JavaScript Gotchas for Beginners

When starting to learn JavaScript, there are a few details and gotchas that are always important to remember. Memorizing and understanding these concepts will make you a better and faster JavaScript programmer overall and it is why I am listing a few of these concepts (which are best learned at the beginner level) in this post.

The Global Object

The global scope is the space in which global variables live and it can also be approached as an object. Each global variable is present as a property of this object. In browsers, the global scope object is stored in the window variable:

var v = 10;
console.log("v" in window);
// > true
// > 10

Objects and Mutability

With JavaScript objects there is a difference between having two references to the same object and having two different objects that contain the same properties. Consider the following code:

var object1 = {value: 10};
var object2 = object1;
var object3 = {value: 10};

console.log(object1 == object2);
// > true
console.log(object1 == object3);
// > false

object1.value = 15;
// > 15
// > 10

This is why it is important to be careful when trying to “copy” variables like above, it leaves a lot of room for unexpected results.


In JavaScript, almost all objects have a prototype, in addition to their set of properties. A prototype is another object that is used as a fallback source of properties. When an object gets a request for a property that it does not have, its prototype will be searched for the property, then the prototype's prototype, and so on.

The entity behind almost all objects is Object.prototype. The Object.getPrototypeOf function returns the prototype of an object.

A prototype can be used at any time to add new properties and methods to all objects based on it.

Function Arguments

It is perfectly possible to call a function and pass more or fewer arguments than the number of parameters the function itself declares. When too many arguments are passed, the extra arguments are ignored:

function noArguments() {}
noArguments(1, 2, 3); // all arguments are ignored

When fewer arguments are passed, the missing parameters simply get assigned the value undefined:

function myFunction(first, second, third) {}
myFunction(1, 2); // 'third' would become undefined

Using a variable name starting with an underscore or consisting entirely of a single underscore is a way to indicate to human readers that this argument is not going to be used.

The arguments Object

In JavaScript, whenever a function is called, a special variable named arguments is added to the environment in which the function body runs. This variable refers to an object that holds all of the arguments passed to the function.

The arguments object has a length property that tells us the number of arguments that were really passed to the function.

function argumentCounter() {
  console.log("You gave me", arguments.length, "arguments.");

argumentCounter("Hello", "Hola", "Ciao");
// => You gave me 3 arguments.

The instanceof Operator

JavaScript provides a binary operator called instanceof which determines whether an object was derived from a specific constructor or not:

console.log(new TextCell("A") instanceof RTextCell);
// => false
console.log([1] instanceof Array);
// => true

Strict Mode and Debugging

When debugging, we can make use of a special mode in JavaScript called strict mode. To use this mode, we can put the string “use strict” at the top of a file or function body. For example:

function canYouSpotTheProblem() {
  "use strict";
  for (counter = 0; counter < 10; counter++)
    console.log("Happy happy");

// => ReferenceError: counter is not defined.

In the example above, JavaScript would normally proceed to create a global variable and use that. In strict mode however, an error is reported instead.

We can also use the debugger keyword to set a breakpoint in the program. When using a browser's developer tools, the program will pause whenever it reachers that statement and we can inspect the program's state.

Selective Exception Catching

When we want to catch specific exceptions in JavaScript, we can do so by first defining a new type of error and use instanceof to identify it. For example:

function InputError(message) {
  this.message = message;
  this.stack = (new Error()).stack;
InputError.prototype = Object.create(Error.prototype); = "InputError";

We can throw the error inside a function:

function throwError() {
  /* ... */
  throw new InputError("Invalid Error - InputError");

And we can catch it from another place in our code:

/* ... */
try {
  var a = throwError();
} catch(e) {
  if (e instanceof InputError)
    console.log("An input error ocurred");
    throw e;


  1. Eloquent JavaScript - Marijn Haverbeke
javascript programming


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