I find it curious that Vuejs does not issue a warning when a prop is mutated in a child component. It makes

Question

I find it curious that Vuejs does not issue a warning when a prop is mutated in a child component. It makes

Recently, I came across an interesting observation regarding prop mutation in Vue 2.6.

It is generally advised to avoid mutating props directly in a child component as it can lead to the well-known warning:

"Avoid mutating a prop directly since the value will be overwritten...."

For example, if I have a parent component with data like this:

example: "Hello World"

and I pass it down as a prop to a child component. If I try to mutate it in the child component like so:

this.example = "Hello World!"

I would receive a warning. However, I noticed that if the data in the parent component is an object like:

example : {message : "Hello World"}

and in the child component I do this:

this.example.message = "Hello World!"

No warning is triggered and the data in the parent component gets updated. This made me wonder why prop mutation propagates to the parent in one case but not in the other. Could it be related to how JavaScript stores these variables? Is it considered good practice to utilize this behavior?

javascript vue.js vuejs2 vue-props

Answer 1

Answer №1

What causes the prop mutation to affect the parent in one scenario but not in another? Could it be related to how JavaScript stores these variables?

Absolutely. In JavaScript, reference variables like objects and arrays do not change their references when you modify their properties or values. This implies that the parent component can also access the updated properties because it shares the same reference. Vue only issues a warning for prop mutations if the reference itself changes.

Warning:

this.prop = {}

No warning:

this.prop.id = 5

Is it advisable to utilize this technique?

No, it is not recommended. It goes against the principle of One way data flow, where data should flow downwards through props (to children) and upwards through events (to parent).

But why doesn't Vue give warnings for such modifications?

Consider an object with a thousand properties or an array with a thousand elements. Vue would need to extensively check all of them for changes, which could have significant performance implications as each item would need to be individually compared to its previous value. This could potentially make many applications run poorly.

Vue team member Bolerodan made a comment regarding this issue:

It is generally discouraged to do so. This practice may lead to unexpected bugs in your code. Instead, it is advised to create a copy of the object, make modifications to that copy, and then emit the changes upwards to the parent (as mentioned in your last paragraph). This approach follows the top-down, emit-up strategy, which is widely endorsed.

Answer 2

What causes the prop mutation to affect the parent in one scenario but not in another? Could it be related to how JavaScript stores these variables?

Absolutely. In JavaScript, reference variables like objects and arrays do not change their references when you modify their properties or values. This implies that the parent component can also access the updated properties because it shares the same reference. Vue only issues a warning for prop mutations if the reference itself changes.

Warning:

this.prop = {}

No warning:

this.prop.id = 5

Is it advisable to utilize this technique?

No, it is not recommended. It goes against the principle of One way data flow, where data should flow downwards through props (to children) and upwards through events (to parent).

But why doesn't Vue give warnings for such modifications?

Consider an object with a thousand properties or an array with a thousand elements. Vue would need to extensively check all of them for changes, which could have significant performance implications as each item would need to be individually compared to its previous value. This could potentially make many applications run poorly.

Vue team member Bolerodan made a comment regarding this issue:

It is generally discouraged to do so. This practice may lead to unexpected bugs in your code. Instead, it is advised to create a copy of the object, make modifications to that copy, and then emit the changes upwards to the parent (as mentioned in your last paragraph). This approach follows the top-down, emit-up strategy, which is widely endorsed.

Answer 3

Answer №2

One explanation is that when you modify a reference value, Vue only keeps track of the reference address and not the entire reference itself. So, if you try to reassign that value, Vue will issue a warning.

Let's illustrate this behavior with the use of the const keyword in variable declaration.

const makes a variable immutable. If the value is primitive, it cannot be changed. However, if it's a reference, you can't change the reference itself but you can update the value stored at that reference address.

const a = 'Foo' // Primitive value

a = 'bar' // This breaks the rule
 
const b = { // Reference value
 name: 'Foo' 
}

b.name = 'Bar' // This is allowed

console.log(b)

b = {} // Trying to reassign violates the rule

Answer 4

One explanation is that when you modify a reference value, Vue only keeps track of the reference address and not the entire reference itself. So, if you try to reassign that value, Vue will issue a warning.

Let's illustrate this behavior with the use of the const keyword in variable declaration.

const makes a variable immutable. If the value is primitive, it cannot be changed. However, if it's a reference, you can't change the reference itself but you can update the value stored at that reference address.

const a = 'Foo' // Primitive value

a = 'bar' // This breaks the rule
 
const b = { // Reference value
 name: 'Foo' 
}

b.name = 'Bar' // This is allowed

console.log(b)

b = {} // Trying to reassign violates the rule

Answer 5

Answer №3

The mutation prevention feature in Vue only detects direct assignments to a prop and not manipulations of its properties. This means that when you assign new values to the prop, you are actually modifying the prop itself and not just a variable within the parent component that is supplying the prop.

Whenever the parent component updates the feeding variable, the prop will be automatically updated with this new value, potentially overwriting any changes made to the prop in the child component.

If you pass an object as a prop and then modify a property of that object in the child component, it will also change the same object in the parent component. However, if you attempt to assign a completely new object to the prop in the child component, you will receive a warning about mutating props.

Answer 6

The mutation prevention feature in Vue only detects direct assignments to a prop and not manipulations of its properties. This means that when you assign new values to the prop, you are actually modifying the prop itself and not just a variable within the parent component that is supplying the prop.

Whenever the parent component updates the feeding variable, the prop will be automatically updated with this new value, potentially overwriting any changes made to the prop in the child component.

If you pass an object as a prop and then modify a property of that object in the child component, it will also change the same object in the parent component. However, if you attempt to assign a completely new object to the prop in the child component, you will receive a warning about mutating props.

I find it curious that Vuejs does not issue a warning when a prop is mutated in a child component. It makes

Answer №1

But why doesn't Vue give warnings for such modifications?

Answer №2

Answer №3

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