x

A number.

A number representing e^x - 1, where e is the base of the natural logarithm.

For very small values of x, adding 1 can reduce or eliminate precision. The double floats used in JS give you about 15 digits of precision. 1 + 1e-15 = 1.000000000000001, but 1 + 1e-16 = 1.000000000000000 and therefore exactly 1.0 in that arithmetic, because digits past 15 are rounded off.

When you calculate $\mathrm{e}^x$ where x is a number very close to 0, you should get an answer very close to 1 + x, because $\lim_{x \to 0} \frac{\mathrm{e}^x - 1}{x} = 1$. If you calculate Math.exp(1.1111111111e-15) - 1, you should get an answer close to 1.1111111111e-15. Instead, due to the highest significant figure in the result of Math.exp being the units digit 1, the final value ends up being 1.1102230246251565e-15, with only 3 correct digits. If, instead, you calculate Math.exp1m(1.1111111111e-15), you will get a much more accurate answer 1.1111111111000007e-15, with 11 correct digits of precision.

Because expm1() is a static method of Math, you always use it as Math.expm1(), rather than as a method of a Math object you created (Math is not a constructor).

• Polyfill of Math.expm1 in core-js
• Math.E
• Math.exp()
• Math.log()
• Math.log10()
• Math.log1p()
• Math.log2()
• Math.pow()