Why doesn't this invalid syntax throw an error in JavaScript?

Discover why JavaScript doesn't throw errors with certain syntaxes. Learn how object properties work in strings and avoid common pitfalls.
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This video is based on the question https://stackoverflow.com/q/73667430/ asked by the user 'Konrad' ( https://stackoverflow.com/u/5089567/ ) and on the answer https://stackoverflow.com/a/73667457/ provided by the user 'Samathingamajig' ( https://stackoverflow.com/u/12101554/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding JavaScript Syntax: Why Doesn't This Invalid Syntax Throw an Error?

When coding in JavaScript, it's not uncommon to come across situations where the syntax you think should raise an error doesn't, leading to confusion. A particular case that often puzzles both novices and experienced developers is when a piece of code appears to be invalid but executes without throwing an error. In this guide, we're going to explore a specific example, dissect the syntax involved, and offer insight into why JavaScript behaves this way.

The Code in Question

Let's look at the following snippet of code:

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At a glance, many might expect this code to generate a syntax error because of the {}] following the string 'some string'. However, when running this code, it executes without any problems, and here’s why.

Delving into the Details

This isn’t Invalid Syntax

The main reason this code does not throw an error is due to how JavaScript interprets the {} following the string. In this case, the {} is not seen as an object initializer; instead, it's interpreted as a property access. Here’s the crux of it:

Property Access: The {} is treated as a string which results in 'some string' [{}] being translated to 'some string' followed by the string representation of the empty object which is "[object Object]".

Outcome: Essentially, the property in the object ends up being a string of 'some string[object Object]', which explains why no syntax error occurs.

Contrasting Example - Valid Syntax Error

Now, consider another piece of code:

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This piece of code indeed throws an error when you try to run it. Why the difference?

Invalid Syntax: The reason for the error here lies in the use of []. JavaScript expects something to select after the property access operator ([]). In this case, because it is empty, it results in a syntax error as there is nothing to access.

Understanding Property Access in Strings

Even with the property being effectively undefined if you attempt to access a custom property on a string, you won’t be able to add properties directly to JavaScript's native String type. This restriction exists because the string primitive type (string) is immutable. However, there are ways to work around this.

Custom Properties via Prototypes

While you can't add properties directly to a string, you can define custom properties on the String prototype, allowing JavaScript to recognize them. Here’s how to do it:

Define the Custom Property:

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Use in Your Object:

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This will now output an object where the property includes the string form with your custom prototype property.

Conclusion

Understanding the nuances of JavaScript syntax can be tricky, but breaking down a complex issue into parts helps clarify things. In summary, while the code const object = { property: 'some string' [{}] } looks invalid at first glance, its behavior rooted in JavaScript's understanding of property access and object representations allows it to execute without error. However, it’s essential to be cautious when working with property access to avoid potential syntax pitfalls.

Now, when you encounter a similar syntax issue in your JavaScript coding journey, you can approach it with confidence and clarity!