Convert long number into abbreviated string in JavaScript, with a special shortness requirement

Approach 1: Built-in library

I would recommend using Javascript’s built-in library method Intl.NumberFormat

Intl.NumberFormat('en-US', {
  notation: "compact",
  maximumFractionDigits: 1
}).format(2500);

Approach 2: No library

But you can also create these abbreviations with simple if statements, and without the complexity of Math, maps, regex, for-loops, etc.

Formatting Cash value with K

const formatCash = n => {
  if (n < 1e3) return n;
  if (n >= 1e3) return +(n / 1e3).toFixed(1) + "K";
};

console.log(formatCash(2500));

Formatting Cash value with K M B T

const formatCash = n => {
  if (n < 1e3) return n;
  if (n >= 1e3 && n < 1e6) return +(n / 1e3).toFixed(1) + "K";
  if (n >= 1e6 && n < 1e9) return +(n / 1e6).toFixed(1) + "M";
  if (n >= 1e9 && n < 1e12) return +(n / 1e9).toFixed(1) + "B";
  if (n >= 1e12) return +(n / 1e12).toFixed(1) + "T";
};

console.log(formatCash(1235000));

Using negative numbers

let format;
const number = -1235000;

if (number < 0) {
  format="-" + formatCash(-1 * number);
} else {
  format = formatCash(number);
}