Templates

template jsFmt(code: untyped)

JavaScript string template literal interpolation for cstring.

This is not meant to be better or even equal than normal stdlib strformat, is just for when you need faster performance and string interpolation for JavaScript targets, otherwise use the normal strformat for better user experience.

Note: To use any Nim symbols inside the string interpolation block, the symbol declaration must have the {.exportc.} pragma.

Warning: This is unsafe, but has zero cost in performance. Debugging with fmt"{expr=}" is NOT supported.

See also:

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals
• https://caniuse.com/template-literals
• https://tc39.es/ecma262/#sec-template-literals
• strformat

Example:

jsFmt:
  doAssert fmt"" == "" ## Empty string is Ok.
  let foo {.exportc.} = "platypus".cstring
  var bar {.exportc.} = "capybara".cstring
  doAssert fmt"${foo}" == "platypus".cstring
  doAssert fmt("foo ${foo} bar ${bar} baz ${8 + 1} !?".cstring) == "foo platypus bar capybara baz 9 !?".cstring

jsFmt:
  let a {.exportc.} = 1
  let b {.exportc.} = 2
  ## Triple-quoted and multi-line string is Ok.
  doAssert &"""\ ${(a + b * a + b)} \t ${3.14} \n /""".cstring == """\ 5 \t 3.14 \n /""".cstring

var str: cstring
jsFmt:
  doAssert fmt"${  -1.234560e+02  } ${-0.0} {42} ${0.0} $0 $$ {{{}}}" == "-123.456 0 {42} 0 $0 $$ {{{}}}".cstring
  doAssert fmt"{123.456=}" == "{123.456=}".cstring ## Debug with `=` NOT supported.
  str = fmt"${         1_000_000         }"
doAssert str == "1000000"  ## Get result back outside of jsFmt block.

jsFmt:
  let x {.exportc.} = 3.14
  let y {.exportc.} = 0.99
  const s {.exportc.} = "${ x + y }".cstring ## The string can be a constant.
  doAssert fmt(s) == "4.13".cstring