Why Does Babel Use `(0, fn)(...)` When Calling Imported Functions?

Why (0, Babel Function Call) in Babel's Compiled Output?

When importing functions from external modules and using them in your code, you may notice an interesting behavior in Babel's transformed output. Functions are wrapped in the form (0, fn)(...). This differs from the original function call without the comma operator when compiled in loose mode.

This transformation has a specific purpose: it ensures that the function's this context is set to either the global object in non-strict mode or undefined in strict mode. When calling _b.a() directly, this would default to the _b module object.

By using (0, _b.a)(), Babel forces the function to be called with this set to the appropriate value. The comma operator (,) essentially discards the return value of the preceding expression (in this case, 0) and evaluates the right-hand side, which is the function call in this instance.

In essence, the transformed code is equivalent to:

0; // Ignore result
var tmp = _b.a;
tmp();

This technique guarantees that the function's this context is consistent with the expected behavior.

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