f the target value is an integer, return true, otherwise return false. If the value is NaN or Infinity, return false. The method will also return true for floating point numbers that can be represented as integer. It will always return false if the value is not a number.

Note that some number literals, while looking like non-integers, actually represent integers — due to the precision limit of ECMAScript floating-point number encoding (IEEE-754). For example, 5.0000000000000001 only differs from 5 by 1e-16, which is too small to be represented. (For reference, Number.EPSILON stores the distance between 1 and the next representable floating-point number greater than 1, and that is about 2.22e-16.) Therefore, 5.0000000000000001 will be represented with the same encoding as 5, thus making Number.isInteger(5.0000000000000001) return true.

In a similar sense, numbers around the magnitude of Number.MAX_SAFE_INTEGER will suffer from loss of precision and make Number.isInteger return true even when it's not an integer. (The actual threshold varies based on how many bits are needed to represent the decimal — for example, Number.isInteger(4500000000000000.1) is true, but Number.isInteger(4500000000000000.5) is false.)