JavaScript syntax quick reference guide

If you know the basics of JavaScript, you can skip this section. Here is a quick overview of JavaScript:

• Use the keyword var to define a variable:

  var myAge = 31;

• Use // for inline comments and /* */ for multiline comments:

  // this is an inline comment
  /* this
  is a
  multi-line
  comment
  */

• Conditional statements:

  if (myAge > 29) {
    console.log("I am not in my twenties anymore!");
  } else {
    console.log("I am still in my twenties!");
  }

• Defining a function:

  function nameOfMyFunction(argument1, argument2) {
    console.log(argument1, argument2);
  }

• Executing a function:

  nameOfMyFunction("First Value", "Second Value");

• A function can also behave as a class and have methods, properties, and instances. Properties are accessed through the keyword this:

  function Car(maxSpeed) {
    this.maxSpeed = maxSpeed;
    this.currentSpeed = 0;
  }

• The standard way to create a prototyped method for a function in JavaScript is by using the property prototype:

  Car.prototype.brake = function() {
    if (this.currentSpeed > 0) {
      this.currentSpeed -= 5;
    }
  };
  
  Car.prototype.accelerate = function() {
    if (this.currentSpeed < this.maxSpeed) {
      this.currentSpeed += 5;
    }
  };

• To create an instance of a class in JavaScript, use the keyword new:

  var car = new Car(100);
  car.accelerate();
  car.accelerate();
  car.brake();

• Arrays and objects:

  var myArray = [];
  var myObject = {};

• Callbacks in JavaScript:

  var friends = ["Karalyn", "Patrik", "Bernardo"];
  friends.forEach(function (name, index) {
    console.log(index + 1, name); // 1 Karalyn, 2 Patrik, 3 Bernardo
  });

A callback in this example is an anonymous function that is passed to another function as a parameter, so it is called (or executed) inside the other function. As you can see in the preceding example, the forEach array method expects a callback function. It executes the provided callback once for each element in the array. It is very common to find asynchronous functions/methods that expect callbacks in JavaScript.

If you want to know more about JavaScript syntax and features, we recommend the Mozilla Developer Network website at https://developer.mozilla.org/en-US/docs/Web/JavaScript.