Your cms function is either going to return a Number or an Object, and neither of them have a slice method.

A better function to use would be Math.abs(), which will convert any negative number into a positive one (and leave any positive number alone) but be aware that it will return NaN if you pass it an object.

The .slice method is for strings and arrays, not for numbers.

function cms(miliseconds, format) {
    let days, hours, minutes, seconds, total_hours, total_minutes, total_seconds;
    total_seconds = parseInt(Math.floor(miliseconds / 1000));
    total_minutes = parseInt(Math.floor(total_seconds / 60));
    total_hours = parseInt(Math.floor(total_minutes / 60));
    days = parseInt(Math.floor(total_hours / 24));
    seconds = parseInt(total_seconds % 60);
    minutes = parseInt(total_minutes % 60);
    hours = parseInt(total_hours % 24);
    switch (format) {
        case 's':
            return total_seconds;
        case 'm':
            return total_minutes;
        case 'h':
            return total_hours;
        case 'd':
            return days;
        default:
            return {d: days, h: hours, m: minutes, s: seconds};
    }
}

const cd3F = cms(...); // Result: -3512
const abs_cd3F = Math.abs(cd3F);

You should use Math.abs. Only if it were a string, you could use .slice(1).

If you only want positive numbers then you might as well put Math.abs inside your function.

function cms(milliseconds, format) {
    milliseconds = Math.abs(milliseconds);
    let days, hours, minutes, seconds, total_hours, total_minutes, total_seconds;
    total_seconds = parseInt(Math.floor(milliseconds / 1000));
    total_minutes = parseInt(Math.floor(total_seconds / 60));
    total_hours = parseInt(Math.floor(total_minutes / 60));
    days = parseInt(Math.floor(total_hours / 24));
    seconds = parseInt(total_seconds % 60);
    minutes = parseInt(total_minutes % 60);
    hours = parseInt(total_hours % 24);
    switch (format) {
        case 's':
            return total_seconds;
        case 'm':
            return total_minutes;
        case 'h':
            return total_hours;
        case 'd':
            return days;
        default:
            return {d: days, h: hours, m: minutes, s: seconds};
    }
}

const cd3F = cms(-3512000, "s"); // Result: 3512

Why so complicated? n * -1

const cd3F = cms(...); // Result: -3512
cd3F = cd3F < 0 ? cd3F * -1 : cd3F;
console.log(cd3F); // 3512