Avoiding a jest compile error and executing the test efficiently

Question

Avoiding a jest compile error and executing the test efficiently

Here is my Vue.js code snippet:

export default {
  name: 'App',
  components: {
  },
  created() {
    let t = typeof t === 'undefined' ? {} : t; // ISSUE LINE
  }
}

The ISSUE LINE runs successfully in the browser without any errors and passes the vue-cli compile process. However, when attempting to run tests for this component in Jest, a compile error is reported:

ReferenceError: Cannot access 't' before initialization

Why does the app work in the browser but not in Jest? Can this compile error be skipped?

javascript vue.js jestjs babeljs babel-jest

Answer 1

Answer №1

ReferenceError occurs at runtime, not during compilation. Writing

let t = typeof t === 'undefined' ? {} : t

is incorrect ES6 syntax because the variable t is in the temporal dead zone until it's assigned a value.

In Jest, which runs on Node.js and fully supports ES6, there is no need to transpile code to ES5 like in browser builds. In browser builds, the code gets transpiled to:

var t = typeof t === 'undefined' ? {} : t;

The use of var allows for hoisting, so the variable t is defined before being assigned a value, making it equivalent to:

var t; // t === undefined
// ...everything that happens in this scope...
t = typeof t === 'undefined' ? {} : t; // always results in t = {}

The conditional check in the original code is unnecessary as it never enters the else block. The correct way to write this would be:

let t = {};

Answer 2

ReferenceError occurs at runtime, not during compilation. Writing

let t = typeof t === 'undefined' ? {} : t

is incorrect ES6 syntax because the variable t is in the temporal dead zone until it's assigned a value.

In Jest, which runs on Node.js and fully supports ES6, there is no need to transpile code to ES5 like in browser builds. In browser builds, the code gets transpiled to:

var t = typeof t === 'undefined' ? {} : t;

The use of var allows for hoisting, so the variable t is defined before being assigned a value, making it equivalent to:

var t; // t === undefined
// ...everything that happens in this scope...
t = typeof t === 'undefined' ? {} : t; // always results in t = {}

The conditional check in the original code is unnecessary as it never enters the else block. The correct way to write this would be:

let t = {};

Avoiding a jest compile error and executing the test efficiently

Answer №1

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