Math.pow 给出错误的结果

Question

I was trying to repeat a character N times, and came across the Math.pow function.

But when I use it in the console, the results don't make any sense to me:

Math.pow(10,15) - 1 provides the correct result 999999999999999

But why does Math.pow(10,16) - 1 provide 10000000000000000 ?

Answer 1

You are producing results which exceed the Number.MAX_SAFE_INTEGER value, and so they are not accurate any more up to the unit.

This is related to the fact that JavaScript uses 64-bit floating point representation for numbers, and so in practice you only have about 16 (decimal) digits of precision.

Since the introduction of BigInt in EcmaScript, you can get an accurate result with that data type, although it cannot be used in combination with Math.pow . Instead you can use the ** operator.

See how the use of number and bigint (with the n suffix) differ:

10  ** 16  - 1  // == 10000000000000000
10n ** 16n - 1n // ==  9999999999999999n

Answer 2

Unlike many other programming languages, JavaScript does not define different types of numbers, like integers, short, long, floating-point etc.

JavaScript numbers are always stored as double-precision floating-point numbers, following the international IEEE 754 standard.

This format stores numbers in 64 bits, where the number (the fraction) is stored in bits 0 to 51, the exponent in bits 52 to 62, and the sign-in bit 63:

Integers (numbers without a period or exponent notation) are accurate up to 15 digits.