Vue.js strange quirks and unpredictable method responses

Question

Vue.js strange quirks and unpredictable method responses

I am currently working on a Vue.js component that has a simple template.

<div @click="createTargets(2)">
text
</div>

Here is the script file for the component:

export default {
  name: 'test',
  data() {
    return {
      targets: [],
    };
  },
  methods: {
    createTargets(targetCount) {
      this.targets = [];
      var emptyTarget = {
          id: null,
      };
      for (var i = 0; i < targetCount; i++) {
        var targetToPush = emptyTarget;
        targetToPush.id = i;
        console.log(targetToPush.id);
        this.targets.push(targetToPush);
        console.log(this.targets);
      }
      return {};
    },
  },
}

After clicking on the text in the component, I noticed an unexpected output:

0
[{"id":1},{"id":1}]
1
[{"id":1},{"id":1}]

I am puzzled by this behavior and cannot seem to find the cause.

My expected output should be:

0
[{"id":0}]
1
[{"id":0},{"id":1}]

Any suggestions or ideas on why this is happening?

javascript vue.js

Answer 1

Answer №1

Understanding how objects are initialized and referenced is key. When assigning an object to a variable, it is initialized only once. By assigning that variable to another, you are essentially linking both references. Any changes made to the original object will reflect in the copied object as well.

To avoid this issue, utilize the spread operator to create a new copy of the object. This way, changes made to one object won't affect the other:

const targets = [];
const common = { commonProp: 'test' };

for (let i = 1; i <= count; i++) {
  const target = { ...common, id: i };
  targets.push(target);
}

this.targets = targets;

It's important to note that mutating the component's state within a loop is not advisable. It's best practice to modify properties only once as shown in the example above.

Deep copying objects is also crucial when dealing with nested objects. The shallow copy method showcased here only duplicates the top level properties. For a thorough copy, a deep copy or creating a new object each time is recommended:

const common = {
  commonProp: { a: 1, b: 2 }
};
const object1 = { ...common, id: 1 };
const object2 = { ...common, id: 2 };
object1.commonProp.a = 2;
console.log(object1); // { commonProp: { a: 2, b: 2 } }
console.log(object2); // { commonProp: { a: 1, b: 2 } }

To tackle this issue, consider using a library for deep copying or creating a factory function/class to generate new objects each time:

// factory
const createTarget = id => ({
  commonProp: { a: 1, b: 2 },
  id,
});

// class
class Target {
  constructor(id) {
    this.id = id;
    this.commonProp = { a: 1, b: 2 };
  }
}

for (let i = 1; i <= count; i++) {
  const target = createTarget(i); // or new Target(i);
  targets.push(target);
}

Hopefully, this explanation sheds some light on the topic. Good luck! ;)

Answer 2

Understanding how objects are initialized and referenced is key. When assigning an object to a variable, it is initialized only once. By assigning that variable to another, you are essentially linking both references. Any changes made to the original object will reflect in the copied object as well.

To avoid this issue, utilize the spread operator to create a new copy of the object. This way, changes made to one object won't affect the other:

const targets = [];
const common = { commonProp: 'test' };

for (let i = 1; i <= count; i++) {
  const target = { ...common, id: i };
  targets.push(target);
}

this.targets = targets;

It's important to note that mutating the component's state within a loop is not advisable. It's best practice to modify properties only once as shown in the example above.

Deep copying objects is also crucial when dealing with nested objects. The shallow copy method showcased here only duplicates the top level properties. For a thorough copy, a deep copy or creating a new object each time is recommended:

const common = {
  commonProp: { a: 1, b: 2 }
};
const object1 = { ...common, id: 1 };
const object2 = { ...common, id: 2 };
object1.commonProp.a = 2;
console.log(object1); // { commonProp: { a: 2, b: 2 } }
console.log(object2); // { commonProp: { a: 1, b: 2 } }

To tackle this issue, consider using a library for deep copying or creating a factory function/class to generate new objects each time:

// factory
const createTarget = id => ({
  commonProp: { a: 1, b: 2 },
  id,
});

// class
class Target {
  constructor(id) {
    this.id = id;
    this.commonProp = { a: 1, b: 2 };
  }
}

for (let i = 1; i <= count; i++) {
  const target = createTarget(i); // or new Target(i);
  targets.push(target);
}

Hopefully, this explanation sheds some light on the topic. Good luck! ;)

Answer 3

Answer №2

Understanding console.log()

It's important to note that when logging objects in Chrome and Firefox, the console may display a reference to the object rather than its current value. This reference reflects the object's value at the time the console is opened, not at the time console.log() is called.

To ensure you are logging the current value of an object, it's recommended to use
console.log(JSON.parse(JSON.stringify(obj)))
instead of console.log(obj).

Answer 4

Understanding console.log()

It's important to note that when logging objects in Chrome and Firefox, the console may display a reference to the object rather than its current value. This reference reflects the object's value at the time the console is opened, not at the time console.log() is called.

To ensure you are logging the current value of an object, it's recommended to use
console.log(JSON.parse(JSON.stringify(obj)))
instead of console.log(obj).

Answer 5

Answer №3

To display the values of the array (not the reference), you can use the following code:

for (let i = 0; i < targetCount; i++) {
  let targetToPush = emptyTarget;
  targetToPush.id = i;
  console.log(targetToPush.id);
  this.targets.push(targetToPush);

  // Using es6:
  console.log([...this.targets]);

  // Using the old way:
  console.log(this.targets.slice());
}

If you use console.log(this.targets), you will see the changing values as it prints the variable reference directly. However, if you use console.log([...this.targets]), you will see the array values of each loop interaction, which will remain the same even if the array changes later.

Answer 6

To display the values of the array (not the reference), you can use the following code:

for (let i = 0; i < targetCount; i++) {
  let targetToPush = emptyTarget;
  targetToPush.id = i;
  console.log(targetToPush.id);
  this.targets.push(targetToPush);

  // Using es6:
  console.log([...this.targets]);

  // Using the old way:
  console.log(this.targets.slice());
}

If you use console.log(this.targets), you will see the changing values as it prints the variable reference directly. However, if you use console.log([...this.targets]), you will see the array values of each loop interaction, which will remain the same even if the array changes later.

Answer 7

Answer №4

The issue lies in updating the reference variable, causing it to always hold the most recent data. To resolve this problem, you can follow the code snippet below:

createTargets(targetCount) {
      this.targets = [];
      for (var i = 0; i < targetCount; i++) {
        var targetToPush = {id: null};
        targetToPush.id = i;
        console.log(targetToPush.id);
        this.targets.push(targetToPush);
        console.log(this.targets);
      }
      return {};
    }

Answer 8

The issue lies in updating the reference variable, causing it to always hold the most recent data. To resolve this problem, you can follow the code snippet below:

createTargets(targetCount) {
      this.targets = [];
      for (var i = 0; i < targetCount; i++) {
        var targetToPush = {id: null};
        targetToPush.id = i;
        console.log(targetToPush.id);
        this.targets.push(targetToPush);
        console.log(this.targets);
      }
      return {};
    }

Vue.js strange quirks and unpredictable method responses

Answer №1

Answer №2

Answer №3

Answer №4

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