Why Does Babel Use the Comma Operator (0, fn)(...) for Imported Function Calls?

Babel's Encapsulation of Imported Function Calls

Babel often generates code that encloses imported function calls with a comma operator, (0, fn)(...), instead of the expected fn(). This seemingly gratuitous comma can be puzzling. Let's delve into why Babel does this.

In JavaScript, invoking a function without explicitly specifying the this context defaults to the global object. However, when a function is defined as a property of an imported module, its this context is automatically set to the module object. To prevent this "lexical binding" and ensure that imported functions always execute in the global context, Babel introduces the (0, fn)(...) syntax.

The comma operator evaluates the expression on its left (0) while discarding its result. This yields 0, which is essentially a placeholder. The purpose of this syntactical construct is to force the execution of the function call as a function application, rather than as a method call on the imported module object.

In essence, (0, fn)(...) achieves the equivalent of:

0; // Ignore result
var tmp = fn;
tmp();

By placing the unnecessary comma, Babel effectively intercepts the function call and executes it with the this context set to the global object. This ensures that the function can access global variables and functions without triggering any unintended lexical bindings.

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