Why Does Babel Insert a Comma Operator Before Imported Function Calls in Strict Mode?

Understanding Babel's Mysterious Comma Operator in Function Calls

In Babel's compilation process, users have observed that imported function calls undergo a transformation where a comma (,) is inserted before the function name. This behavior raises questions about the purpose and rationale behind this change.

When examining the input and output code examples, it's clear that Babel's strict mode compilation results in the following syntax:

(0, _b.a)();

However, in loose mode, this transformation is absent, leaving only the regular function call:

_b.a();

The mystery lies in the comma operator's insertion. To unravel this enigma, we need to dive into the code responsible for this transformation.

Upon investigation, we discover that Babel uses the comma operator to ensure that the imported function is invoked in the context of the global object or undefined if strict mode is enabled. This is achieved by the following JavaScript code:

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

In essence, the comma operator creates a temporary variable (tmp) that holds the reference to the imported function (_b.a). By calling tmp() instead of _b.a(), Babel ensures that the function is invoked with the correct execution context.

In other words, "(0, _b.a)() is equivalent to calling _b.a with this set to undefined (or the global object in non-strict mode). This prevents any accidental binding of this to _b, which could lead to unexpected behavior.

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