'TypeError: Cannot set property 'xxx' of undefined' in Vue application - how to solve it?

"TypeError: Cannot set property 'xxx' of undefined" in Vue application - how to solve it?

In the process of developing Vue applications, developers often encounter such errors: "TypeError: Cannot set property 'xxx' of undefined". This error usually occurs during page rendering. For example, when operating props inside a component, this error will be reported because the props have not yet been assigned a value. So how should we deal with this error? This article will provide you with solutions.

Error analysis
First of all, we need to understand the meaning of this error, its causes and how to avoid it in Vue applications.

Error meaning
This error is a type error (TypeError), which means that we are trying to set a property to an "undefined" value. In Vue applications, usually this "undefined" value is a Key that has not been assigned a value in props or data.

Cause of error
Vue component instances will undergo some preprocessing before being created, including props initialization, data initialization, etc. In these initialization steps, if the Key has not been set, its value is undefined. If we set it at this stage, since its value is undefined, "TypeError: Cannot set property 'xxx' will be reported of undefined" error.

Avoidance method
It is very simple to avoid this error when making Vue components. A common avoidance method is to use default values. In Vue, this error can be avoided by setting default values ​​in props. For example, if we define a props: "message" in a component, we can use a default value to avoid this error when it is not assigned a value.

Solution
If we do not use the default value in the avoidance method, but this error occurs, then we can try the following method to solve this error.

1. Check whether the Key is correct
First, we need to check whether we have correctly referenced the corresponding Key in the component where the error occurred. We need to check whether the Key exists and whether it is spelled correctly. At the same time, we also need to ensure that this Key is correctly passed to the current component in the parent component.

2. Set the value of the Key correctly
If we reference this Key correctly in the component, then we need to ensure that it is assigned the value correctly in the component. To avoid this error, we should use default values ​​in the component or ensure that its value is set correctly in the component.

3. Add a v-if instruction to the component
If our component needs to wait for asynchronous data, then we can add a v-if instruction to the component. This instruction can ensure that the component arrives when the asynchronous data arrives. It won't render before. This prevents components from trying to set undefined Key values ​​when data has not yet arrived.

For example:
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{{ data }}

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export default {
data() {

return {
  data: null,
};

},
created() {

fetchData().then((res) => {
  this.data = res;
});

},
};
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Summary
In the process of developing Vue applications, we often encounter the error "TypeError: Cannot set property 'xxx' of undefined". This error may be because we operated on an undefined Key, or it may be because we performed the operation without waiting for the asynchronous data to arrive. We can avoid this error through avoidance methods and workarounds. If an undefined Key is referenced in a component, then we need to check whether the component refers to the Key correctly and make sure that the default value is used or its value is set correctly. We can also add the v-if instruction to wait for the asynchronous data to arrive before operating.

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