Examples of default values for TypeScript input parameters and analysis of the generated JavaScript code

const fun = ({ aa = 1, bb = 0 } = {}) = > {
      console.log('aa: ' + aa);
      console.log('bb: ' + bb);
      return 'hello';
    }

document.body.innerHTML = fun(); 
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Compiling with TSC:

Generated JavaScript code:

var fun = function (_a) {
    var _b = _a === void 0 ? {} : _a, _c = _b.aa, aa = _c === void 0 ? 1 : _c, _d = _b.bb, bb = _d === void 0 ? 0 : _d;
    console.log('aa: ' + aa);
    console.log('bb: ' + bb);
    return 'hello';
};
document.body.innerHTML = fun();
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The readability is mediocre, so Jerry adds some notes:

var fun = function (_a) {
	// Is the input parameter _a undefined? If so, assign the default value {}, an empty object. If it's not empty,
	// use temporary _b to store the value of input variable _a
	// now _b and _A are equivalent.
    var _b = _a === void 0 ? {} : _a;

    // _c stores the aa field of the input variable
    var _c = _b.aa;

    // Is the aa field of the input variable undefined? If so, the default value 1 is assigned to the temporary variable aa, otherwise the value of the AA field passed in by the variable is used
    var aa = _c === void 0 ? 1 : _c;

    // _d stores the bb field of the input variable
    var _d = _b.bb;

    // Is the input field bb undefined? If so, use the default value 0; otherwise, use the actual value of the bb field of the input variable
    var bb = _d === void 0 ? 0 : _d;

    console.log('aa: ' + aa);
    console.log('bb: ' + bb);
    return 'hello';
};
document.body.innerHTML = fun();
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