JSFiddle - React, Tailwind, and code Playground

JavaScript

Number.prototype.toNDigits = function (n) {
    return (Math.abs(this) < 1) ?
        this.toFixed(n - 1) :
        this.toPrecision(n);
};

alert( (100).toNDigits(5) );
alert( (100.45485485).toNDigits(5) );
alert( (0.00034700).toNDigits(5) );