How do I chop/slice/trim off last character in string using Javascript?

The easiest method is to use the slice method of the string, which allows negative positions (corresponding to offsets from the end of the string):

const s = "your string";
const withoutLastFourChars = s.slice(0, -4);

If you needed something more general to remove everything after (and including) the last underscore, you could do the following (so long as s is guaranteed to contain at least one underscore):

const s = "your_string";
const withoutLastChunk = s.slice(0, s.lastIndexOf("_"));
console.log(withoutLastChunk);

A regular expression is what you are looking for:

let str = "foo_bar";
console.log(str.replace(/_bar$/, ""));

1. (.*), captures any character multiple times:

console.log("a string".match(/(.*).$/)[1]);

2. ., matches last character, in this case:

console.log("a string".match(/(.*).$/));

3. $, matches the end of the string:

console.log("a string".match(/(.*).{2}$/)[1]);

If you want to do generic rounding of floats, instead of just trimming the last character:

var float1 = 12345.00,
    float2 = 12345.4567,
    float3 = 12345.982;

var MoreMath = {
    /**
     * Rounds a value to the specified number of decimals
     * @param float value The value to be rounded
     * @param int nrDecimals The number of decimals to round value to
     * @return float value rounded to nrDecimals decimals
     */
    round: function (value, nrDecimals) {
        var x = nrDecimals > 0 ? 10 * parseInt(nrDecimals, 10) : 1;
        return Math.round(value * x) / x;
    }
}

MoreMath.round(float1, 1) => 12345.0
MoreMath.round(float2, 1) => 12345.5
MoreMath.round(float3, 1) => 12346.0

EDIT: Seems like there exists a built in function for this, as Paolo points out. That solution is obviously much cleaner than mine. Use parseFloat followed by toFixed