There are several ways you could rejig this.
The simplest way I can think of would be to make a global object to hold all of your class base values:
var class_bases = {};
class_bases.Fighter = {Str:20, Dex:20, Con:18, Int:15, Wis:15, Chrm:16};
class_bases.Thief = {Str:16, Dex:20, Con:18, Int:18, Wis:16, Chrm:20};
class_bases.WhiteMage = {Str:20, Dex:20, Con:18, Int:15, Wis:15, Chrm:16};
class_bases.BlackMage = {Str:15, Dex:13, Con:18, Int:20, Wis:20, Chrm:14};
Then you could reference into it as follows:
//in the place of all of the if/else statements:
this.Str = InitVal(class_bases[this.CharClass].Str);
this.Dex = InitVal(class_bases[this.CharClass].Dex);
this.Con = InitVal(class_bases[this.CharClass].Con);
this.Int = InitVal(class_bases[this.CharClass].Int);
this.Wis = InitVal(class_bases[this.CharClass].Wis);
this.Chrm = InitVal(class_bases[this.CharClass].Chrm);