TypeError: Cannot read property 'XXX' of null in Vue, how to deal with it?

Vue is a popular JavaScript framework used by many developers to build modern, interactive web applications. However, when used in Vue, we may encounter TypeError: Cannot read property 'XXX' of null exception. In this article, we will explore the root causes of this problem and how to fix it.

Root cause of the problem

In Vue, we often use reactive data, which means that when the data changes, Vue will automatically update the view. However, in some cases, we may access a property or element from a null or undefined object or array, which will cause an exception.

For example, we have a data object person, which contains a name attribute name and a hobby array hobbies. If we try to access person.hobbies[0], but the person object itself is null or undefined, JavaScript will throw a TypeError exception, prompting "cannot read property '0' of null".

Workaround

1. Check if the data is null or undefined

Check if the data object exists and make sure it contains all the properties you are trying to access or element. These situations can be handled in scripts using if statements or conditional operators.

For example:

if(person && person.hobbies && person.hobbies.length > 0){
console.log(person.hobbies[0]);
}

2. Use default values

If you cannot ensure that the data object exists, or you don't want to check every time you access it, you can use default values. In Vue's templates, you can use the v-if or v-show directive to avoid accessing non-existent objects during rendering.

For example, use the default value in the Vue template:

{{ person && person.hobbies ? person.hobbies[0] : 'none' }}

This The expression can render the value of person.hobbies[0], or a default value of 'none' if person does not exist or hobbies does not exist.

3. Reasonable use of the computed attribute

Vue's computed attribute computed can perfectly avoid this problem. We can process the default value or numerical value of the data in the computed attribute. When we access a computed property, if the data object does not exist, Vue will automatically handle it and return a default value.

For example:

computed:{
firstHobby(){

  return this.person && this.person.hobbies ? this.person.hobbies[0] : 'none';

}
}

Now, when we access this.firstHobby , if person or hobbies does not exist, Vue will automatically return a default value of 'none'.

In short, when we encounter a TypeError: Cannot read property 'XXX' of null exception in a Vue application, we can check whether the data object exists and apply if statements, v-if/v-show appropriately. Directive, default value, or computed property. With these workarounds, we can better ensure the validity of data objects, prevent exceptions, and improve the stability of our applications.

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