Using Javascript to subtract large strings

Function subtract (str1, str2 ){
// Alert (a + "-" + B + "=" + (a-B ));
// Subtraction result
Var c = "";
// Debit sign
Var flog = true;
// The length of the subtrahend and subtrahend.
Var I = str1.length-1;
Var j = str2.length-1;
If (j> I ){
Var c = '-' + subtract (str2, str1 );
Return c;
}
If (j = I & str1 <str2 ){
Var c = '-' + subtract (str2, str1 );
Return c;
}
For (; I> = 0; I --, j --){
// Obtain the low position
Var charb = 0;
If (j> = 0 ){
Charb = str2.charAt (j );
}

// Obtain the subtrahend
Var chara = str1.charAt (I );
// If the digit is used
If (flog! = True ){
// If the borrow value is 0
If (chara = 0 ){
// Borrow another one and then subtract one
Flog = false;
Chara = 9;
} Else if (chara-1) <charb ){
// If not, borrow one.
Flog = false;
Chara = 1 + chara-1;
} Else {
Chara = chara-1;

Flog = true;
}
}
// Low position Subtraction
If (chara> = charb ){
C = ''+ (chara-charb) + c;
// Alert ('enough reduction' + c );
} Else {
// If the borrow space is not enough
Var x = 1 + chara-charb;
// Alert (x );
C = ''+ x + c;
Flog = false;
// Alert ('undiminished '+ c );
}
}
// String
Return c;
}