JSFiddle - React, Tailwind, and code Playground

JavaScript

function init() {
            var x = prompt("How many meters?");
            convertMetricMeters(x);

            function convertDown(x) {
                
                // checks the number of digits after decimal point
                decimals = (x.split('.')[1] || []).length
                
                if (decimals == 1 || decimals == 2) {
                    console.log("deci");
                }

                else if (decimals == 3) {
                    console.log("centi");
                }

                else if (decimals == 4) {
                    console.log("milli");
                }

                else {
                    console.log("error");
                }   
            }               

            function convertUp(x) {
                if (1 <= x && x <= 99) {
                    console.log("deca");
                }

                else if (100 <= x && x <= 999) {
                    console.log("hecto");
                }

                else if (1000 <= x) {
                    console.log("kilo");
                }

                else {
                    console.log("error");
                }
            }

            function convertMetricMeters(x) {
                if (x < 1) {
                    convertDown(x);
                }

                else if (x > 9) {
                    convertUp(x);
                }

                else {
                    console.log("Appropriate Metric Unit");
                }
            }
        }
        init();