Format a number with SI Prefix, with fixed number of decimals

Question

I'm looking over the available type for d3.format

The available type values are:

exponent ("e") - use Number.toExponential.

general ("g") - use Number.toPrecision.

fixed ("f") - use Number.toFixed.

integer ("d") - use Number.toString, but ignore any non-integer values.

rounded ("r") - like fixed, but round to precision significant digits.

percentage ("%") - like fixed, but multiply by 100 and suffix with "%".

rounded percentage ("p") - like rounded, but multiply by 100 and suffix with "%".

SI-prefix ("s") - like rounded, but with a unit suffixed such as "9.5M" or "1.00µ".

https://github.com/mbostock/d3/wiki/Formatting#wiki-d3_format

What I would like is an SI-prefix that is like fixed not rounded does such a format option exist?

Some examples:

var format = d3.format('.1s');
format(12600000); // Would like 12.6M get 10M
format(12400000); // Would like 12.4M get 10M
format(1240000); // Would like 1.2M get 1M
format(1290000); // Would like 1.3M get 1M

Answer 1

You almost got it right. Using d3.formatPrefix() one can get the SI prefix. To get the rounded number without decimals, I used Javascript's .toFixed():

var prefix = d3.formatPrefix(137594020);
console.log(prefix.symbol); // "M"
console.log(prefix.scale(137594020).toFixed()); // 138

var prefix = d3.formatPrefix(13759402);
console.log(prefix.symbol); // "M"
console.log(prefix.scale(13759402).toFixed()); // 14

var prefix = d3.formatPrefix(1375);
console.log(prefix.symbol); // "k"
console.log(prefix.scale(1375).toFixed()); // 1

You can try it yourself at jsfiddle.