9007199254740991 (9,007,199,254,740,991, or ~9 quadrillion).

Double precision floating point format only has 52 bits to represent the mantissa, so it can only safely represent integers between -(2⁵³ – 1) and 2⁵³ – 1. "Safe" in this context refers to the ability to represent integers exactly and to compare them correctly. For example, Number.MAX_SAFE_INTEGER + 1 === Number.MAX_SAFE_INTEGER + 2 will evaluate to true, which is mathematically incorrect. See Number.isSafeInteger() for more information.

Because MAX_SAFE_INTEGER is a static property of Number, you always use it as Number.MAX_SAFE_INTEGER, rather than as a property of a number value.

Number.EPSILON is 2^-52, while MAX_SAFE_INTEGER is 2⁵³ – 1 — both of them are derived from the width of the mantissa, which is 53 bits (with the highest bit always being 1). Multiplying them will give a value very close — but not equal — to 2.

Number.MAX_SAFE_INTEGER * Number.EPSILON; // 1.9999999999999998

• Polyfill of Number.MAX_SAFE_INTEGER in core-js
• Number.MIN_SAFE_INTEGER
• Number.isSafeInteger()
• BigInt