CODEWITHSUNDEEP
javascript
gipinani
gipinani

Reputation: 14418

Detect if a variable has been used in assignment

I'm developing a dynamic form, where the defined cells can either:

I need a way to provide a hint to the user, highlighting the cells he/she must fill to have a valid form.

In other works I could have

cellA -> input by user cellB -> input by user cellC -> formula = cellA + cellB cellA and cellB are required (highlighted). This case is pretty easy to handle.

I can also have more complex formulas, call them scripts. For example

cellC =
    if(cellD == true)
    {
        cellC = cellA;
    }
    else 
    {
        cellC = cellB;
    }

I'm wondering if in javascript it would be possible to understand whether the vars cellA and cellB have been used as a right part in any assignment.

Upvotes: 0

Views: 268

Answers (2)

GMaiolo
GMaiolo

Reputation: 4628

You could achieve something like this but you'd need to use an object (as @T.J Crowder did in his answer) or by using a Proxy like this

const grid = new Proxy({
  cellA: 'CELL-A',
  cellB: 'CELL-B',
  cellC: 'CELL-C',
  cellD: 'CELL-D'
}, {
  get (obj, prop) {
    if (prop in obj) {
      // here you can use an object or whatever you need
      // to keep track of used properties/values
      console.log(`${prop} has been retrieved.`)
      return obj[prop]
    }
  },
  set (obj, prop, val) {
    console.log(`${prop} has been set with value ${val}.`)
    obj[prop] = val
  }
})

Therefore doing

grid.cellA = grid.cellB

Yields

// cellB has been retrieved.
// cellA has been set with value CELL-B.

And sets the correct value of cellB to cellA

Upvotes: 1

T.J. Crowder
T.J. Crowder

Reputation: 1075337

I'm wondering if in javascript it would be possible to understand whether the vars cellA and cellB have been used as a right part in any assignment.

No, you can't hook into a variable being read like that.

You could make them accessor properties of an object, though, which would give you a means of doing that:

const used = { a: false, b: false, c: false};
const values = { };
const cells = {
    get a() {
        used.a = true;
        return values.a;
    },
    set a(value) {
        values.a = value;
    },
    
    get b() {
        used.b = true;
        return values.b;
    },
    set b(value) {
        values.b = value;
    },
    
    get c() {
        used.c = true;
        return values.c;
    },
    set c(value) {
        values.c = value;
    }
};

// You'd only give `cells` to the code doing this, so it doesn' thave
// access to `used` or `values`.
cells.a = 2;
cells.b = 21;
const result = cells.a * cells.b;
console.log("result: " + result);
console.log("a used? " + used.a);
console.log("b used? " + used.b);
console.log("c used? " + used.c);

Alternatively, you could do something similar with a Proxy.

Or you can store the flags and values on the object itself if you're not worried about them being accessible by other code. (Or use a class instance and private fields, which are being implemented in JavaScript engines now...)

Upvotes: 5

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