Math.round(num * 10) / 10 works, here is an example…
var number = 12.3456789
var rounded = Math.round(number * 10) / 10
// rounded is 12.3
if you want it to have one decimal place, even when that would be a 0, then add…
var fixed = rounded.toFixed(1)
// fixed is always to 1 d.p.
// NOTE: .toFixed() returns a string!
// To convert back to number format
parseFloat(number.toFixed(2))
// 12.34
// but that will not retain any trailing zeros
// So, just make sure it is the last step before output,
// and use a number format during calculations!
EDIT: Add round with precision function…
Using this principle, for reference, here is a handy little round function that takes precision…
function round(value, precision) {
var multiplier = Math.pow(10, precision || 0);
return Math.round(value * multiplier) / multiplier;
}
… usage …
round(12345.6789, 2) // 12345.68
round(12345.6789, 1) // 12345.7
… defaults to round to nearest whole number (precision 0) …
round(12345.6789) // 12346
… and can be used to round to nearest 10 or 100 etc…
round(12345.6789, -1) // 12350
round(12345.6789, -2) // 12300
… and correct handling of negative numbers …
round(-123.45, 1) // -123.4
round(123.45, 1) // 123.5
… and can be combined with toFixed to format consistently as string …
round(456.7, 2).toFixed(2) // “456.70”
var number = 123.456;
console.log(number.toFixed(1)); // should round to 123.5