JavaScript Anomalies.

javascript-anomalies.js

// #1 - Context. The `this` variable case:

var Person = function(firstname) {
  this.firstname = firstname;
  this.print = function() {
    setTimeout(function () {
      console.log("My name is " + this.firstname);
    });
  }
}

var mau = new Person("Mauricio");
console.log(mau.firstname); // Prints "Mauricio".
mau.print(); // We expect "My name is Mauricio" but instead it prints "My name is undefined".

// A possible solution:

Person = function(firstname) {
  this.firstname = firstname;
  this.print = function() {
    setTimeout(() => {
      console.log("My name is " + this.firstname);
    });
  }
}

mau = new Person("Mauricio");
console.log(mau.firstname); // Prints "Mauricio".
mau.print(); // We finally printed "My name is Mauricio".

// #2 - Object Reference.

// Extending the code above:

function setAge(person, age) {
	person.age = age;
}

var javi = mau;
setAge(mau, 26);
console.log(mau.age); // It prints 26 as expected.
console.log(javi.age); // But it also changed the age for `javi`. This could be tricky.

// A possible solution: deep clone.

javi = Object.assign(new Person(), mau);
javi.firstname = "Javi";
setAge(javi, 28);

console.log(mau); // The instance `mau` keeps their original properties.
console.log(javi); // Changes in `javi` only affected this specific instance.