Looping variable conditions in an if statement

Question

I have a lot of condition statements stored in a table, each of which has some wildly different formats (if x > Y, if Z = true, if Z <= 12, etc). I've stored them in such a way that I could easily loop through the condition statements and execute a function to update the relevant attributes.

However, as I've found out, it won't let me store my conditional statement as a variable. Meaning, I have to explicitly declare each conditional statement, rather than declare it once and loop through all possible conditions.

Is there a way to do this I'm missing, or simply not understanding?

In the below example, I'd like to use eTable[i].condition to loop through all of the conditions stored at that location. Is this possible somehow?

function checkEvents () {
    eventConditionLength = Object.keys(eTable).length;

    for (let i = 0; i < eventConditionLength; i++) {
        if (eTable[i].condition) {
            alert(i);
        };
    };
};

eTable

var eTable = {
0: {
        sceneTitle: '', //Descriptive/organizational only
        sceneHeader: ['Event 1', 'Event 1', 'Event 1'],
        sceneImage: ['/images/1.jpg', '/images/2.jpg', '/images/3.jpg'],
        sceneText: ['Test Scene Text 1', 'Test Scene Text 1', 'Test Scene Text 1'],
        sceneControls: ['myFunction1', 'myFunction2', 'myFunction3'],
        visible: false,
        completed: false,
        condition: 'cEvent == "e0002"'
    },
1: {
        sceneTitle: '', //Descriptive/organizational only
        sceneHeader: ['Event 1', 'Event 1', 'Event 1'],
        sceneImage: ['/images/1.jpg', '/images/2.jpg', '/images/3.jpg'],
        sceneText: ['Test Scene Text 1', 'Test Scene Text 1', 'Test Scene Text 1'],
        sceneControls: ['myFunction1', 'myFunction2', 'myFunction3'],
        visible: false,
        completed: false,
        condition: 'stat1 > 15 && completed == false'
    }
};

Answer 1

As you probably know, as you've coded it, it'll always work (as long as the condition isn't an empty string), because any non-empty string is always truthy.

The best and most secure way would be to write some kind of parser which will analyze the condition by breaking it up cleanly and analyzing it. These typically look something like:

const condition = 'x < y';

/**
 * condition is the full string
 * a is the left-hand variable, if needed
 * b is the right-hand variable, if needed
 */
function resolve(condition, a, b) {
  // regex will get the non-alphanumeric operator and the left and right operands
  const [, left, operator, right] = condition.match(/([a-z0-9]+)\s*([^a-z0-9\s]+)\s*([a-z0-9]+)/);
  
  // you'd need a case for each vald operator
  switch (operator) {
    case '<':
      return a < b;
  }
}

console.log(resolve(condition, 3, 5));
console.log(resolve(condition, 5, 3));

Obviously, that's a simple example, but the basic process is the same. There are likely also libraries out there that will handle it for you.

Then you could just use it as needed:

if (resolve('a > b', a, b)) {
  // do something
}

You'd also want some logic that would see if the operands are numbers, and if so use those values instead of the passed in variables.

Another option that I hesitate to provide is to use the eval() function. This function is generally a really big, super bad no-no, as it executes arbitrary code and can open all kinds of security and logic holes. However, it is worth talking about for scenarios where it may be appropriate (like a really small, quick Node.js script for personal use).

const x = 3;
const y = 5;

if (eval('x < y')) {
  console.log('condition hit');
}

This will work, and in your example, just wrap your eTable[i].condition in the eval (and make sure variables it is looking for are declared in the current scope).

Again though, this is generally really bad to use, so if you use it, be sure to read up all about it before moving forward with it.

Answer 2

You can use eval().

function checkEvents () {
    eventConditionLength = Object.keys(eTable).length;


    for (let i = 0; i < eventConditionLength; i++) {
        if (eval(eTable[i].condition))
        {
            alert(i);
        };
    };
};

As Vasan pointed out in the comments, there are a few downsides to eval to be aware of.

Answer 3

I believe you passing strings to if condition statement, because eTable[i].condition it is not logical condition, it is just string (I believe you have strings). You need to evaluate your strings as code using eval():

for (let i = 0; i < eventConditionLength; i++) {
    if (eval(eTable[i].condition)) {
        alert(i);
    };
};

P.S.: eval() is bad code style and dangerous. Be careful with that.

Answer 4

I can give you 2 solutions for you problem.

1. The better way, store functions. Each function will hold your condition, for example, instead of store eTable['someCondition] = "x > y", store it like eTable['someCondition] = () => x > y, Then you can invoke it simple by if (eTable[i].condition()) { /* handle true */ } else { /* handle false */ } .
2. the bad way, use eval() and store string that will represent js code to excute.

I think that first suggestion can achive your goal.