Question

I’m teachimg myself Javascript by porting a C# algebra expression compiler to Javascript. I have an OpBank object that stores the attributes of each operator, e.g., it’s symbol and a pointer to a function that implements the operator. I know JS doesn’t have function pointers but I don’t know what else to call it.

When I try to load the page, I get the following error:

“Microsoft JScript runtime error: Unable to get value of the property ‘Plus’: object is null or undefined”

The error occurs when the browser tries to parse the Operators array at the bottom, it’s complaining about the OpBank.Plus. I tried OpBank.Plus, OpBank.Plus(), just plain old Plus, and this.Plus. In any case I can’t get it to recognize the Plus function when the Operators array gets loaded.

var OpBank = {
    Plus: function (left, right) {
        return (left + right);
    },

    Minus: function (left, right) {
        return (left - right);
    },

    Operators: [
      (new KeyValuePair("+", new ABinaryOperator("+", 1, OpType.Binary, "+", OpBank.Plus))),
      (new KeyValuePair("-", new ABinaryOperator("-", 1, OpType.Binary, "-", OpBank.Minus)))
      ]
}

Is what I’m attempting not possible? Should I use the constructor syntax for OpBank instead, i.e., var ObBank = function(){} then use prototype to add the functions and array? I didn’t want to do that because I need only one OpBank for the life of the app and I don’t want every method to have to instantiate a new OpBank because it’s a pretty big object (only part of it is shown above).

Answer 1

OpBank does not exist yet while the object literal is evaluated.

You can simply split it into two steps though:

var OpBank = {
    Plus: function (left, right) {
        return (left + right);
    },

    Minus: function (left, right) {
        return (left - right);
    }
}

OpBank.Operators = [
    (new KeyValuePair("+", new ABinaryOperator("+", 1, OpType.Binary, "+", OpBank.Plus))),
    (new KeyValuePair("-", new ABinaryOperator("-", 1, OpType.Binary, "-", OpBank.Minus)))
];