Javascript toFixed() is not working as expected

Question

I am using toFixed but the method does not operate as expected

parseFloat(19373.315).toFixed(2);

//19373.31   Chrome 

Expected Output : 19373.32

parseFloat(9373.315).toFixed(2);
// 9373.32  Working fine

Why does the first example round down, whereas the second example round up?

Answer 1

The problem is that binary floating point representation of most decimal fractions is not exact. The internal representation of 19373.315 may actually be something like 19373.314999999, so toFixed rounds down, while 19373.315 might be 19373.315000001, which rounds up.

Answer 2

Why does the first example round down, whereas the second example round up?

Look at the binary representation of the two values in memory.

const farr = new Float64Array(2);
farr[0] = 19373.315;
farr[1] = 9373.315;
const uarr = new Uint32Array(farr.buffer);
console.log(farr[0], uarr[1].toString(2).padStart(32, 0) + uarr[0].toString(2).padStart(32, 0));
console.log(farr[1], uarr[3].toString(2).padStart(32, 0) + uarr[2].toString(2).padStart(32, 0));

Without diving into the details, we can see that the second value has an additional '1' at the end, which is lost in the first larger value when it is fit into 64 bits.