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The format function produces a string by formatting a number of other values according to a specification string. It is similar to the printf function in C, and other similar functions in other programming languages.

format(spec, values...)


> format("Hello, %s!", "Ander")
"Hello, Ander!"
> format("There are %d lights", 4)
"There are 4 lights"

The format function is most useful when you use more complex format specifications.

Specification Syntax

The specification is a string that includes formatting verbs that are introduced with the % character. The function call must then have one additional argument for each verb sequence in the specification. The verbs are matched with consecutive arguments and formatted as directed, as long as each given argument is convertible to the type required by the format verb.

By default, % sequences consume successive arguments starting with the first. Introducing a [n] sequence immediately before the verb letter, where n is a decimal integer, explicitly chooses a particular value argument by its one-based index. Subsequent calls without an explicit index will then proceed with n+1, n+2, etc.

The function produces an error if the format string requests an impossible conversion or accesses more arguments than are given. An error is also produced for an unsupported format verb.


The specification may contain the following verbs.

%%Literal percent sign, consuming no value.
%tConvert to boolean and produce true or false.
%bConvert to integer number and produce binary representation.
%dConvert to integer and produce decimal representation.
%oConvert to integer and produce octal representation.
%xConvert to integer and produce hexadecimal representation with lowercase letters.
%XLike %x, but use uppercase letters.
%eConvert to number and produce scientific notation, like -1.234456e+78.
%ELike %e, but use an uppercase E to introduce the exponent.
%fConvert to number and produce decimal fraction notation with no exponent, like 123.456.
%gLike %e for large exponents or like %f otherwise.
%GLike %E for large exponents or like %f otherwise.
%sConvert to string and insert the string’s characters.
%qConvert to string and produce a JSON quoted string representation.