The traditional way of teaching on truncation from when I was at school, college & night school...
Using the following example
var x = 123.456789; // the number to truncate
console.log("The number : " + x);
var y = x.toFixed(2); // not recommended by lots of sources because of spurious rounding
var z = Math.floor( x * 100 ) / 100; // the time honoured way
var x = x.toString(); // convert to a string
x = x.slice(0, x.indexOf( ".") + 3 ); // find decimal and move two places to right
console.log("toFixed : " + y);
console.log("traditional : " + z);
console.log("unconventional : " + x);
outputs the following
The number : 123.45678901
toFixed : 123.46
traditional : 123.45
unconventional : 123.45
You can see how toFixed rounds up values, Math.floor is used to ensure that the 3rd value has no impact on the truncation value, IMHO taking the value, making it a string and slicing out your value is a better option, it can be cast back in to a number by subtracting zero.
x = x - 0; or as some others would say is a better option, x = x * 1; which I was taught that in computers addition and subtraction is much faster than multiplication or division because they only add multiples and take longer processing times.