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Exercise 9

What does the following program log to the console? Why?

let foo = {
  a: 'hello',
  b: 'world',
};

let qux = 'hello';

function bar(argument1, argument2) {
  argument1.a = 'hi';
  argument2 = 'hi';
}

bar(foo, qux);

console.log(foo.a);
console.log(qux);

Solution

This program logs 'hi' and 'hello' to the console. This is because objects are mutable and primitives are immutable.

When bar is invoked on line 13, argument1 is initialized to an object and argument2 is initialized to a string value.

Line 9 reassigns an object property (argument1.a), which will mutate the object. Line 10 reassigns a variable, which completely replaces the value of the local argument2 variable.

Thus, line 9 mutates foo by reassigning its a property to a new value ('hi'), and line 10's variable reassignment doesn't affect any variables outside of bar.

Because foo was mutated, the access to foo.a on line 15 logs 'hi'. On the other hand, the variable reassignment on line 10 does not mutate qux's value, so line 16 logs hello - the original value of the qux variable.

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