What is the proper way to create a function that accepts the parameter fct_x and can access the variable a, which must be defined within the function?

Question

What is the proper way to create a function that accepts the parameter fct_x and can access the variable a, which must be defined within the function?

function myFunction() {
    return a + 1; // any variable accessing var-a here can be anything.
}

function anotherFunction(callback) {
    var a = 2;
    callback(); // no exception thrown, a is defined in the scope
}

anotherFunction(myFunction);   // no exceptions raised
        // myFunction is any function that accesses var-a, which must be defined in anotherFunction

Question: How can I write anotherFunction similar to the example above so that calling the callback within it will not result in an exception?
Note: The callback is any user-provided function that accesses var-a. Var-a is not defined in the callback, but must be defined in anotherFunction.

I have attempted different approaches like this, and also explored alternatives such as this, but none of them worked.

The inspiration for this question comes from the concept of "MongoDB mapReduce", where one of the functions, emit(key, value), is provided within the map-reduce operation. Refer to (MongoDB documentation).

Consider this scenario:

function mapperFunction() { emit(this.fieldA, this.fieldB) };
db.collection.mapReduce(mapperFunction, reducerFunction, {...});  // invoking mapReduce operation

In the example above, the emit function used within mapperFunction is not defined within the scope of mapperFunction itself. It is somehow provided or defined within the db.collection.mapReduce function. How can the db.collection.mapReduce function be implemented to provide such functionality for a user-defined mapperFunction?

[var a] is equivalent to [emit function]
[anotherFunction] corresponds to [mapReduce function]
[myFunction] corresponds to [mapperFunction]

javascript mongodb

Answer 1

Answer №1

If you've correctly understood the query, you seem to be interested in dynamic scoping. In Javascript, variables are lexically scoped, so a closure must be textually within its scope to capture a variable. Otherwise, achieving this is not possible, unless you resort to somewhat unconventional methods like the following:

function createClosure(context) {
    return function() {
        return context("a") + 1;
    };
}

function y(evaluateThis) {
    var a = 2;

    if(evaluateThis)
        return eval(evaluateThis);

    return makeClosure(y);
}

closure = y();
document.write(closure()) // Output: 3

Refer to Is it possible to achieve dynamic scoping in JavaScript without using eval? for further insights and illustrations.

Regarding your MongoDB inquiry specifically, directly injecting a variable into a function's scope using pure javascript is not feasible (unless employing eval), as Mongo's map-reduce functionalities are primarily set in C++ which allows manipulation of scope in various ways:

_scope->setFunction("emit", etc

Check out the source code.

For demonstration purposes, here's an example involving the use of eval:

function applyMapping(array, functionality) {

    // declaring local variables
    var LOCAL = 42;

    // re-evaluating the function within the specified scope
    eval("functionality=" + functionality);

    // now the function has access to our locals
    return array.map(functionality);
}

document.write(
    applyMapping([1,2,3], function(x) { return x * LOCAL }) // Result: [ 42, 84, 126 ]
)

Answer 2

If you've correctly understood the query, you seem to be interested in dynamic scoping. In Javascript, variables are lexically scoped, so a closure must be textually within its scope to capture a variable. Otherwise, achieving this is not possible, unless you resort to somewhat unconventional methods like the following:

function createClosure(context) {
    return function() {
        return context("a") + 1;
    };
}

function y(evaluateThis) {
    var a = 2;

    if(evaluateThis)
        return eval(evaluateThis);

    return makeClosure(y);
}

closure = y();
document.write(closure()) // Output: 3

Refer to Is it possible to achieve dynamic scoping in JavaScript without using eval? for further insights and illustrations.

Regarding your MongoDB inquiry specifically, directly injecting a variable into a function's scope using pure javascript is not feasible (unless employing eval), as Mongo's map-reduce functionalities are primarily set in C++ which allows manipulation of scope in various ways:

_scope->setFunction("emit", etc

Check out the source code.

For demonstration purposes, here's an example involving the use of eval:

function applyMapping(array, functionality) {

    // declaring local variables
    var LOCAL = 42;

    // re-evaluating the function within the specified scope
    eval("functionality=" + functionality);

    // now the function has access to our locals
    return array.map(functionality);
}

document.write(
    applyMapping([1,2,3], function(x) { return x * LOCAL }) // Result: [ 42, 84, 126 ]
)

Answer 3

Answer №2

The answer below has been reviewed by Is it possible to achieve dynamic scoping in JavaScript without resorting to eval?

function x() {
    return a + 1; // any value accessing var-a is valid.
}

function y(fct_x) {
    var a = 2;
    var x = eval("(" + String(fct_x) + ")"); // using this format to avoid returning undefined
                                             // the function will be returned properly
    console.log(x()); // Output: 3 
}

y(x);   // This will throw an exception as mentioned above
        // x can be any function that requires access to var-a, which needs to be defined within the scope of function-y

Answer 4

The answer below has been reviewed by Is it possible to achieve dynamic scoping in JavaScript without resorting to eval?

function x() {
    return a + 1; // any value accessing var-a is valid.
}

function y(fct_x) {
    var a = 2;
    var x = eval("(" + String(fct_x) + ")"); // using this format to avoid returning undefined
                                             // the function will be returned properly
    console.log(x()); // Output: 3 
}

y(x);   // This will throw an exception as mentioned above
        // x can be any function that requires access to var-a, which needs to be defined within the scope of function-y

What is the proper way to create a function that accepts the parameter fct_x and can access the variable a, which must be defined within the function?

Answer №1

Answer №2

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