Note about strict mode and destructuring parameters.

strict-mode-and-destructuring-parameters.js

// 30 August 2023
// Note about strict mode and destructuring parameters.

// Setting strict mode at the top of the console...

"use strict";

// ...makes the following assignment in the try block fail:

console.group("init");
  try { undeclared_variable = 'should fail'; }
  catch (e) { console.warn(e); }
  finally { }
console.groupEnd("init");

// That results in the error message,
// "Uncaught ReferenceError: assignment to undeclared variable
// undeclared_variable"

// If we do that outside a try block, that halts the console. No subsequent
// statements are evaluated or executed.

// We can test this behavior better by moving statements into functions that
// declare "use strict"; at the top.

// Here we use console.groups, IIFEs (immediately invoked function expressions),
// and try-catch-finally blocks to avoid crashing the console...

(function test(value) {
  console.group(value);

  "use strict";

  try { undeclared_variable = value; }
  catch(e) { console.warn(e) }
  finally { console.log("done"); }

  console.groupEnd(value);
})("should fail");


// Another one.

(function test(value) {
  console.group(value);

  "use strict";

  try { another_undeclared_variable = value; }
  catch(e) { console.warn(e) }
  finally { console.log("done"); }

  console.groupEnd(value);
})("should fail again");


// Now for the quirk.

// A function with destructuring parameters may *not* declare "use strict"!

// The following nested IIFE in its own try block results in a *syntax* error,
// preventing the console from completing the evaluation step, so that no code
// executes.

(function (value) {
  console.group(value);

  try {
    (function test({ value }) {

      // un-comment the next line to experience the error...
      // "use strict";

    })({ value });
  }
  catch(e) { console.warn(e) }
  finally { console.log("done"); }

  console.groupEnd(value);
})("destructuring parameter");

// Uncaught SyntaxError: "use strict" not allowed in function with destructuring
// parameter.


// Now you know.