The JavaScript Math.round() function will round a decimal to the nearest whole number, but I found that I needed was to round a number to the nearest 10. So after a bit of thinking I realised that I could divide the value by 10, round it using the round() function and then multiply the result by 10.

So taking it a step further I decided to create a function that would round a number to the nearest 10, 100, 1000 or whatever value is entered. If this value is less than 0 then do the reverse by multiplying and dividing the number by the given value of accuracy. Here is the function.

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function roundNearest(num, acc){
    if ( acc < 0 ) {
        num *= acc;
        num = Math.round(num);
        num /= acc;
        return num;
    } else {
        num /= acc;
        num = Math.round(num);
        num *= acc;
        return num;
    }
}

Here are some tests of the function.

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alert( roundNearest(12345, 1000) ); // 1200
alert( roundNearest(12345.1234, -100) ); //12345.12

This function can be simplified a little by putting all of the calculations onto one line.

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function roundNearest(num, acc){
    if ( acc < 0 ) {
        return Math.round(num*acc)/acc;
    } else {
        return Math.round(num/acc)*acc;
    }
 }