Tips for dynamically assigning arguments to a method call within a loop

Question

Tips for dynamically assigning arguments to a method call within a loop

Encountered an unusual issue that can be best described through code:

var fruits = ["apples", "oranges", "pears"];

var Breakfast = {
  _consume : function (fruit) {
    Breakfast[fruit]._numConsumed++;
  }
};

for (var f in fruits) {
  var fruit = fruits[f];
  Breakfast[fruit] = {
    consume : function () {
      Breakfast._consume(fruit);
    },
    _numConsumed: 0
  }
}

Breakfast.pears.consume();
Breakfast.pears.consume();

Breakfast.apples.consume();
Breakfast.apples.consume();
Breakfast.apples.consume();


console.log("Pears eaten: " + Breakfast.pears._numConsumed);
console.log("Apples eaten: " + Breakfast.apples._numConsumed);

This results in:

$ node example.js
Pears eaten: 5
Apples eaten: 0

Not sure how to address this behavior?

Is there a mistake in my code? Or should I follow a different approach? (considering that I want the "consume" function to work for all fruits)

Thank you!

javascript

Answer 1

Answer №1

The issue lies in the fact that your variable will always end up being the last one in the for loop. A more straightforward approach would be to directly define the property within the object you are creating.

For example:

var fruits = ["apples", "oranges", "pears"];

var Breakfast = {
  _consume : function (fruit) {
    Breakfast[fruit]._numConsumed++;
  }
};

for (var f in fruits) {
  var fruit = fruits[f];
  Breakfast[fruit] = {
    id: fruit,
    consume: function () {
      Breakfast._consume(this.id);
    },
    _numConsumed: 0
  }
}

Answer 2

The issue lies in the fact that your variable will always end up being the last one in the for loop. A more straightforward approach would be to directly define the property within the object you are creating.

For example:

var fruits = ["apples", "oranges", "pears"];

var Breakfast = {
  _consume : function (fruit) {
    Breakfast[fruit]._numConsumed++;
  }
};

for (var f in fruits) {
  var fruit = fruits[f];
  Breakfast[fruit] = {
    id: fruit,
    consume: function () {
      Breakfast._consume(this.id);
    },
    _numConsumed: 0
  }
}

Answer 3

Answer №2

Having difficulty understanding function closures? There's a great explanation on this topic in the following question: How do JavaScript closures work?

The issue you're facing is with this line of code:

  Breakfast._consume(fruit);

It will always refer to:

  Breakfast._consume(fruits[2]);

once your loop has completed its execution.

A potential solution would be to create a unique context for each iteration within the loop:

for (var f in fruits) {
  (function(f) {
    var fruit = fruits[f];
    Breakfast[fruit] = {
      consume : function () {
        Breakfast._consume(fruit);
      },
      _numConsumed: 0
    }
  })(f);
}

This modification will yield the desired outcome:

"Pears eaten: 2"
"Apples eaten: 3"

However, there may be more elegant solutions available once you grasp the concept of function closures better.

Answer 4

Having difficulty understanding function closures? There's a great explanation on this topic in the following question: How do JavaScript closures work?

The issue you're facing is with this line of code:

  Breakfast._consume(fruit);

It will always refer to:

  Breakfast._consume(fruits[2]);

once your loop has completed its execution.

A potential solution would be to create a unique context for each iteration within the loop:

for (var f in fruits) {
  (function(f) {
    var fruit = fruits[f];
    Breakfast[fruit] = {
      consume : function () {
        Breakfast._consume(fruit);
      },
      _numConsumed: 0
    }
  })(f);
}

This modification will yield the desired outcome:

"Pears eaten: 2"
"Apples eaten: 3"

However, there may be more elegant solutions available once you grasp the concept of function closures better.

Answer 5

Answer №3

Revised response:

let fruitsList = ["bananas", "grapes", "kiwis"];

let BreakfastMenu = {
  _consume : function (fruit) {
    console.log('more'+JSON.stringify(fruit));
    BreakfastMenu[fruit]._numConsumed++;
  }
};

for (let f in fruitsList) {
  //let fruitName = fruits[f];
  BreakfastMenu[fruitsList[f]] = {
    fruit: fruitsList[f],
    consume : function () {
      BreakfastMenu._consume(this.fruit);
    },
    _numConsumed: 0
  }
}

BreakfastMenu.kiwis.consume();
BreakfastMenu.kiwis.consume();

BreakfastMenu.bananas.consume();
BreakfastMenu.bananas.consume();
BreakfastMenu.bananas.consume();


console.log("Kiwis eaten: " + BreakfastMenu.kiwis._numConsumed);
console.log("Bananas eaten: " + BreakfastMenu.bananas._numConsumed);

Answer 6

Revised response:

let fruitsList = ["bananas", "grapes", "kiwis"];

let BreakfastMenu = {
  _consume : function (fruit) {
    console.log('more'+JSON.stringify(fruit));
    BreakfastMenu[fruit]._numConsumed++;
  }
};

for (let f in fruitsList) {
  //let fruitName = fruits[f];
  BreakfastMenu[fruitsList[f]] = {
    fruit: fruitsList[f],
    consume : function () {
      BreakfastMenu._consume(this.fruit);
    },
    _numConsumed: 0
  }
}

BreakfastMenu.kiwis.consume();
BreakfastMenu.kiwis.consume();

BreakfastMenu.bananas.consume();
BreakfastMenu.bananas.consume();
BreakfastMenu.bananas.consume();


console.log("Kiwis eaten: " + BreakfastMenu.kiwis._numConsumed);
console.log("Bananas eaten: " + BreakfastMenu.bananas._numConsumed);

Tips for dynamically assigning arguments to a method call within a loop

Answer №1

Answer №2

Answer №3

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