JavaScript Common Mistakes

This chapter points at some common JavaScript mistakes.

Accidentally Using the Assignment Operator

This statement only returns true, if x1 equals 10:

if (x1 == 10)

Try it yourself »

This statement always returns true, because the assignment is always true:

if (x1 = 10)

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Confusing Addition & Concatenation

Addition is about adding numbers.

Concatenation is about adding strings.

In JavaScript both operations use the same + operator.

Because of this, when adding a number as a number, will produce a different result from adding a number as a string:

var x = 10 + 5;          // the result in x is 15
var x = 10 + "5";        // the result in x is "105"

Try it yourself »

When adding two variables, it can be difficult to anticipate the result:

var x = 10;
var y = 5;
var z = x + y;           // the result in z is 15

var x = 10;
var y = "5";
var z = x + y;           // the result in z is "105"

Try it yourself »

Misunderstanding Floats

All numbers in JavaScript are stored as 64-bits Floating point numbers (Floats).

All programming languages, including JavaScript, have difficulties with precise floating point values:

var x = 0.1;
var y = 0.2;
var z = x + y            // the result in z will not be 0.3
if (z == 0.3)            // this if test will fail

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Breaking a JavaScript String

JavaScript will allow you to break a statement into two lines:

Example 1

var x =
"Hello World!";

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But, breaking a statement in the middle of a string will not work:

Example 2

var x = "Hello
World!";

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You must use a "backslash" if you must break a statement in a string:

Example 3

var x = "Hello \
World!";

Try it yourself »

Misplacing Semicolon

Because of a misplaced semicolon, this code block will execute regardless of the value of x:

if (x == 19);
{
    // code block 
}

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Breaking a Return Statement

It is a default JavaScript behavior to close a statement automatically at the end of a line.

Because of this, these two examples will return the same result:

Example 1

function myFunction(a) {
    var power = 10 
    return a * power
}

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Example 2

function myFunction(a) {
    var power = 10;
    return a * power;
}

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JavaScript will also allow you to break a statement into two lines.

Because of this, example 3 will also return the same result:

Example 3

function myFunction(a) {
    var
    power = 10; 
    return a * power;
}

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But, what will happen if you break the return statement in two lines like this:

Example 4

function myFunction(a) {
    var
    power = 10; 
    return
    a * power;
}

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The function will return undefined!

Why? Because JavaScript thinks you meant:

Example 5

function myFunction(a) {
    var
    power = 10; 
    return;
    a * power;
}

Try it yourself »

Explanation

If a statement is incomplete like:

var

JavaScript will try to complete the statement by reading the next line:

power = 10;

But since this statement is complete:

return

JavaScript will automatically close it like this:

return;

This happens because closing (ending) statements with semicolon is optional in JavaScript.

JavaScript will close the return statement at the end of the line, because it is a complete statement.

Note Never break a return statement.

Accessing Arrays with Named Indexes

Many programming languages support arrays with named indexes.

Arrays with named indexes are called associative arrays (or hashes).

JavaScript does not support arrays with named indexes.

In JavaScript, arrays use numbered indexes:  

Example:

var person = [];
person[0] = "John";
person[1] = "Doe";
person[2] = 46;
var x = person.length;         // person.length will return 3
var y = person[0];             // person[0] will return "John"

Try it Yourself »

In JavaScript, objects use named indexes.

If you use a named index, when accessing an array, JavaScript will redefine the array to a standard object.

After the automatic redefinition, array methods and properties will produce undefined or incorrect results:

 Example:

var person = [];
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
var x = person.length;         // person.length will return 0
var y = person[0];             // person[0] will return undefined

Try it Yourself »

Ending an Array Definition with a Comma

Incorrect:

points = [40, 100, 1, 5, 25, 10,];

Some JSON and JavaScript engines will fail, or behave unexpectedly.

Correct:

points = [40, 100, 1, 5, 25, 10];

Ending an Object Definition with a Comma

Incorrect:

person = {firstName:"John", lastName:"Doe", age:46,}

Some JSON and JavaScript engines will fail, or behave unexpectedly.

Correct:

person = {firstName:"John", lastName:"Doe", age:46}

Undefined is Not Null

With JavaScript, null is for objects, undefined is for variables, properties, and methods.

To be null, an object has to be defined, otherwise it will be undefined.

If you want to test if an object exists, this will throw an error if the object is undefined:

Incorrect:

if (myObj !== null && typeof myObj !== "undefined") 

Because of this, you must test typeof() first:

Correct:

if (typeof myObj !== "undefined" && myObj !== null) 

Expecting Block Level Scope

JavaScript does not create a new scope for each code block.

It is true in many programming languages, but not true in JavaScript.

It is a common mistake, among new JavaScript developers, to believe that this code returns undefined:

Example:

for (var i = 0; i < 10; i++) {
    // some code
}
return i;

Try it Yourself »


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