Clever Geek Handbook

How a function callback works

I have the following code

$.map( [ 0, 1, 2 ], function( n ) {
  return n > 0 ? n + 1 : null;
});

Out Put: [2, 3]

I know $.map

Translate all items in an array or object to new array of items.

( Documentation ).
What I want to know how call back function

to .map

work (internal implants)?
One possible answer might be

  • .map

    has some loop that passes each element of the array into call back method

    , which returns some value.
  • .map

    manages every return value from call back method

    . In this case, push into some internal array.
  • .map

    Return an array at the end of the loop .
    EDIT

But I'm not sure how it works, does it work as I explained?

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2 answers


But I'm not sure how it works?

Yes, that's basically how it works. Complete information, as always, in the source code (line number will rot over time ...). It currently looks like this:



map: function( elems, callback, arg ) {
    var value,
        i = 0,
        length = elems.length,
        isArray = isArraylike( elems ),
        ret = [];

    // Go through the array, translating each of the items to their new values
    if ( isArray ) {
        for ( ; i < length; i++ ) {
            value = callback( elems[ i ], i, arg );

            if ( value != null ) {
                ret.push( value );
            }
        }

    // Go through every key on the object,
    } else {
        for ( i in elems ) {
            value = callback( elems[ i ], i, arg );

            if ( value != null ) {
                ret.push( value );
            }
        }
    }

    // Flatten any nested arrays
    return concat.apply( [], ret );
},
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Well, there is no better way to test than by looking at the source code

function (elems, callback, arg) {
    var value, i = 0,
        length = elems.length,
        isArray = isArraylike(elems),
        ret = [];

    // Go through the array, translating each of the items to their new values
    if (isArray) {
        for (; i < length; i++) {
            value = callback(elems[i], i, arg);

            if (value != null) {
                ret.push(value);
            }
        }

        // Go through every key on the object,
    } else {
        for (i in elems) {
            value = callback(elems[i], i, arg);

            if (value != null) {
                ret.push(value);
            }
        }
    }

    // Flatten any nested arrays
    return concat.apply([], ret);
}


  • Yes, be it version or object Array

    , it is a loop and a call callback

    to set the value
  • Yes, for both loops the value is pressed
  • Yes, it returns a flattened array by callingconcat

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