", 65536); // 00 00 10000 10000
Values are restricted by first limiting the result to a specified number of bytes (appropriate bit-and) and then adding or subtracting to ensure the value is signed or unsigned according to the conversion specifier. If a length is specified, it overrides the implied length of the conversion. The following table describes the behavior of this implementation:
| implied C type | ctypes.json | length | conv default |
|---|---|---|---|
int or unsigned int | int | (none) | d i o u x X |
char or unsigned char | char | hh | |
short or unsigned short | short | h | |
long or unsigned long | long | l | D U O |
long long or unsigned long long | longlong | L ll q | |
intmax_t or uintmax_t | intmax_t | j | |
size_t or ssize_t | size_t | z Z | |
ptrdiff_t or unsigned variant | ptrdiff_t | t |
num.toString(10) produces the correct result for exact integers.
"u" conversion restricts values to int; "U" restricts to long.
Even though num.toString(8) is implementation-dependent, all browser
implementations use standard form for integers in the exact range.
The alternate form (#) prints a "0" prefix.
"o" conversion restricts values to int; "O" restricts to long.
Even though num.toString(16) is implementation-dependent, all browser
implementations use standard form for integers in the exact range.
The alternate form (#) prints a "0x" or "0X" prefix.
Unlike "U" "O" "D", "X" conversion uses A-F instead of a-f in hex.
num.toString(10) produces the correct result for exact integers. The flags
" +" control prefixes for positive integers.
"di" conversions restrict values to int; "D" restricts to long.
This section covers the conversions fFeEgGaA.
Due to C variadic argument promotion rules, float types are always promoted to
double. None of the conversions or length specifiers signal that an argument
is to be interpreted as a float. There is no JS canonical representation of
an extended floating point number, so JS Number suffices.
JS recognizes a few special IEEE754 values, as described in the following table:
| JS value | JS Expression | Description |
|---|---|---|
Infinity | 1./0. | Positive limiting value lim{x->0+} 1/x |
-Infinity | -1./0. | Negative limiting value lim{x->0+} -1/x |
NaN | 0./0. | Placeholder for "not-a-number" e.g. 0./0. |
-0. | -1/Infinity | Negative limiting value lim{x->0-} x |
JS Number methods render different strings from the POSIX spec:
| JS value | POSIX string | JS string |
|---|---|---|
Infinity | "inf" "INF" or "infinity" "INFINITY" | "Infinity" |
-Infinity | "-inf" "-INF" or "-infinity" "-INFINITY" | "-Infinity" |
NaN | "[-]nan" "[-]NAN" w/opt parenthesized chars | "NaN" |
-0. | uses negative sign (e.g. "-0" under "%f") | same as +0. |
This implementation performs the required adjustments.
Aside from the special cases discussed above, JS num.toExponential(prec)
differs from POSIX printf("%1$.*2$e", num, prec) in the exponent field: JS
writes exponents with the fewest digits (POSIX requires 2+ digits). This is
easily fixed by inspecting the output string and inserting a "0" when needed.
The optional # flag forces the decimal point to appear when precision is 0.
This is also easily corrected by adding a decimal point just before the "e".
The POSIX spec only requires that the number of digits after the decimal point is equal to the precision. It does not specify how many digits appear before the decimal point, nor does it specify how to handle numbers that cannot be exactly represented.
For values less than 1e21 the JS num.toFixed(n) generally matches %f with
the specified precision. However, for larger values toFixed defaults to the
exponential form.
The final form (exponential or standard) is determined based on the value. The
threshold is different from the JS toString / toPrecision thresholds and
depends on the specified precision as well as the base-10 exponent:
| Value | "%.3g" | toPrecision(3) |
|---|---|---|
| 1.2345e-4 | 0.000123 | 0.000123 |
| 1.2345e-5 | 1.23e-05 | 0.0000123 |
| 1.2345e-6 | 1.23e-06 | 0.00000123 |
| 1.2345e-7 | 1.23e-07 | 1.23e-7 |
According to JS spec, toPrecision uses standard form when precision > E and
E >= -6. For printf standard form is used when precision > E and E >= -4.
A general exponential form involves 3 parameters: radix of the mantissa, base of
the exponent expression, and radix of the exponent expression. The standard
exponential form uses decimal for all three parts. For base 16, there are quite
a few reasonable combinations. Consider the value 1.234567e-80:
| Mant | Exp Base | Radix-10 (sigil ";") | Radix-16 (sigil ";") |
|---|---|---|---|
| 10 | 10 | 1.234567;-80 | 1.234567;-50 |
| 16 | 10 | 1.3c0c9539b8887;-80 | 1.3c0c9539b8887;-50 |
| 16 | 16 | 5.daf8c8f5f4104;-67 | 5.daf8c8f5f4104;-43 |
| 16 | 4 | 1.76be323d7d041;-133 | 1.76be323d7d041;-85 |
| 16 | 2 | 1.76be323d7d041;-266 | 1.76be323d7d041;-10a |
POSIX "%a" uses a hex mantissa (16), decimal exponent radix (10), and binary
exponent base (2). The general normalized form requires that the integral part
of the mantissa to exceed 0 and not to exceed exponent base - 1 except in the
special case of 0. The sigil is p and exponent sign is always used.
JS num.toString(radix) is implementation-dependent for valid non-10 radices
(2-9, 11-36). IE uses hex-mantissa decimal-hex-exponent form when the
absolute value of the base-2 exponent exceeds 60. Otherwise, IE uses an exact
standard hexadecimal form. Chrome, Safari and other browsers always use the
exact standard hexadecimal form. Both forms are easily converted to "%a" by
calculating and dividing by the appropriate power of 2.
For each non-zero normal floating point value, there are 4 acceptable strings that represent the value, derived by multiplying the normalized value by powers of 2 and adjusting the exponent accordingly:
| Value | Normalized | Alternate *2 | Alternate *4 | Alternate *8 |
|---|---|---|---|---|
1 | 1p+0 | 2p-1 | 4p-2 | 8p-3 |
.2 | 1.9999999p-3 | 3.3333333p-4 | 6.6666666p-5 | c.cccccccp-6 |
.69 | 1.6147ae1p-1 | 2.c28f5c2p-2 | 5.851eb85p-3 | b.0a3d70ap-4 |
6.e20 | 1.043561p+69 | 2.086ac3p+68 | 4.10d586p+67 | 8.21ab0dp+66 |
JS engines follow the glibc model: multiply by a suitable power of 16 so that
the mantissa is between 1 and 16, render left to right one digit at a time, then
fix the result at the end. FreeBSD and OSX always show the normalized form.
This implementation defaults to the normalized form. To switch to the glibc
form, define DO_NOT_NORMALIZE in the JSFLAGS variable when building:
$ JSFLAGS=-DDO_NOT_NORMALIZE make
This section covers the conversions sScC.
JS has no concept of "wide strings" (wchar_t * in C), so the length modifiers
are ignored. s and S are treated as equivalent.
Arguments are first interpreted as strings by calling the String function.
Implementing toString on the argument to be converted may lead to unexpected
results:
var O = {valueOf:function() {return 456;}, toString:function() {return "123"}};
printf("%1$s %1$d", O); // "123 456"
If a positive precision is specified, up to that many characters will be taken from the string. Otherwise the entire string will be used:
printf("|%s|", "sheetjs"); // '|sheetjs|' (no precision)
printf("|%.9s|", "sheetjs"); // '|sheetjs|' (string shorter than precision)
printf("|%.5s|", "sheetjs"); // '|sheet|' (string truncated)
Lengths are measured using the JS string length accessor. Since there is no attempt to correct for multi-character sequences like combining marks, the results may be unexpected:
printf("%.1s","ñ"); // 'n' not "ñ"
If the width is specified and is greater than the width of the string to be
rendered, padding will be applied. If the "-" flag is specified, then the
string will be right-padded, otherwise it will be left-padded. If the "0"
flag is specified, the final string is left-padded with zeroes. The "-" flag
takes precedence over 0.
printf( "|%s|", "sheetjs"); // '|sheetjs|' (no width)
printf( "|%5s|", "sheetjs"); // '|sheetjs|' (string longer than width)
printf( "|%9s|", "sheetjs"); // '| sheetjs|' (no flag = left pad spaces)
printf( "|%09s|", "sheetjs"); // '|00sheetjs|' ("0" = left pad "0")
printf( "|%-9s|", "sheetjs"); // '|sheetjs |' ("-" = right pad space)
printf("|%-09s|", "sheetjs"); // '|sheetjs |' ("0" ignored)
JS has no concept of "wide characters" (wchar_t in C). The length modifier is
used in determining whether the number should be interpreted as one or two
16-bit character codes (when the "C" format or the "l" or "ll" specifiers are
used) or a single 8-bit char code. Precision and flags are ignored.
All other parameters are ignored.
JS has no true concept of pointers. In array and typed array contexts, it is
common to associate a position object that stores the address relative to the
start of the array. This implementation reads the l key and interprets as a
32-bit or 52-bit unsigned integer depending on size_t in the data model.
The normal output format is equivalent to "%#x" but the alternate form emits
using the "%d" format. When the pointer is invalid, -1 is rendered. Only
the "#" flag is interpreted.
var x = {}, y = {l:3};
printf("%1$p %1$#p", y); // 0x3 3
printf("%1$p %1$#p", x); // 0xFFFFFFFF -1
C printf permits a special n conversion which interprets the argument as an
integral pointer (interpreted size controlled by the length specifier) and
writes the number of characters printed to that pointer.
JS has no true concept of pointers in the C sense. The library works around
the limitation by interpreting the argument as an object and assigning to the
len key. The conversion does not write any characters to the output string:
var x = {};
printf("%1$s %2$J%2$n abc", "foo", x); // "foo {} abc", also sets x.len = 6
// |........| |......| (6 chars at that point)
This implementation mutates the object while processing:
var x = {};
printf("%1$s %2$J%2$n %3$s %2$J", "foo", x, "bar"); // 'foo {} bar {"len":6}'
glibc supports an m conversion that does not consume arguments. It renders
the string strerror(errno) where strerror is the libc function and errno
is the global error number.
JS has no equivalent of errno and no standard JS runtime exposes a similar
global error variable, so %m will write the default message "Success". A
positional parameter or # flag changes the behavior:
| form | position | behavior |
|---|---|---|
| main | no | do not read argument, emit "Success" |
| alt (flag #) | no | read and process next argument |
| main or alt | yes | read and process specified argument |
In all forms other than "%m", an argument will be processed as follows:
Error, emit "Success"message field is set, emit the error message.errno field is set, emit "Error number " followed by the errnovar x = new Error("sheetjs");
x.errno = 69; x.toString = function() { return "SHEETJS"; };
printf("|%#m|", x); // |sheetjs|
delete x.message;
printf("|%#m|", x); // |Error number 69|
delete x.errno;
printf("|%#m|", x); // |Error SHEETJS|
These additional conversions take advantage of unused format characters:
Values are converted to Boolean and tested for truthiness. The Y rendering
is the uppercase version of the equivalent rendering with format y.
| form | truthy value y (Y) | falsy value y (Y) |
|---|---|---|
| main | true (TRUE) | false (FALSE) |
| alt (flag #) | yes (YES) | no (NO) |
Width and precision are applied in the same manner as the s conversion.
printf("|%1$y|%2$Y|%1$#Y|%2$#y|%2$.1y|", 1, 0); // |true|FALSE|YES|no|f|
printf("|%05.2Y|%-5.2y|", 1, 0); // |000TR|fa |
The default rendering is the standard output from JSON.stringify. Alternate
form ("#" flag) renders using util.inspect if available.
var x = {
a: [1,[2,3,4],5,6,7],
b: {
c: {
d: { e:"f" },
g:"h",
i:"j"
},
k:"l",
m:"n",
o:"p"},
q: "r"
};
printf("%J", x) // '{"a":[1,[2,3,4],5,6,7],"b":{"c":{"d":{"e":"f"}, ..(ctnd)..
printf("%#J", x) // '{ a: [ 1, [ 2, 3, 4 ], 5, 6, 7 ],\n b: { c: { ..(ctnd)..
Width, precision and other flags are ignored.
Under the "T" conversion, the result of typeof arg is rendered. If the #
flag is specified, the type is derived from Object.prototype.toString:
printf("%1$T %1$#T", 1); // 'number Number'
printf("%1$T %1$#T", 'foo'); // 'string String'
printf("%1$T %1$#T", [1,2,3]); // 'object Array'
printf("%1$T %1$#T", null); // 'object Null'
printf("%1$T %1$#T", undefined); // 'undefined Undefined'
Under the "V" conversion, the result of arg.valueOf() is rendered:
var _f = function() { return "f"; };
var _3 = function() { return 3; };
printf("%1$d %1$s %1$V", {toString:_f}); // '0 f f'
printf("%1$d %1$s %1$V", {valueOf:_3}); // '3 [object Object] 3'
printf("%1$d %1$s %1$V", {valueOf:_3, toString:_f}); // '3 f 3'
The implementation is similar to the octal "o" and "O" conversions, except
for the radix (2 for "b" and "B") and the alternate-form prefix ("0b")
For compatibility purposes, format characters must be printable ASCII characters
(ASCII codes 0x20 - 0x7E). The 95 eligible characters are listed below:
| C | Type | C | Type | C | Type | C | Type |
|---|---|---|---|---|---|---|---|
a | conversion | A | conversion | | flag | ! | |
b | conversion | B | conversion | " | # | flag | |
c | conversion | C | conversion | $ | other | % | conversion |
d | conversion | D | conversion | & | ' | flag | |
e | conversion | E | conversion | ( | ) | ||
f | conversion | F | conversion | * | other | + | flag |
g | conversion | G | conversion | , | - | flag | |
h | length | H | . | other | / | ||
i | conversion | I | length | 0 | digit | 1 | digit |
j | length | J | conversion | 2 | digit | 3 | digit |
k | K | 4 | digit | 5 | digit | ||
l | length | L | length | 6 | digit | 7 | digit |
m | conversion | M | 8 | digit | 9 | digit | |
n | conversion | N | : | ; | |||
o | conversion | O | conversion | < | = | ||
p | conversion | P | > | ? | |||
q | length | Q | @ | [ | |||
r | R | \ | ] | ||||
s | conversion | S | conversion | ^ | _ | ||
t | length | T | conversion | ~ | { | ||
u | conversion | U | conversion | ` | ` | } | |
v | V | conversion | ` | ||||
w | length | W | |||||
x | conversion | X | conversion | ||||
y | conversion | Y | conversion | ||||
z | length | Z | length |
C provides no guidance on the actual character set. According to K&R all valid characters in source code must be in a character set that is a subset of the 7-bit ASCII set. This implementation falls back on the UTF-16 base required by JS. When converting C literal strings, there are a few differences in escaping:
| C escape sequence | Equivalent JS | Notes |
|---|---|---|
"\a" | "\007" | BEL character will not ring in browser |
"\?" | "?" | JS does not handle trigraphs |
"\ooo" (octal) | "\ooo" | JS uses Latin-1 for non-ASCII codes |
"\xhh" (hex) | "\xhh" | JS uses Latin-1 for non-ASCII codes |
Opera does not always include the last significant digit in base 16 rendering.
For example, (-6.9e-11).toString(16) is "0.000000004bddc5fd160168" in every
other browser but is "0.000000004bddc5fd16017" in Opera. The test suite skips
the %a/%A precision-less formats in Opera.
Object.prototype.toString.call gives unexpected results in older browsers, and
no attempt is made to correct for them. The test suite ignores those cases:
| value | %#T expected | %#T IE < 9 | %#T Android < 4.4 |
|---|---|---|---|
null | "Null" | "Object" | "global" |
undefined | "Undefined" | "Object" | "global" |
quad_t and u_quad_t conversionZ length conversion and extended m error supportI32/I64 fixed lengthsLP64 data model but can be configured to support ILP32 or LLP64FAQs
Pure-JS printf
The npm package printj receives a total of 281,093 weekly downloads. As such, printj popularity was classified as popular.
We found that printj demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 1 open source maintainer collaborating on the project.
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