How do I reset the setInterval timer?

If by "restart", you mean to start a new 4 second interval at this moment, then you must stop and restart the timer.

function myFn() {console.log('idle');}var myTimer = setInterval(myFn, 4000);// Then, later at some future time, // to restart a new 4 second interval starting at this exact moment in timeclearInterval(myTimer);myTimer = setInterval(myFn, 4000);

You could also use a little timer object that offers a reset feature:

function Timer(fn, t) {    var timerObj = setInterval(fn, t);    this.stop = function() {        if (timerObj) {            clearInterval(timerObj);            timerObj = null;        }        return this;    }    // start timer using current settings (if it's not already running)    this.start = function() {        if (!timerObj) {            this.stop();            timerObj = setInterval(fn, t);        }        return this;    }    // start with new or original interval, stop current interval    this.reset = function(newT = t) {        t = newT;        return this.stop().start();    }}

Usage:

var timer = new Timer(function() {    // your function here}, 5000);// switch interval to 10 secondstimer.reset(10000);// stop the timertimer.stop();// start the timertimer.start();

Working demo: https://jsfiddle.net/jfriend00/t17vz506/

Once you clear the interval using clearInterval you could setInterval once again. And to avoid repeating the callback externalize it as a separate function:

var ticker = function() {    console.log('idle');};

then:

var myTimer = window.setInterval(ticker, 4000);

then when you decide to restart:

window.clearInterval(myTimer);myTimer = window.setInterval(ticker, 4000);