CODEWITHSUNDEEP
javascriptevalscopedynamic-scope
Samuel BR
Samuel BR

Reputation: 51

Local variable to external function

I'm trying to access a variable local to a function in an external function as a free variable.

This is what I'm trying to achieve:

function try_evaluate() {
    var i = 0;
    show_r("i <= 10");
}

function show_r(expression) {
    if (eval(expression)) {
        i++;
        show_r(expression);
    } else alert(eval(expression));
}

I get an error because the variable i is not defined in the scope of show_r. Making i a global variable works but I need i to be a local variable.

Upvotes: 2

Views: 870

Answers (5)

zardilior
zardilior

Reputation: 2978

You should add i t the string but like this. Pass it as a paremeter and integrate it in the expression.

function try_evaluate() {
   var i = 0;
   show_r(i," <= 10");
}

function show_r(i,expression) {
    if (eval(i+expression)) {
        i++;
        show_r(i+expression);
    } else alert(eval(i+expression));
 }

or you could try replacing it in the expression to allow for && expresions with multiple i's

function try_evaluate() {
   var i = 0;
   show_r(i,"i <= 10");
}

function show_r(i,expression) {
    if (eval(expression.replace("i",i)) {
        i++;
        show_r(expression.replace("i",i));
    } else alert(eval(expression.replace("i",i)));
 }

Upvotes: 0

Aadit M Shah
Aadit M Shah

Reputation: 74234

This seems like the kind of thing dynamic scoping in JavaScript is useful for. See this StackOverflow thread: Is it possible to achieve dynamic scoping in JavaScript without resorting to eval?

This is what I would do:

function try_evaluate() {
    var i = 0;
    var show_r_dynamic = eval("(" + show_r + ")");
    show_r_dynamic("i <= 10");
}

Everything else remains the same:

function show_r(expression) {
    if (eval(expression)) {
        i++;
        show_r(expression);
    } else alert(eval(expression));
}

You can see the demo here: http://jsfiddle.net/53Qsu/

Upvotes: 1

Lydon
Lydon

Reputation: 2975

This will work better:

function try_evaluate(){
    var i = 0;
    show_r('$ <= 10', i);
    show_r('false', i);
}

function show_r(expression, i){
    if (eval(expression.replace('$', i))){
        i += 1
        show_r(expression, i);
    } else {
        alert(eval(expression.replace('$', i)));
    }       
}

Upvotes: 0

smk
smk

Reputation: 5912

function try_evaluate(){
    var i = 0;
    show_r(i);
}
function show_r(x){
    //console.log(eval.expression);
    if (x<=10){
       x++;
        console.log(x);
        show_r(x);
    } else {
        console.log(false);
    }       
}`

Why are you doing eval? I think you can achieve what you intend to do with above code.

Upvotes: 1

Explosion Pills
Explosion Pills

Reputation: 191789

With var i, that variable is only accessible in the scope of the function in which it is defined. An anonymous function in the same scope does have access to it, but a call to a function defined externally does not have access. Instead, you need to pass the variable or define it in a scope that is available to both contexts (e.g. the global scope, but I'm not suggesting you use that).

I'd also be extremely wary about what you're trying to do specifically. If you have to use eval to do something, it's probably the wrong thing.

Upvotes: 2

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