zloirock/core-js · error · SyntaxError

Failed to parse number's exponent value at: ${i}

Error message

Failed to parse number's exponent value at: ${i}

What it means

This SyntaxError is thrown by core-js's polyfilled JSON.parse when a number contains an 'e' or 'E' exponent marker but no digits follow it (after an optional sign). JSON requires at least one exponent digit, so inputs like '1e', '2e+' are invalid and parsing stops at the reported index.

Source

Thrown at packages/core-js/modules/es.json.parse.js:218

  number: function () {
    var source = this.source;
    var startIndex = this.index;
    var i = startIndex;
    if (at(source, i) === '-') i++;
    if (at(source, i) === '0') i++;
    else if (exec(IS_NON_ZERO_DIGIT, at(source, i))) i = this.skip(IS_DIGIT, i + 1);
    else throw new SyntaxError('Failed to parse number at: ' + i);
    if (at(source, i) === '.') {
      var fractionStartIndex = i + 1;
      i = this.skip(IS_DIGIT, fractionStartIndex);
      if (fractionStartIndex === i) throw new SyntaxError("Failed to parse number's fraction at: " + i);
    }
    if (at(source, i) === 'e' || at(source, i) === 'E') {
      i++;
      if (at(source, i) === '+' || at(source, i) === '-') i++;
      var exponentStartIndex = i;
      i = this.skip(IS_DIGIT, i);
      if (exponentStartIndex === i) throw new SyntaxError("Failed to parse number's exponent value at: " + i);
    }
    return this.node(PRIMITIVE, Number(slice(source, startIndex, i)), startIndex, i);
  },
  keyword: function (value) {
    var keyword = '' + value;
    var index = this.index;
    var endIndex = index + keyword.length;
    if (slice(this.source, index, endIndex) !== keyword) throw new SyntaxError('Failed to parse value at: ' + index);
    return this.node(PRIMITIVE, value, index, endIndex);
  },
  skip: function (regex, i) {
    var source = this.source;
    for (; i < source.length; i++) if (!exec(regex, at(source, i))) break;
    return i;
  },
  until: function (array, i) {
    i = this.skip(IS_WHITESPACE, i);
    var chr = at(this.source, i);

View on GitHub (pinned to 84e45fba09)

Solutions

1. Fix the source so the exponent marker is always followed by digits ('1e' -> '1e0', '3E-' -> '3E-1') or remove the marker
2. Validate numeric strings with a regex like /^-?\d+(\.\d+)?([eE][+-]?\d+)?$/ before embedding them in JSON
3. Build payloads with JSON.stringify on real numbers instead of manual string templates
4. Use the reported index to find and inspect the malformed token in the payload
5. Wrap JSON.parse in try/catch to convert the SyntaxError into an application-level parse failure with context

Example fix

// before
JSON.parse('{"n": 1e}'); // SyntaxError: Failed to parse number's exponent value at: 9
// after
JSON.parse('{"n": 1e0}');

// validate exponent-bearing numeric strings before use
function isValidExponent(numStr) {
  return /^[+-]?\d+(\.\d+)?([eE][+-]?\d+)?$/.test(numStr) && !/[eE][+-]?$/.test(numStr);
}

try {
  return JSON.parse(input);
} catch (e) {
  if (e instanceof SyntaxError && /exponent value/.test(e.message)) {
    throw new Error('JSON number has exponent marker without digits: ' + e.message);
  }
  throw e;
}

When it happens

Trigger: JSON.parse on text containing a number with a dangling exponent, e.g. '{"n": 1e}', '3E-', '5e+,'; caused by truncation or by string-building code that appends 'e' + possibly-empty variable.

Common situations: Truncated scientific-notation output from logs or streams; constructing payloads with `"v": ${mantissa}e${exp}` where exp is empty/undefined; hand-edited data files; legacy engines running the core-js JSON.parse polyfill.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• Failed to parse number at: ${i}
• Failed to parse number's fraction at: ${i}
• Failed to parse value at: ${index}
• String should only contain hex characters
• Unexpected extra character: "${chr}" after the parsed data a

AI-assisted analysis of zloirock/core-js@84e45fba09 (2026-08-30). Data as JSON: /api/errors/d6d928d4124bb27f. Report an issue: GitHub.