Difference between Date.toJSON() and Date.toISOString()

Internally, toJSON() calls toISOString() if it's available, so no difference.

15.9.5.44 Date.prototype.toJSON ( key )

This function provides a String representation of a Date object for use by JSON.stringify (15.12.3).

When the toJSON method is called with argument key, the following steps are taken:

  1. Let O be the result of calling ToObject, giving it the this value as its argument.

  2. Let tv be ToPrimitive(O, hint Number).

  3. If tv is a Number and is not finite, return null.

  4. Let toISO be the result of calling the [[Get]] internal method of O with argument "toISOString".

  5. If IsCallable(toISO) is false, throw a TypeError exception.

  6. Return the result of calling the [[Call]] internal method of toISO with O as the this value and an empty argument list.


JSON date format follows ISO 8601, which is what toISOString produces as well. There is no functional difference between the values returned by either method.


One convenient difference is that if you have an invalid date, .toJSON() will output null. However, .toISOString()'s behavior can vary. In firefox this outputs a string "Invalid Date" but in chrome it raises an exception.

Edit: Recent versions of Firefox have fixed the behavior to be the same as chrome (raising exception). However, the difference between .toJSON() and .toISOString() remains. (outputting null vs. raising exception)

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