http://www.musicdsp.org/showone.php?id=29

Audio filter.js

const LOWPASS=0,HIGHPASS=1,BANDPASS=2;
class Filter{
	constructor(mode){
		this.cutoff=.99;
		this.resonance=0;
		this.mode=mode;
		this.b0=0;this.b1=0;this.b2=0;this.b3=0;
		
		this.feedbackAmount=0;
		this.cutoffMod=0;
	}
	process(input){
		if(input===0){return 0;}
		var calculatedCutoff=this.getCalculatedCutoff();
		this.b0+=calculatedCutoff*(input-this.b0+this.feedbackAmount*(this.b0-this.b1));
		this.b1+=calculatedCutoff*(this.b0-this.b1);
		this.b2+=calculatedCutoff*(this.b1-this.b2);
		this.b3+=calculatedCutoff*(this.b2-this.b3);
		switch(this.mode){
			case LOWPASS:return this.b3;
			case HIGHPASS:return input-this.b3;
			case BANDPASS:return this.b0-this.b3;
		}
		return 0;
	}
	getCalculatedCutoff(){
		return Math.max(Math.min(this.cutoff+this.cutoffMod,.99),.001);
	}
	calculateFeedbackAmount(){
		this.feedbackAmount=this.resonance+this.resonance/(1-this.getCalculatedCutoff());
	}
	setCutoffMod(value){this.cutoffMod=value;this.calculateFeedbackAmount();}
	setCutoff(value){this.cutoff=value;this.calculateFeedbackAmount();}
	setResonance(value){this.resonance=value;this.calculateFeedbackAmount();}
}