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The Javascript standard is actually very clear about the difference between "mathematical values" and Numbers as represented in the language (https://262.ecma-international.org/11.0/#sec-mathematical-operations)

From the mathematical standpoint, 100.0 is definitely an integer. From the JS standpoint, isInteger(100.0) is true, because isInteger is defined to return true for Numbers whose mathematical value is integer.

Your uncertainty comes from the fact in some other languages 100 and 100.0 are indeed quite different things, with different storage mechanisms and the nature of operations performed on them. In Javascript, we actually don't know how a particular number is stored, and have no control over that. The standard says that all Numbers should behave as if they all were IEEE 754-2019 double values, but what sequence of operations a particular engine actually performs when you compute 1+2 and how they are different from 1.2+3.4 is up to that engine.