Euler
You will find this code in the EJS console at page Initialization.
lines marked // include remarks that do not influence the calculation.
xE[0]=0;
yE[0]=valueFrom;
derivativeE[0]=yE[0];//for exponential
for (var i=1; i<n; i++) {//loop definitionn)
xE[i]=xE[i-1]+delta;
yE[i]=yE[i-1]+delta*derivativeE[i-1];
derivativeE[i]=yE[i];//for exponential
//back to start of loop
FE[i]=(valueFrom*Math.exp(xE[i])-yE[i])/(valueFrom*Math.exp(xE[i]));
}
Heun
xH[0]=0;
yH[0]=valueFrom;
derivativeH_a[0]=yH[0];//for exponential
for (var i=1; i<n; i++) {//loop definition)
xH[i]=xH[i-1]+delta;
yH_a[i]=yH[i-1]+delta*derivativeH_a[i-1];
derivativeH_b[i]=yH_a[i];////for exponential
yH[i]=yH[i-1]+delta/2*(derivativeH_a[i-1]+derivativeH_b[i]);
derivativeH_a[i]=yH[i];////for exponential
//back to loop
FH[i]=(valueFrom*Math.exp(xH[i])-yH[i])/(valueFrom*Math.exp(xH[i]));
}