Trailing commas in function arguments

Trailing commas are commas that are used after the final argument of a function. Using a comma after the last parameter of a function can be useful because it is very common to forget a comma when we modify an existing function by adding additional parameters.

For example, the following function only takes one parameter and doesn't use trailing commas:

function greetWithoutTralingCommas(
    name: string
): string {
    return 'Hi! ${name}';
}

Some time after the initial implementation, we might be required to add a parameter to the previous function. A common mistake is to declare the new parameter and forget to add a comma after the first parameter:

function updatedGreetWithoutTralingCommas(
    name: string
    surname: string, // Error
): string {
    return 'Hi! ${name} ${surname}';
}

Using a trailing comma in the first version of the function could have helped us to prevent this common mistake:

function greetWithTralingCommas(
    name: string,
): string {
    return 'Hi! ${name}';
}

Using a trailing comma eliminates the possibility of forgetting the comma when adding a new argument:

function updatedGreetWithTralingCommas(
    name: string,
    surname: string,
): string {
    return 'Hi! ${name} ${surname}';
}