Precision is still fixed

[thany]

Precision is still fixed

[MikeMcl]

As the documentation makes clear, the result of the sqrt method is calculated and rounded according to the current DECIMAL_PLACES setting.

So your

BigNumber(2).sqrt()

returns the square root of 2 to what looks like 20 decimal places, then toPrecision displays that number to the requested precision.

As you state, the square root of 2 has an infinite decimal expansion, so the sqrt method needs to know how many digits to be calculated before, not after, it returns.

BigNumber.config(50);

BigNumber(2).sqrt().toPrecision(50);
// '1.4142135623730950488016887242096980785696718753770'

Examples of JavaScript Number type toPrecision:

// Firefox
Math.sqrt(2).toPrecision(70);
// '1.414213562373095145474621858738828450441360473632812500000000000000000'

// IE
Math.sqrt(2).toPrecision(21);    // '1.41421356237309510000'