Why Explicit Semicolons Are a Must in JavaScript: A Comprehensive Guide

JavaScript is a powerful and flexible language that has become ubiquitous in web development. However, with great flexibility comes the potential for confusion and subtle bugs, especially when it comes to the language‘s automatic semicolon insertion (ASI) mechanism. In this in-depth guide, we‘ll explore the ins and outs of ASI, make the case for why explicitly terminating your statements with semicolons is a best practice, and address some common objections and counterarguments.

Automatic Semicolon Insertion: The Good, the Bad, and the Ugly

One of JavaScript‘s most controversial features is its handling of semicolons. Unlike many other programming languages, JavaScript doesn‘t require you to end each statement with a semicolon. If you omit a semicolon, the JavaScript parser will attempt to automatically insert one for you using a set of ASI rules defined in the ECMAScript specification.

The basic rules for ASI, as described in the spec, are:

1. A semicolon is inserted before, when a Line terminator or "}" is encountered that is not allowed by the grammar.
2. A semicolon is inserted at the end, when the end of the input stream of tokens is detected and the parser is unable to parse the single input stream as a complete Program.

While these rules may seem straightforward, there are numerous edge cases and gotchas where ASI can fail to insert a semicolon where you might expect, or insert one in a surprising place, leading to bugs that can be tricky to diagnose and fix.

For example, consider the following code:

function returnObject() {
  return
  {
    key: ‘value‘
  }
}

console.log(returnObject());

If you‘re not familiar with ASI, you might expect this to log the object { key: ‘value‘ }. However, it actually logs undefined! This is because ASI inserts a semicolon immediately after the return statement, so the function returns undefined and the object literal on the following lines is never reached.

Here‘s another common pitfall:

const x = 5
[1, 2, 3].forEach(console.log)

You may expect this to log each number in the array, but instead, it throws a TypeError: Cannot read property ‘forEach‘ of undefined. The reason is that ASI inserts a semicolon after 5, so the array on the next line is interpreted as an array literal being passed as an argument to the const declaration, which is invalid.

These are just a couple examples of how ASI can cause hard-to-diagnose bugs and make your code less predictable and maintainable, especially for other developers who may not be as familiar with ASI‘s intricacies.

The Case for Explicit Semicolons

Given the potential pitfalls of ASI, many JavaScript experts recommend using explicit semicolons to terminate your statements. One of the most prominent advocates for this approach is Douglas Crockford, a renowned JavaScript guru and the author of "JavaScript: The Good Parts".

In his book and his popular "Effective JavaScript" training courses, Crockford makes a strong case for using semicolons. He argues that relying on ASI is error-prone and makes your code harder to reason about, since you have to constantly be aware of the ASI rules and make sure your line breaks are in the right places.

By contrast, using explicit semicolons eliminates any ambiguity and makes your code more robust and readable. It clearly communicates your intent and removes the burden of having to worry about ASI edge cases.

Crockford isn‘t alone in this view. Many other prominent JavaScript experts and resources, such as Kyle Simpson‘s "You Don‘t Know JS" series and the widely-used Airbnb JavaScript Style Guide, also recommend using semicolons. In fact, according to a 2019 analysis of over 137,000 JavaScript projects on GitHub, 85% of them used semicolons consistently.

The use of semicolons is also enforced by most popular JavaScript linting and formatting tools, such as ESLint and Prettier. These tools can automatically insert semicolons for you and/or flag any missing semicolons as errors, making it easy to ensure consistency across your codebase.

Addressing Common Objections

Despite the strong arguments in favor of explicit semicolons, some developers still prefer to omit them. Let‘s take a look at some of the most common objections and counterarguments.

"Semicolons are unnecessary clutter"

One common argument against semicolons is that they add visual noise and make your code less clean and concise. However, this is largely a matter of personal preference and familiarity. Many developers find that explicit semicolons actually make code easier to read, since they provide a clear visual indication of where each statement ends.

Moreover, the idea that semicolons are purely optional in JavaScript is somewhat misleading. While ASI allows you to omit semicolons in many cases, there are still situations where they are required, such as when you want to write multiple statements on the same line. For example:

let x = 5; console.log(x);

In this case, the semicolon is necessary to separate the two statements. So even if you choose to rely on ASI most of the time, you still need to be aware of the cases where semicolons are mandatory.

"I never have problems with ASI"

Some developers argue that ASI is fine as long as you are consistent and follow certain conventions, such as always starting a new line after a return, throw, break, continue, or yield statement. However, this still puts the burden on you to be constantly vigilant and make sure your code follows these conventions.

Moreover, even if you never make a mistake, other developers who work with your code may not be as familiar with the ASI rules and conventions. Explicit semicolons make your code more self-explanatory and reduce the cognitive overhead of having to think about ASI.

There‘s also the risk of accidentally introducing ASI-related bugs when copying and pasting code snippets or examples that don‘t follow your conventions. By consistently using semicolons, you can avoid these potential pitfalls and make your code more robust and portable.

"Semicolons are a matter of personal preference"

While it‘s true that the choice of whether to use semicolons is ultimately up to each individual developer or team, it‘s important to consider the broader implications of this choice. When working on a team or contributing to an open-source project, following consistent conventions is important for maintainability and collaboration.

If your team or project has a convention of using semicolons, deviating from that convention can create inconsistency and make the codebase harder to navigate and understand. It can also create unnecessary merge conflicts and make it harder to use automated tools like linters and formatters.

In general, it‘s a good idea to follow the established conventions of the project or community you‘re working with. And if you‘re starting a new project or team, it‘s worth considering the benefits of using explicit semicolons from the outset.

Conclusion

In conclusion, while JavaScript‘s ASI feature provides some flexibility and convenience, relying on it can lead to subtle bugs and make your code harder to reason about and maintain. Explicitly terminating your statements with semicolons eliminates these potential pitfalls and makes your code more robust, readable, and self-explanatory.

The use of semicolons is a widely-accepted best practice in the JavaScript community, and for good reason. It‘s recommended by many experts and style guides, enforced by popular tools, and used consistently in the majority of JavaScript codebases.

While it may take some getting used to if you‘re not accustomed to using semicolons, the benefits are well worth the effort. By embracing this practice, you can write clearer, more maintainable code and avoid unnecessary bugs and confusion.

At the end of the day, the goal of any coding convention or best practice is to make your code easier to understand, modify, and collaborate on. Explicit semicolons serve this goal admirably, and are a small but important way to be a more effective JavaScript developer. So let‘s embrace the semicolon and write code that is as unambiguous and intentional as possible!