Because JavaScript is permissive, developers have the temptation to access uninitialized values. I'm guilty of such bad practice too.

Often such risky actions generate undefined related errors:

• TypeError: 'undefined' is not a function
• TypeError: Cannot read property '<prop-name>' of undefined
• and alike type errors.

JavaScript developers can understand the irony of this joke:

To reduce such errors, you have to understand the cases when undefined is generated. Let's explore undefined and its effect on code safety.

1. What is undefined

JavaScript has 6 primitive types:

• Boolean: true or false
• Number: 1, 6.7, 0xFF
• String: "Gorilla and banana"
• Symbol: Symbol("name") (starting ES2015)
• Null: null
• Undefined: undefined.

And a separated object type: {name: "Dmitri"}, ["apple", "orange"].

From 6 primitive types undefined is a special value with its own type Undefined. According to ECMAScript specification:

Undefined value primitive value is used when a variable has not been assigned a value.

The standard clearly defines that you will receive undefined when accessing uninitialized variables, non-existing object properties, non-existing array elements, and alike.

A few examples:

The above example demonstrates that accessing:

• an uninitialized variable number
• a non-existing object property movie.year
• or a non-existing array element movies[3]

are evaluated as undefined.

The ECMAScript specification defines the type of undefined value:

Undefined type is a type whose sole value is the undefined value.

In this sense, typeof operator returns 'undefined' string for an undefined value:

Of course typeof works nicely to verify whether a variable contains an undefined value:

2. Scenarios that create undefined

2.1 Uninitialized variable

A declared variable but not yet assigned with a value (uninitialized) is undefined.

Plain and simple:

myVariable is declared and not yet assigned with a value. Accessing the variable evaluates to undefined.

An efficient approach to solve the troubles of uninitialized variables is whenever possible to assign an initial value. The less the variable exists in an uninitialized state, the better.

Ideally, you would assign a value right away after declaration const myVariable = 'Initial value'. But that's not always possible.

Tip 1: Favor const, otherwise use let, but say goodbye to var

const and let are block scoped (contrary to older function scoped var) and exist in a temporal dead zone until the declaration line.

If you want to define a variable, always start with const, which creates an immutable binding.

One of the nice features of const is that you must assign an initial value to the variable const myVariable = 'initial'. The variable is not exposed to the uninitialized state and accessing undefined is impossible.

Let's check the function that verifies whether a word is a palindrome:

length and half variables are assigned with a value once. It seems reasonable to declare them as const since these variables aren't going to change.

Use let declaration for variables whose value can change. Whenever possible assign an initial value right away, e.g. let index = 0.

What about the old-school var? My suggestion is to stop using it.

var declaration problem is the variable hoisting within the function scope. You can declare a var variable somewhere at the end of the function scope, but still, you can access it before declaration: and you'll get an undefined.

myVariable is accessible and contains undefined even before the declaration line: var myVariable = 'Initial value'.

Contrary, a const or let variable cannot be accessed before the declaration line — the variable is in a temporal dead zone before the declaration. And that's nice because you have less chance to access an undefined.

The above example updated with let (instead of var) throws a ReferenceError because the variable in the temporal dead zone is not accessible.

Encouraging the usage of const for immutable bindings or let otherwise ensures a practice that reduces the appearance of the uninitialized variable.

Tip 2: Increase cohesion

Cohesion characterizes the degree to which the elements of a module (namespace, class, method, block of code) belong together. The cohesion can be high or low.

A high cohesion module is preferable because the elements of such a module focus solely on a single task. It makes the module:

• Focused and understandable: easier to understand what the module does
• Maintainable and easier to refactor: the change in the module affects fewer modules
• Reusable: being focused on a single task, makes the module easier to reuse
• Testable: it's easier to test a module that's focused on a single task

High cohesion accompanied by loose coupling is the characteristic of a well-designed system.

A code block can be considered a small module. To profit from the benefits of high cohesion, keep the variables as close as possible to the code block that uses them.

For instance, if a variable solely exists to form the logic of block scope, then declare and make the variable exist only within that block (using const or let declarations). Do not expose this variable to the outer block scope, since the outer block shouldn't need this variable.

One classic example of the unnecessarily extended life of variables is the usage of for cycle inside a function:

index, item, and length variables are declared at the beginning of the function body. However, they are used only near the end. What's the problem with this approach?

Between the declaration at the top and the usage in for statement the variables index and item are uninitialized and exposed to undefined. They have an unreasonably long lifecycle in the entire function scope.

A better approach is to move these variables as close as possible to their usage place:

index and item variables exist only in the block scope of for statement. They don't have any meaning outside of for.
length variable is declared close to the source of its usage too.

The refactored code is better because:

• The variables are not exposed to an uninitialized state, thus you have no risk of accessing undefined
• Moving the variables as close as possible to their usage place increases the code readability
• High cohesive chunks of code are easier to refactor and extract into separate functions, if necessary

2.2 Accessing a non-existing property

When accessing a non-existing object property, JavaScript returns undefined.

Let's demonstrate that in an example:

favoriteMovie is an object with a single property title. Accessing a non-existing property actors using a property accessor favoriteMovie.actors evaluates to undefined.

Accessing a non-existing property does not throw an error. The problem appears when trying to get data from the non-existing property (e.g. favoriteMovie.actors.length) — the most common undefined issue.

"TypeError: Cannot read property <prop> of undefined" is a very common error message generated by accessing data from non-existing properties.

Let's slightly modify the previous code snippet to illustrate a TypeError throw:

favoriteMovie does not have the property actors, so favoriteMovie.actors evaluates to undefined.

As a result, accessing the length of an undefined value using the expression favoriteMovie.actors.length throws a TypeError.

A good way to bypass this problem is to restrict the object to have always defined the properties that it holds.

Unfortunately, often you don't have control over the objects. Such objects may have a different set of properties in diverse scenarios. So you have to handle all these scenarios manually.

Let's implement a function append(array, toAppend) that adds at the beginning and/or at the end of an array new elements. toAppend parameter accepts an object with properties:

• first: element inserted at the beginning of array
• last: element inserted at the end of array.

The function returns a new array instance, without altering the original array.

The first version of append(), a bit naive, may look like this:

Because toAppend object can omit first or last properties, it is obligatory to verify whether these properties exist in toAppend.

A property accessor evaluates to undefined if the property does not exist. The first temptation to check whether first or last properties are present is to verify them against undefined. This is performed in conditionals if(toAppend.first){} and if(toAppend.last){}...

Not so fast. This approach has a drawback. undefined, as well as false, null, 0, NaN, and '' are falsy values.

In the current implementation of append(), the function doesn't allow to insert falsy elements:

0 and false are falsy. Because if(toAppend.first){} and if(toAppend.last){} actually compare against falsy, these elements are not inserted into the array. The function returns the initial array [10] without modifications, instead of the expected [0, 10, false].

The tips that follow explain how to correctly check the property's existence.

Tip 3: Check the property existence

Fortunately, JavaScript offers a bunch of ways to determine if the object has a specific property:

• obj.prop !== undefined: compare against undefined directly
• typeof obj.prop !== 'undefined': verify the property value type
• obj.hasOwnProperty('prop'): verify whether the object has its own property
• 'prop' in obj: verify whether the object has an own or inherited property

My recommendation is to use in operator. It has a short and readable syntax. in operator presence suggests a clear intent of checking whether an object has a specific property, without accessing the actual property value.

obj.hasOwnProperty('prop') is a nice solution too. It's slightly longer than in operator and verifies only the object's own properties.

Let's improve append(array, toAppend) function using in operator:

in operator fixes the problem by inserting falsy elements 0 and false. Now, adding these elements at the beginning and the end of [10] produces the expected result [0, 10, false].

Tip 4: Destructuring to access object properties

When accessing an object property, sometimes it's necessary to set a default value if the property does not exist.

You might use in accompanied with a ternary operator to accomplish this:

Ternary operator syntax becomes daunting when the number of properties to check increases. For each property, you have to create a new line of code to handle the defaults, increasing an ugly wall of similar-looking ternary operators.

To use a more elegant approach, let's get familiar with a great ES2015 feature called object destructuring.

Object destructuring allows inline extraction of object properties directly into variables and sets a default value if the property does not exist. A convenient syntax to avoid dealing directly with undefined.

Indeed, the property extraction is better:

To see things in action, let's define a useful function that wraps a string in quotes.

quote(subject, config) accepts the first argument as the string to be wrapped. The second argument config is an object with the properties:

• char: the quote char, e.g. ' (single quote) or " (double quote). Defaults to ".
• skipIfQuoted: the boolean value to skip quoting if the string is already quoted. Defaults to true.

Applying the benefits of object destructuring, let's implement quote():

const { char = '"', skipIfQuoted = true } = config destructuring assignment in one line extracts the properties char and skipIfQuoted from config object.

If some properties are missing in the config object, the destructuring assignment sets the default values: '"' for char and false for skipIfQuoted.

Fortunately, the function still has room for improvement.

Let's move the destructuring assignment into the parameters section. And set a default value (an empty object { }) for the config parameter, to skip the second argument when default settings are enough.

The destructuring assignment replaces the config parameter in the function's signature. I like that: quote() becomes one line shorter.

= {} on the right side of the destructuring assignment ensures that an empty object is used if the second argument is not indicated during the call: quote('Sunny day').

Object destructuring is a powerful feature that handles efficiently the extraction of properties from objects. I like the possibility to specify a default value to be returned when the accessed property doesn't exist. As a result, you avoid undefined and the hassle around it.

Tip 5: Fill the object with default properties

If there is no need to create variables for every property, as the destructuring assignment does, the object that misses some properties can be filled with default values.

Object.assign(target, source1, source2, ...) copies the values of all enumerable own properties from one or more source objects into the target object. The function returns the target object.

For instance, you need to access the properties of unsafeOptions object that doesn't always contain its full set of properties.

To avoid undefined when accessing a non-existing property from unsafeOptions, let's make some adjustments:

• Define an object defaults that holds the default property values
• Call Object.assign({ }, defaults, unsafeOptions) to build a new object options. The new object receives all properties from unsafeOptions, but the missing ones are taken from defaults.

unsafeOptions contains only fontSize property. defaults object defines the default values for properties fontSize and color.

Object.assign() takes the first argument as a target object {}. The target object receives the value of fontSize property from unsafeOptions source object. And the value of color property from defaults source object, because unsafeOptions doesn't contain color.

The order in which the source objects are enumerated does matter: later source object properties overwrite earlier ones.

You are now safe to access any property of options object, including options.color that wasn't available in unsafeOptions initially.

Fortunately, an easier alternative to fill the object with default properties exists. I recommend using the spread properties in object initializers.

Instead of Object.assign(), use the object spread syntax to copy into the target object all own and enumerable properties from source objects:

The object initializer spreads properties from defaults and unsafeOptions source objects. The order in which the source objects are specified is important: later source object properties overwrite earlier ones.

Filling an incomplete object with default property values is an efficient strategy to make your code safe and durable. No matter the situation, the object always contains the full set of properties: and undefined cannot be generated.

Bonus tip: nullish coalescing

The operator nullish coalescing evaluates to a default value when its operand is undefined or null:

Nullish coalescing operator is convenient to access an object property while having a default value when this property is undefined or null:

styles object doesn't have the property color, thus styles.color property accessor is undefined. styles.color ?? 'black' evaluates to the default value 'black'.

styles.fontSize is 18, so the nullish coalescing operator evaluates to the property value 18.

2.3 Function parameters

The function parameters implicitly default to undefined.

Usually, a function defined with a specific number of parameters should be invoked with the same number of arguments. That's when the parameters get the values you expect:

When calling multiply(5, 3), the parameters a and b receive 5 and respectively 3 values. The multiplication is calculated as expected: 5 * 3 = 15.

What does happen when you omit an argument on invocation? The corresponding parameter inside the function becomes undefined.

Let's slightly modify the previous example by calling the function with just one argument:

The invocation multiply(5) is performed with a single argument: as a result a parameter is 5, but the b parameter is undefined.

Tip 6: Use default parameter value

Sometimes a function does not require the full set of arguments on invocation. You can set defaults for parameters that don't have a value.

Recalling the previous example, let's make an improvement. If b parameter is undefined, let default it to 2:

The function is invoked with a single argument multiply(5). Initially, a parameter is 2 and b is undefined.

The conditional statement verifies whether b is undefined. If it happens, b = 2 assignment sets a default value.

While the provided way to assign default values works, I don't recommend comparing directly against undefined. It's verbose and looks like a hack.

A better approach is to use the ES2015 default parameters feature. It's short, expressive, and has no direct comparisons with undefined.

Adding a default value to parameter b = 2 looks better:

b = 2 in the function signature makes sure that if b is undefined, the parameter defaults to 2.

The default parameters feature is intuitive and expressive. Always use it to set default values for optional parameters.

2.4 Function return value

Implicitly, without return statement, a JavaScript function returns undefined.

A function that doesn't have return statement implicitly returns undefined:

square() function does not return any computation results. The function invocation result is undefined.

The same situation happens when return statement is present, but without an expression nearby:

return; statement is executed, but it doesn't return any expression. The invocation result is also undefined.

Of course, indicating near return the expression to be returned works as expected:

Now the function invocation is evaluated to 4, which is 2 squared.

Tip 7: Don't trust the automatic semicolon insertion

The following list of statements in JavaScript must end with semicolons (;):

• empty statement
• let, const, var, import, export declarations
• expression statement
• debugger statement
• continue statement, break statement
• throw statement
• return statement

If you use one of the above statements, be sure to indicate a semicolon at the end:

At the end of both let declaration and return statement an obligatory semicolon is written.

What happens when you don't want to indicate these semicolons? In such a situation ECMAScript provides an Automatic Semicolon Insertion (ASI) mechanism, which inserts for you the missing semicolons.

Helped by ASI, you can remove the semicolons from the previous example:

The above text is a valid JavaScript code. The missing semicolons are automatically inserted for you.

At first sight, it looks pretty promising. ASI mechanism lets you skip the unnecessary semicolons. You can make the JavaScript code smaller and easier to read.

There is one small, but annoying trap created by ASI. When a newline stands between return and the returned expression return \n expression, ASI automatically inserts a semicolon before the newline return; \n expression.

What does it mean to have return; statement inside of a function? The function returns undefined. If you don't know in detail the mechanism of ASI, the unexpectedly returned undefined is misleading.

For instance, let's study the returned value of getPrimeNumbers() invocation:

Between return statement and the array literal expression exists a new line. JavaScript automatically inserts a semicolon after return, interpreting the code as follows:

The statement return; makes the function getPrimeNumbers() return undefined instead of the expected array.

The problem is solved by removing the newline between return and array literal:

My recommendation is to study how exactly Automatic Semicolon Insertion works to avoid such situations.

Of course, never put a newline between return and the returned expression.

2.5 void operator

void <expression> evaluates the expression and returns undefined no matter the result of the evaluation.

One use case of void operator is to suppress expression evaluation to undefined, relying on some side-effect of the evaluation.

3. undefined in arrays

You get undefined when accessing an array element with an out-of-bounds index.

colors array has 3 elements, thus valid indexes are 0, 1, and 2.

Because there are no array elements at indexes 5 and -1, the accessors colors[5] and colors[-1] are undefined.

In JavaScript, you might encounter so-called sparse arrays. These are arrays that have gaps, i.e. at some indexes no elements are defined.

When a gap (aka empty slot) is accessed inside a sparse array, you also get an undefined.

The following example generates sparse arrays and tries to access their empty slots:

sparse1 is created by invoking an Array constructor with a numeric first argument. It has 3 empty slots.

sparse2 is created with an array literal with the missing second element.

In any of these sparse arrays accessing an empty slot evaluates to undefined.

When working with arrays, to avoid undefined, be sure to use valid array indexes and prevent the creation of sparse arrays.

4. undefined and null differences

What is the main difference between undefined and null? Both special values imply an empty state.

undefined represents the value of a variable that hasn't been yet initialized, while null represents an intentional absence of an object.

Let's explore the difference in some examples.

The variable number is defined, however, is not assigned with an initial value:

number variable is undefined, which indicates an uninitialized variable.

The same uninitialized concept happens when a non-existing object property is accessed:

Because lastName property does not exist in obj, JavaScript evaluates obj.lastName to undefined.

On the other side, you know that a variable expects an object. But for some reason, you can't instantiate the object. In such case null is a meaningful indicator of a missing object.

For example, clone() is a function that clones a plain JavaScript object. The function is expected to return an object:

However clone() might be invoked with a non-object argument: 15 or null. The function cannot create a clone from these values, so it returns null — the indicator of a missing object.

typeof operator makes the distinction between undefined and null:

Also the strict quality operator === correctly differentiates undefined from null:

5. Conclusion

undefined existence is a consequence of JavaScript's permissive nature that allows the usage of:

• uninitialized variables
• non-existing object properties or methods
• out-of-bounds indexes to access array elements
• the invocation result of a function that returns nothing

Comparing directly against undefined is unsafe because you rely on a permitted but discouraged practice mentioned above.

An efficient strategy is to reduce the appearance of undefined keyword in your code by applying good habits such as:

• reduce the usage of uninitialized variables
• make the variables' lifecycle short and close to the source of their usage
• whenever possible assign initial values to variables
• favor const, otherwise use let
• use default values for insignificant function parameters
• verify the properties' existence or fill the unsafe objects with default properties
• avoid the usage of sparse arrays

Is it good or bad that JavaScript has undefined and null to represent empty values?