Can vue modify the value in prop?

You cannot directly modify the value in prop in vue. If you directly modify vue, a warning will be generated, and modifying the property value cannot modify the variable corresponding to the parent component; however, you can trigger the child component event and the parent component listens This event executes the function that modifies the parent component, and achieves two-way data binding by listening to the child component event to modify the value in the prop.

The operating environment of this tutorial: Windows 10 system, Vue3 version, Dell G3 computer.

Can vue modify the value in prop

In Vue, prop can accept attributes passed from parent components to child components, but prop cannot be modified.

If a prop is forcibly modified, there will be a warning

All props form a one-way downward binding between their parent and child props: The update of the parent prop will Flows downward to subcomponents, but not the other way round. This will prevent the child component from accidentally changing the state of the parent component, making the data flow of your application difficult to understand.

In addition, every time the parent component changes, all props in the child component will be refreshed to the latest values. This means you should not change props inside a child component. If you do this, Vue will issue a warning in the browser's console.

Try to directly modify the attribute value through the following method

this.$props.modelValue = true;

Vue will raise a warning

Set operation on key “modelValue” failed: target is readonly.

And modifying the value of this attribute cannot modify the variable corresponding to the parent component, it is only applicable to the basic data type.

Note that in JavaScript objects and arrays are passed in by reference, so for an array or object type prop, Changing the object or array itself in the child component will affect the parent The state of the component, and Vue cannot warn you about it. As a general rule, you should avoid modifying any props, including objects and arrays, as this ignores one-way data binding and may lead to unexpected results.

Vue3 Props Document

The correct modification method is to trigger the child component event. The parent component listens to the event and executes the function that modifies the parent component. Take v-model as an example. , v-model itself is the modelValue attribute passed to the child component, and then listens to the update:modelValue event to achieve two-way data binding, so we can also use this method to modify modelValue

this.$emit("update:modelValue", !this.$props.modelValue);

v-model will automatically listen to the update:modelValue event And modify modelValue to the parameter value passed when the event is triggered (that is, the second parameter of $emit).

It should be noted that this data flow mode of triggering events and monitoring events does not take effect immediately. If you use the following code

this.$emit("update:modelValue", !this.$props.modelValue);console.log(this.$props.modelValue);

, you will find that the output to the console is still modelValue The original value is because it takes time to modify the attribute value in the trigger event-listen event mode. If you use the following code instead, you will find that the output is normal:

this.$emit("update:modelValue", !this.$props.modelValue);setTimeout(()=> {console.log(this.$props.modelValue)}, 1000)

Practice

Since Vant only implements an input box with label on the left, I would like to implement an input box with label on the right based on this

Vant3 Field Document

The problem encountered here is that in addition to moving label to the right and adapting the width according to the content of label, it is <van-field />## in Vant # I need to implement the data flow between and the parent component where is placed.

Mistakes

To be able to use

v-model consistently with , I < Set the modelValue attribute under right-label-input />, and then v-model="modelValue" of

<template>
<van-field :placeholder="placeholder" :name="name" v-model="modelValue" :disabled="disabled"/>
</template>

<script>
export default {
    name: "RightLabelInput",
    props: {
        modelValue: "",
    },
}
</script>

Undoubtedly, this approach violates the principle of one-way data flow. When

VanField is modified, it will directly affect the modelValue value of RightLabelInput. And modelValue is a property of RightLabelInput and cannot be modified directly.

Possibly the right approach

<template>
    <van-field :placeholder="placeholder"
               :name="name"
               v-model="input"

               :disabled="disabled"
    />
</template>

<script>
export default {
    name: "RightLabelInput",
    props: {
        placeholder: "",
        name: "",
        label: "",
        modelValue: "",
        disabled: {
            type: Boolean,
            default: false
        }
    },
    data() {
        return {
            input: this.modelValue, // 绑定到 data 的变量，防止直接修改 Props
        }
    },
    beforeMount() {
        this.input = this.modelValue
    },
    watch: {
        input(newInput, oldInput) {
            this.$emit("update:modelValue", newInput); // 监听 input 在修改时发起事件，由父组件修改 modelValue 的值
        },
        modelValue(newValue, oldValue) {
            this.input = newValue;
        }
    }
}
</script>

<style scoped>

</style>

Keywords: One-way data flow

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