Misplacing Semicolon
Because of a misplaced semicolon, this code block will execute regardless of the value of x:
{
// code block
}
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
var power = 10
return a * power
}
Example 2
var power = 10;
return a * power;
}
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
var
power = 10;
return a * power;
}
But, what will happen if you break the return statement in two lines like this:
Example 4
var
power = 10;
return
a * power;
}
The function will return undefined!
Why? Because JavaScript thought you meant:
Example 5
var
power = 10;
return;
a * power;
}
If a statement is incomplete like:
varJavaScript will try to complete the statement by reading the next line:
power = 10;But since this statement is complete:
returnJavaScript 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.
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
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"
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:
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
Ending Definitions with a Comma
Trailing commas in object and array definition are legal in ECMAScript 5.
Object Example:
Array Example:
JSON:
JSON:
Undefined is Not Null
JavaScript objects, variables, properties, and methods can be undefined.
In addition, empty JavaScript objects can have the value null.
This can make it a little bit difficult to test if an object is empty.
You can test if an object exists by testing if the type is undefined:
Example:
But you cannot test if an object is null, because this will throw an error if the object is undefined:
Incorrect:
To solve this problem, you must test if an object is not null, and not undefined.
But this can still throw an error:
Incorrect:
Because of this, you must test for not undefined before you can test for not null:
Correct:
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