Features

Everything here is additive and opt-in — the default behaviour is unchanged, so existing callers see identical output. Try any of it live in the playground.

Fallbacks

Add || literal to a token and the literal is used whenever the token would otherwise render empty:

parser({ hi: 'Hi {{ user.name || Guest }}' }, {});
// { hi: 'Hi Guest' }

Quote the literal to keep surrounding spaces:

parser({ hi: 'Hi {{ user.name || "no name on file" }}' }, { user: {} });
// { hi: 'Hi no name on file' }

Filters (pipes)

Pipe a value through one or more named transforms with | name. Transforms are supplied via options.transforms and can take colon-separated arguments (numbers are parsed as numbers, everything else as strings):

const transforms = {
  upper: (v) => String(v).toUpperCase(),
  round: (v, digits) => Number(v).toFixed(Number(digits)),
};

parser(
  { line: '{{ name | upper }} costs $ {{ price | round: 2 }}' },
  { name: 'pen', price: 9.5 },
  {
    transforms,
  },
);
// { line: 'PEN costs $ 9.50' }

Filters chain left to right. Referencing a transform that was not registered throws Unknown transform "<name>".

Escaping

Prefix a token with a backslash to emit the braces literally instead of interpolating:

parser({ doc: 'Use \\{{ token }} to interpolate {{ what }}' }, { what: 'values' });
// { doc: 'Use {{ token }} to interpolate values' }

passthrough

By default, parameter values that are neither string nor number are dropped. Set passthrough: true to keep them as-is:

parser({ enabled: true, greeting: '{{ name }}' }, { name: 'Felippe' }, { passthrough: true });
// { enabled: true, greeting: 'Felippe' }

raw

When a parameter string is exactly one token, raw: true returns the resolved value with its real type instead of stringifying it — handy for building payloads rather than display strings:

parser(
  { user: '{{ account.user }}' },
  { account: { user: { id: 1, name: 'Felippe' } } },
  {
    raw: true,
  },
);
// { user: { id: 1, name: 'Felippe' } }

A raw token still honours fallbacks ({{ account.user || none }}), and resolves to undefined when absent with no fallback.