How to get a leading '+' for positive numbers in Intl.NumberFormat?

Question

I'm using Intl.NumberFormat to convert a number type to a formatted string in typescript/javascript in Angular2. I want a native solution and this is ideal, but I need a leading plus sign to be included for positive numbers.

If this is not possible with Intl.NumberFormat how else might I do it natively?

@Input() amount : number;

drawLabel() {
   var formatter = new Intl.NumberFormat("en-GB",
                                         {
                                            style: "decimal",
                                            minimumFractionDigits:1
                                         });
   ...

   this.label = formatter.format(this.amount)
}

Answer 1

In 2019 you do this:

var formatter = new Intl.NumberFormat("en-GB", { style: "decimal",  signDisplay: 'always' });

console.log(formatter.format(-100.123456));
// output -100.123
console.log(formatter.format(100.123456));
// output +100.123
console.log(formatter.format(0));
// output +0.

Answer 2

Try something like this:

class FormatterWithSign extends Intl.NumberFormat {
  constructor(...args) {
    super(...args);
  }
  
  format(x) {
    var res = super.format(x);
    return x < 0 ? res : "+" + res;
  }
}

var formatter = new FormatterWithSign("en-GB", { style: "decimal", minimumFractionDigits:1 });

console.log(formatter.format(-100.123456));
console.log(formatter.format(100.123456));
console.log(formatter.format(0));