Why Does a JavaScript Function Return Undefined When a Line Break Occurs Between the Return Statement and Returned Object?

JavaScript Function Returns Undefined When a Line Break Occurs Between Return Statement and Object

In JavaScript, a function is expected to return an object when the return statement is followed directly by the object definition, as seen in the following example:

<code class="javascript">function foo1() {
  return { msg: "hello1" };
}</code>

console.log(JSON.stringify(foo1())); // Output: { msg: "hello1" }
However, if a line break separates the return statement from the object definition, the function returns undefined, as shown below:

<code class="javascript">function foo2() {
 return
 { msg: "hello2" };
}</code>

console.log(JSON.stringify(foo2())); // Output: undefined

This behavior occurs due to the Automatic Semicolon Insertion (ASI) mechanism in JavaScript. ASI assumes semicolons in certain contexts to ensure the proper execution and interpretation of code. In the case of foo2(), the line break is interpreted as a semicolon, which terminates the return statement and introduces an empty statement before the object definition.

Solution

To resolve this issue, you can use a grouping operator to ensure that the return statement and object definition are evaluated as a single expression. This prevents ASI from intervening:

<code class="javascript">function foo2() {
  return ({ msg: "hello2" });
}</code>

Aesthetic Considerations

While it's important to avoid ASI issues, you may want to consider aesthetic factors when writing your code. Some developers prefer to group expressions for clarity or to emphasize the structure of their code. For example:

<code class="javascript">return ([
  "ul",
  ["li",
    ["span", {class: "name"}, this.name],
    ["span", {id: "x"}, "x"]
  ]
]);</code>

or

<code class="javascript">function() {
  return (
    doSideEffects(),
    console.log("this is the second side effect"),
    1 + 1
  );
}</code>

In these examples, the grouping operator is used to keep the expressions organized and to improve readability.

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