VueJS: why is “this” undefined?

In Vue.js, the value of this can sometimes be undefined within certain methods or callbacks. This issue typically arises when attempting to access this within functions that do not preserve the correct context. The specific reasons why this may be undefined in Vue.js can vary. Here are a few common scenarios:

• Arrow functions: If you’re using an arrow function (() => {}) for a method or callback, this will not refer to the Vue instance. Arrow functions do not bind their own this value but instead, inherit it from the surrounding context. Therefore, this will likely be undefined or reference the wrong object. To access the Vue instance in this situation, you should use a regular function expression (function() {}) or the bind() method to bind the correct context explicitly.
• Event handlers: When using event listeners like @click or v-on:click in templates, the method is invoked with the event as the first argument, not the Vue instance. In this case, this will refer to the DOM element, not the Vue instance. To access the Vue instance within the event handler, you can use @click=”myMethod” syntax and access the Vue instance via this.$root, this.$parent, or emit a custom event to communicate with the parent component.
• Callback functions: If you pass a callback function to an external library or API, the context of this can be lost or changed. Some libraries may provide ways to bind the correct context explicitly, like using .bind(this) or passing a context parameter. Alternatively, you can capture the Vue instance reference (const self = this) before entering the callback function and use self within it.

In general, when encountering issues with this being undefined, it’s essential to understand the context in which you’re using this and ensure that it refers to the correct object (in this case, the Vue instance).

Examples

Instead of doing this

mounted: () => {
    console.log(this); // logs "undefined"
  },

Do this

mounted() {
    console.log(this); // logs Vue Instance
  }

Instead of doing this

<template>
  <div>
    <button @click="arrowFunctionMethod">Click me</button>
  </div>
</template>


<script>
export default {
  methods: {
    arrowFunctionMethod: () => {
      console.log(this); // 'this' will be undefined
    },
  },
};
</script>

Do this

<template>
  <div>
    <button @click="regularFunctionMethod">Click me</button>
  </div>
</template>


<script>
export default {
  methods: {
    regularFunctionMethod() {
console.log(this); // 'this' will refer to the Vue instance
    },
  },
};
</script>