Given a set of variables, how to find undefined variables in a literal expression?

Question

Purpose: Find undefined variables

Suppose we have a set of variables: varA, varB, varC

we know the following expression is valid:

varA + varB - varC

and the following expression is not valid(because VARC is not in the set of variables):

varA + varB - VARC

I've made it working by:

function validate(comma separated variables)
{
    the literal expression
    return 'PASS'
}

Now I simply run "validate()" without passing in any argument(Yes, it is supported by javascript), if the function returns 'PASS', it means the literal expression is valid. Otherwise it has error.

In this example, the function will not return 'PASS' because we have a typo in "varA + varB - VARC" and "VARC" will be reported by the javascript debugger.

function validate(varA, varB, varC)
{
    varA + varB - VARC
    return 'PASS'
}
validate();  //call the function WITHOUT passing in any argument

---------------------All good so far. I can validate arbitrary expression ! ---------------------

However, if I use javascript objects as a variable in the set, this way does not work. In the following code, 'PASS' is returned (while I'm expecting the debugger report error).

function MyClass()
{
    this.attr1 = null
    this.attr2 = null
}

myClass = new MyClass();

function validate(varA, varB, varC)
{
    varA + varB - varC + myClass.attr3
    return 'PASS'
}

Question 1: How do I find a way to report "myClass.attr3" is not defined?

NOTE: I can not tokenize (actually it is more complicated, has to be a grammar parser) the EXPRESSION and validate each token via "undefine test". I have to validate at the EXPRESSION level.

Edit:

I've tested the code and found that:

varA === undefined
VARC can not be evaluated, exception thrown  //the only case that is helping me
myClass.attr1 === null
myClass.attr3 === undefined

Answer 1

the expected result is: validate(...) function throw exception and not returning 'PASS' to the spidermonky engine.

One issue with the approach at Question is that the code within validate function does not throw an exception.

You use + and - operators, which can convert undefined to NaN, which is the result of

varA + varB - varC + myClass.attr3

For example

console.log(undefined + void 0 - undefined + void 0); // NaN

The solution is to re-evaluate and adjust your code if you are expecting an exception to be thrown if an expression includes undefined. The expression at validate() call currently evaluates to NaN, not an exception or undefined.

function myClass() {}

function validate(varA, varB, varC) {
  var x = varA + varB - varC + myClass.attr3;

  console.assert(!isNaN(x), x, [isNaN(x)]);
  return 'PASS'
}

validate()

Answer 2

The correct way to check if variables are defined is using

if (typeof var === "undefined") {
    // some kind of error
}

as discussed here.

Answer 3

In order to find out undefined variables and null values there is a way to find it in javascript.
Go [here](Is there a standard function to check for null, undefined, or blank variables in JavaScript?"this text appears when you mouse over")! This will help.

Answer 4

function MyClass()
{
    this.attr1 = null
    this.attr2 = null
}

myClass = new MyClass();

function validate(varA, varB, varC, myClass_attr1, myClass_attr2)
{
    varA + varB - varC + myClass_attr3
    return 'PASS'
}

Since this code will report myClass_attr3 is not defined, all I need to do is:

1. flatten myClass to all leaves: myClass.attr1 and myClass.attr2

2. Then preprocess the expression and set of variables by replacing "." With "_".

3. Compose the validate function and run

4. The engine will report because exception is thrown and caught in the engine.

I am doing more test...