Why Do Arrow Functions in Vue Computed Properties Sometimes Cause Unexpected Behavior?

Using Arrow Functions in Vue Computed Properties: Understanding the Pitfalls

In Vue.js, arrow functions can be used to simplify the syntax of computed properties. However, using arrow functions in computed properties can lead to unexpected behavior if not used correctly.

Incorrect Function Definition

Consider the following Vue code snippet:

computed: {
  switchRed: () => {
    return { red: this.turnRed }
  },
  switchGreen: () => {
    return { green: this.turnGreen }
  },
  switchBlue: () => {
    return { blue: this.turnBlue }
  }
}

When using arrow functions in computed properties, it's crucial to be aware of their scoping behavior. Arrow functions do not bind the this keyword to the Vue instance. Instead, they inherit the this binding from the surrounding context, which in this case is the global scope of the Vue component.

Consequences of Incorrect Definition

As a result of the incorrect function definition, the this keyword within the computed properties does not refer to the Vue instance, but rather to the global Vue.js instance. This leads to an undefined reference error for the properties turnRed, turnGreen, and turnBlue. Consequently, the computed properties will return an empty object, and the color change behavior will not work as expected.

Correct Function Definition

To fix this issue, we need to bind the this context explicitly to the Vue instance. To do this, we can use the traditional function syntax:

computed: {
  switchRed: function() {
    return { red: this.turnRed }
  },
  switchGreen: function() {
    return { green: this.turnGreen }
  },
  switchBlue: function() {
    return { blue: this.turnBlue }
  }
}

By using the traditional function syntax, we ensure that the this keyword within the computed properties refers to the Vue instance. This allows the computed properties to access and manipulate the Vue instance's data as intended.

Important Note

It's important to adhere to the Vue.js documentation's guidance when using arrow functions. According to the documentation, "Don't use arrow functions on an instance property or callback (e.g. vm.$watch('a', newVal => this.myMethod())). As arrow functions are bound to the parent context, this will not be the Vue instance as you'd expect and this.myMethod will be undefined."

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