What are the Alternatives to eval in JavaScript?

Question

I have a little bit of code that looks just like this:

function StrippedExample(i1, i2, i3, i4, i5, i6, i7, i8) {
    this.i = [];
    for (var i=1,j=0 ;i<9;i++) {
        var k = eval("i"+i);
        if (k > 0) {
            this.i[j++] = k;
        }
    }
}

FireBug profiler claims that second longest function is eval(), taking up to nearly 6% of the run time.

Everyone says eval is EVIL (as in bad) and slow (as I have found), but I can't really do anything else - the server simply pulls the data out the database and pushes to the browser.

What alternatives do I have? I could do the same as I am doing here on the server but that just shifts the burden higher up the chain. I can't change the database layout since everything hooks into those 8 variables and is a massive undertaking.

Answer 1

function StrippedExample(i1, i2, i3, i4, i5, i6, i7, i8) {
    var args = [i1, i2, i3, i4, i5, i6, i7, i8]; // put values in an array
    this.i = [];
    for (var i=0,j=0 ;i<8;i++) { // now i goes from 0-7 also
        var k = args[i]; // get values out
        if (k > 0) {
            this.i[j++] = k;
        }
    }
}

The above code can be simplified further, I just made the minimal change to get rid of eval. You can get rid of j, for example:

function StrippedExample(i1, i2, i3, i4, i5, i6, i7, i8) {
    var args = [i1, i2, i3, i4, i5, i6, i7, i8];
    this.i = [];
    for (var i = 0; i < args.length; i++) {
        var k = args[i];
        if (k > 0) { this.i.push(k); }
    }
}

is equivalent. Or, to use the built-in arguments object (to avoid having your parameter list in two places):

function StrippedExample(i1, i2, i3, i4, i5, i6, i7, i8) {
    this.i = [];
    for (var i = 1; i < arguments.length; i++) {
        var k = arguments[i];
        if (k > 0) { this.i.push(k); }
    }
}

Even if you weren't filtering the list, you don't want to do something like this.i = arguments because arguments is not a real Array; it has a callee property that you don't need and is missing some array methods that you might need in i. As others have pointed out, if you want to quickly convert the arguments object into an array, you can do so with this expression:

Array.prototype.slice.call(arguments)

You could use that instead of the var args = [i1, i2 ... lines above.

Answer 2

Eval alternative:

exp = '1 + 1'
x = Function('return ' + exp)()
console.log(x)