Why Does Babel Add a Comma Operator When Calling Imported Functions in Strict Mode?

Why Babel Rewrites Imported Function Calls in Strict Mode

In strict mode, when Babel encounters an imported function call, it rewrites it to include the comma operator, as seen in the example below:

import { a } from 'b';

function x () {
  a()
}

The compiled output in strict mode becomes:

'use strict';

var _b = require('b');

function x() {
  (0, _b.a)();
}

This transformation ensures that the imported function is called with this set to the global object (or to undefined if strict mode is enabled). Without the comma operator, the imported function would be called with this set to its module.

Explanation of the Comma Operator

The comma operator (,) is used in JavaScript to evaluate multiple expressions, separated by commas. It returns the value of the last expression. In the rewritten code, the comma operator is used as follows:

(0, _b.a)();

This is equivalent to the following code:

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

The first expression, 0, is ignored. The second expression, _b.a, assigns the imported function to the tmp variable. The third expression, tmp(), calls the imported function with this set to the global object (or to undefined in strict mode).

By using the comma operator, Babel ensures that the imported function is always called with the correct this value, regardless of the execution context.

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