Whats the difference between JS Number.MAX_SAFE_INTEGER and MAX_VALUE?

Number.MAX_SAFE_INTEGER is the largest integer which can be used safely in calculations.

For example, Number.MAX_SAFE_INTEGER + 1 === Number.MAX_SAFE_INTEGER + 2 is true — any integer larger than MAX_SAFE_INTEGER cannot always be represented in memory accurately. All bits are used to represent the digits of the number.

Number.MAX_VALUE on the other hand is the largest number possible to represent using a double precision floating point representation. Generally speaking, the larger the number the less accurate it will be.

More information double-precision floating point numbers on Wikipedia

JS numbers are internally 64-bits floats (IEEE 754-2008).

MAX_SAFE_INTEGER is the maximum integer that can be safely represented in that format, meaning that all the numbers below that value (and above MIN_SAFE_INTEGER) can be represented as integers.

MAX_VALUE comes from 2^1023 (11 bits mantissa minus mantissa sign), thats about 10^308.

Is it the number that comes about if you're using all 63 bits for the exponent instead of the safe 11 bits?

The mantissa (exponent) is always 11 bits, (not so) surprinsingly that's enough for up to 10^308.

Basically floating point numbers are represented as:

digits * 2 ** movement

digits (the mantissa) has 52 bits (and 1 "hidden bit"), movement has 11 bits, and both together form a 64bit number (with 1 sign bit). Through that, you can represent all kinds of different numbers as you can store very large numbers (large positive movement), very small numbers (large negative movement), and integers (digits).

What is Number.MAX_SAFE_INTEGER ?

Integers can just be represented with movement being set in a way that the mantissa is actually the number itself, then digits contains the 52 + 1 bit number, and that can hold up to 2 ** 53 - 1 numbers (which is Number.MAX_SAFE_INTEGER).

Now for larger numbers, you have to use movement, which basically means that you move the digits left or right, and therefore you lose accuracy.

(Imagine digits would just take 8 bits)

  number     >     digits | movement > result  // savely represented  11111111   >  11111111  | 0        > 11111111  // lost the last 1  111111111  >  11111111  | 1        > 111111110  // lost the two last 1s  1111111111 >  11111111  | 10       > 1111111100

What is Number.MAX_VALUE ?

If you set all bits of digits and all bits of movement, you get a number (2 ** 53 - 1) that is moved by 2 ** 10 - 1 to the left, and that is the largest number that can be stored in the 64 bit, everything that is larger is Infinity (which is represented as the movement being 2 ** 10 and the mantissa being 0).