lowpassfilter.js

export default function LowpassFilter(Fc) {
    var Q = 0.707;
    var K = Math.tan(Math.PI * Fc);
    var norm = 1 / (1 + K / Q + K * K);
    this.a0 = K * K * norm;
    this.a1 = 2 * this.a0;
    this.a2 = this.a0;
    this.b1 = 2 * (K * K - 1) * norm;
    this.b2 = (1 - K / Q + K * K) * norm;
    this.z1 = this.z2 = 0;
    this.value = 0;
    this.tick = function(value) {
        var out = value * this.a0 + this.z1;
        this.z1 = value * this.a1 + this.z2 - this.b1 * out;
        this.z2 = value * this.a2 - this.b2 * out;
        return out;
    };
}

// Fc is the frequency that it cuts off. 
// Lower value will smooth more. As you get closer to 1 it wont smooth at all.
// For animation a value between 0.01 and 0.1 isnt uncommon. 

// create a filter (once) that you will re-use
var filter = new LowpassFilter(0.1);

...

// In you loop, call the filter
var filteredValue = filter.tick(unfilteredValue);

This code is ported from ofxBiquadFilter written of openFrameworks and is super useful for animation smoothing