TypeError: can't convert x to BigInt

The JavaScript exception "x can't be converted to BigInt" occurs when attempting to convert a Symbol, null, or undefined value to a BigInt, or if an operation expecting a BigInt parameter receives a number.

Message

TypeError: Cannot convert null to a BigInt (V8-based)
TypeError: can't convert null to BigInt (Firefox)
TypeError: Invalid argument type in ToBigInt operation (Safari)

Error type

TypeError.

What went wrong?

When using the BigInt() function to convert a value to a BigInt, the value would first be converted to a primitive. Then, if it's not one of BigInt, string, number, and boolean, the error is thrown.

Some operations, like BigInt.asIntN, require the parameter to be a BigInt. Passing in a number in this case will also throw this error.

Examples

Using BigInt() on invalid values

const a = BigInt(null);
// TypeError: can't convert null to BigInt
const b = BigInt(undefined);
// TypeError: can't convert undefined to BigInt
const c = BigInt(Symbol("1"));
// TypeError: can't convert Symbol("1") to BigInt

const a = BigInt(1);
const b = BigInt(true);
const c = BigInt("1");
const d = BigInt(Symbol("1").description);

Note: Simply coercing the value to a string or number using String() or Number() before passing it to BigInt() is usually not sufficient to avoid all errors. If the string is not a valid integer number string, a SyntaxError is thrown; if the number is not an integer (most notably, NaN), a RangeError is thrown. If the range of input is unknown, properly validate it before using BigInt().

Passing a number to a function expecting a BigInt

const a = BigInt.asIntN(4, 8);
// TypeError: can't convert 8 to BigInt
const b = new BigInt64Array(3).fill(3);
// TypeError: can't convert 3 to BigInt

const a = BigInt.asIntN(4, 8n);
const b = new BigInt64Array(3).fill(3n);

See also