JSFiddle - React, Tailwind, and code Playground

JavaScript

function shortenNum(num){
    
    if(num.length > 6){
        
        var first = num.slice(0, 3); 
        var last = num.slice(-3); 
        //console.log(first + "..." + last );
        alert(first + "..." + last );
    } else {
        alert(num)
    }
}

// should return 112...899
shortenNum("112233445566778899");
shortenNum("19999");
shortenNum("3536355");