mozilla/pdf.js · error · Error

Invalid number of params in printf

Error message

Invalid number of params in printf

What it means

Thrown by `Util.printf()` (src/scripting_api/util.js:66) when called with zero arguments. `printf` mirrors C's `printf`: the first argument is the format string and subsequent arguments fill its conversion specifiers. No arguments means no format string at all.

Solutions

1. Always pass at least the format string: `util.printf('%s', value)`.
2. Default the format argument: `util.printf(fmt || '%s', value)`.
3. Guard the call site: skip `printf` entirely when there is nothing to format.

Example fix

// before
util.printf(...args); // args may be []
// after
if (args.length === 0) return '';
return util.printf(...args);

function printfSafe(util, ...args) {
  if (args.length === 0) return '';
  return util.printf(...args);
}

function hasFormatArg(args) {
  return Array.isArray(args) && args.length > 0;
}

When it happens

Trigger: Calling `util.printf()` with no arguments, or `util.printf(...emptyArray)` where the array was unexpectedly empty. Often a missing return value from a helper that was supposed to supply the format.

Common situations: Dynamic format strings built from optional configuration where the configuration object was empty; refactoring that introduced an extra call layer swallowing the format argument.

Related errors

• Not enough arguments in printf
• First argument of printf must be a string
• Invalid rotation: must be a multiple of 90
• app.activeDocs is read-only
• app.constants is read-only

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/8988b7f961c8c79b. Report an issue: GitHub.

Appendix: source

Thrown at src/scripting_api/util.js:66

    "Thursday",
    "Friday",
    "Saturday",
  ];

  MILLISECONDS_IN_DAY = 86400000;

  MILLISECONDS_IN_WEEK = 604800000;

  constructor(data) {
    super(data);

    // used with crackURL
    this._externalCall = data.externalCall;
  }

  printf(...args) {
    if (args.length === 0) {
      throw new Error("Invalid number of params in printf");
    }

    if (typeof args[0] !== "string") {
      throw new TypeError("First argument of printf must be a string");
    }

    // eslint-disable-next-line regexp/no-misleading-capturing-group
    const pattern = /%(,[0-4])?([+ 0#]+)?(\d+)?(\.\d+)?(.)/g;
    const PLUS = 1;
    const SPACE = 2;
    const ZERO = 4;
    const HASH = 8;
    let i = 0;
    return args[0].replaceAll(
      pattern,
      function (_, nDecSep, cFlags, nWidth, nPrecision, cConvChar) {
        // cConvChar must be one of d, f, s, x
        if (

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