Function returning array item for assignment

Question

I am implementing a drag'n'drop interface in js which has two tables of objects and a single object as source or destination. To execute the "move" operation, I thought of using these two class methods:

    _getVar(id) {
        var ind;
        switch (id[0]) {
            case 'c':
                return this.clipboard;
            case 's':
                ind = Number(id.substr(1)) - 1;
                return this.SynthPatches[ind];
            case 'f':
                ind = Number(id.substr(1)) - 1;
                return this.FilePatches[ind];
        }
    }

    
    move(from, to) {
        var fv = this._getVar(from);
        var tv = this._getVar(to);
        if (fv == undefined) return "Source empty";
        if (tv == undefined) return "Destination undefined";
        tv = fv;
        return "Ok";
    }

This does not work. fv and tv are local references to the correct objects. The reference fv is copied to tv and then both are forgotten. I would like something like a deep copy in python, but it looks like the only solution is to inline _getVar() nine times, which makes for really ugly code.

tv = fv; that will not behave as you expect, it will just assign the local variable tv the value of fv, so tv will no more point to it's original object, and nothing happens outside of that function, you need a refference example: mainObj.tv = fv; or mainObj[index].tv = fv; — Saadi Toumi Fouad
– Saadi Toumi Fouad, Commented Aug 26, 2020 at 8:04
The challenge here is, that there is no single "mainObj". Instead it is _getVars task to select one of three possible mainObjects (this.SynthPatches, this.FilePatches or this.clipboard). — tommiport5
– tommiport5, Commented Aug 26, 2020 at 10:53

Jonas Høgh · Accepted Answer · 2020-08-26 12:24:03Z

1

The easiest way to reduce duplication here is probably to add an optional second parameter to the function and allow it to set as well as get, e.g.:

_getOrSetVar(id, value) {
    var ind;
    switch (id[0]) {
        case 'c':
            if (value !== undefined) this.clipboard = value
            return this.clipboard;
            break;
        case 's':
            ind = Number(id.substr(1)) - 1;
            if (value !== undefined) this.SynthPatches[ind] = value
            else return this.SynthPatches[ind];
            break;
        case 'f':
            ind = Number(id.substr(1)) - 1;
            if (value !== undefined) this.FilePatches[ind] = value
            else return this.FilePatches[ind];
            break;
    }
}

And perform your update like this:

this._getOrSetVar(to, this._getOrSetVar(from))

edited Aug 26, 2020 at 12:24

answered Aug 26, 2020 at 8:07

Jonas Høgh

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2 Comments

tommiport5 Over a year ago

Nice solution! The cases now need break statements and I need to handle the "undefined" situations with exceptions, but the idea is great.

Jonas Høgh Over a year ago

Good catch, I've added breaks. The undefined checks are for when you're using the method to get the value.

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