Translating Arduino Code into Javascript

The first attempt

/** 'readAccel() reads X,Y,Z accelerometer values'*/
LSM9DS1.prototype.readAccel=function(){
 var temp=this.xgReadBytes(Regs.OUT_X_L_XL, 6);
 var i;var mm=[0,0,0];
 this.a[0]=this.twos_comp(temp[0],temp[1]­);
 this.a[1]=this.twos_comp(temp[2],temp[3]­);
 this.a[2]=this.twos_comp(temp[4],temp[5]­);
etc etc
}
/** 'xgReadBytes(subAddress,count) read count bytes from gyro/accelerometer at subAddress'*/
LSM9DS1.prototype.xgReadBytes=function(s­ubAddress,count){
 var dest= new Uint8Array(count);
 var x=this.xgAddress;
 this.i2c.writeTo(x, subAddress|0x80);
 dest=this.i2c.readFrom(x, count);
 return dest;
};//end xgReadBytes

/** 'prototype.twos_comp(low,high) converts low and high bytes'*/
LSM9DS1.prototype.twos_comp=function(low­,high){
 var t=(high << 8) | low;
  return(t & 0x8000 ? t - 0x10000 : t);
};//end twos_comp

But it can be done differently using ArrayBuffer and DataView
Read the byte stream into an ArrarBuffer, then use a DataView to convert to signed integers, note the littleEndian flag.

var A=new ArrayBuffer(12);
A[0]=1;
A[1]=0;
A[2]=0xfe;
A[3]=0xff;
var B= new Int16Array(A,0,6);
console.log(A);
console.log(B);
console.log("B0= "+B[0]+" B[1]= "+B[1]);
//var C=dataview.getUint16(byteOffset [, littleEndian])
var dataview = new DataView(A);
console.log("A "+dataview.getInt16(0, true)); console.log("A1 "+dataview.getInt16(0, false)); 
console.log("A2 "+dataview.getInt16(2, true)); console.log("A3 "+dataview.getInt16(2, false)); 
B[0]=23;B[1]=-23;B[2]=56;B[3]=-56;
console.log(A);
console.log(B);
A[0]=2;A[1]=0;
A[2]=0xfe;A[3]=0xff;
console.log(A);
console.log(B);

https://developer.mozilla.org/en-US/docs­/Web/JavaScript/Reference/Global_Objects­/DataView/getUint16
https://developer.mozilla.org/en-US/docs­/Web/JavaScript/Reference/Global_Objects­/DataView/getInt16>