Dimiwiz Dimiwiz - 1 month ago 44
Javascript Question

Accessing 'this' inside the callback should reference my array

im kinda new at javascript so i have an function which is given an array and it filters its elements from the arguments it is passed. What i need though is to make 'this' inside the callback function to refference at my originalArray so that

console.log(this);
would acctually print [1,2,3]. Have in mind that i acctually need to this in these lines because i can only edit all those lines not add some more.

Here is my code:

function makeMultiFilter(array) {
// What we track
var originalArray = array;
var currentArray = originalArray;
return (function arrayFilterer(pred, callback) {


// If filter not a function return current Array
if (typeof pred !== "function")
return currentArray;
// Filter out things
currentArray = currentArray.filter(pred);

// If callback is a function, execute callback
if (typeof callback === "function")
// TO BE COMPLETED
return arrayFilterer;
});
}


And the test code:

var arrayFilterer = makeMultiFilter([1, 2, 3]);
// call arrayFilterer to filter out all the numbers not equal to 2
arrayFilterer(function(elem) {

return elem != 2; // check if element is not equal to 2
}, function(currentArray) {
console.log(this); // prints [1,2 3]
console.log(currentArray);
}); // prints [1, 3]`

Answer

You can use call to execute a function in a given this context:

var callback = function() {
  console.log(this);
  console.log(arguments);
};

// Using call
callback.call([1,2,3], "A", "B", "C");

// Using apply
callback.apply([1,2,3], ["A", "B", "C"]);

// Using bind:
//  creates a new function bound to [1,2,3]
callback.bind([1,2,3])("A", "B", "C");
callback.bind([1,2,3], "A")("B", "C"); // etc.

I guess that in your example code, that would mean that instead of doing callback(), you'd use callback.call(originalArray):

function makeMultiFilter(array) {
    // What we track
    var originalArray = array;
    var currentArray = originalArray;
    return (function arrayFilterer(pred, callback) {
        // If filter not a function return current Array
        if (typeof pred !== "function")
            return currentArray;
        // Filter out things
        currentArray = currentArray.filter(pred);

        // If callback is a function, execute callback
        if (typeof callback === "function")
            callback.call(originalArray, currentArray);
            return arrayFilterer;
    });
}

var arrayFilterer = makeMultiFilter([1, 2, 3]);

// call arrayFilterer to filter out all the numbers not equal to 2
arrayFilterer(function(elem) {
    return elem != 2; // check if element is not equal to 2
}, function(currentArray) {
    console.log(this); // prints [1,2 3]
    console.log(currentArray); // prints [1,3]
}); // prints [1, 3]`