Why Do JavaScript Functions Return \'undefined\' When Return Statement Is Line-Broken?

JavaScript Functions Return "undefined" When Return Statement Is Line-Broken

In certain scenarios, JavaScript functions may fail to return expected objects due to line breaks after the return statement. This issue arises when the object definition is separated from the return statement by a line break, as seen in the example code:

function foo1(){
    return {msg: "hello1"};
}
function foo2(){
    return
    {msg: "hello2"};
}

// output = "foo1 =  {"msg":"hello1"}"
console.log('foo1 = ' , JSON.stringify(foo1())); 

//output = " foo2 =  undefined "
console.log('foo2 = ' , JSON.stringify(foo2()));

The key difference between these functions is the line break between the return statement and the object definition in foo2. JavaScript's automatic semicolon insertion (ASI) mechanism falsely inserts a semicolon after the return statement in foo2, effectively ending the function's execution without returning an object. This leads to the output of "undefined" when stringifying foo2().

To avoid this issue, several approaches can be taken:

• Explicit Semicolons: Add a semicolon after the return statement to prevent ASI from inserting an unintended semicolon.
• Parentheses (Grouping Operator): Wrap the return statement and object definition in parentheses to force JavaScript to interpret the object as a part of the return expression.
• Place Object on Same Line: Avoid line breaks between the return statement and object definition to prevent ASI from interfering.

In cases where aesthetic considerations or code readability outweigh the potential for ASI issues, developers may choose to group expressions using the grouping operator, as seen in the example provided in the response. However, this approach is primarily a matter of preference and does not affect the functionality of the code.

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