JavaScript undefined Value – Overview and Proper Understanding

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Definition and Usage

The undefined value is the initial value automatically assigned by the JavaScript engine to indicate that a variable has not been assigned a value.

In other words, before a value is assigned to a variable, the JavaScript engine assigns undefined as the variable's initial value.

Therefore, referencing a variable after it has been declared but before a value is assigned returns undefined. However, if a value is assigned to the variable, the assigned value is returned instead of undefined.

# undefined in Regular Variables

let x; console.log(x); // undefined

Assigning a value returns the assigned value.

let x; console.log(x); // undefined /* Assigning a value returns the assigned value. */ x = 1; console.log(x); // 1

# undefined in Functions

In JavaScript, functions can also be treated as values.
If a function does not return a value using the return keyword after execution, it returns undefined.
If a value is returned using the return keyword, that value is returned.

function x(a, b) { a + b; // No value returned via the return keyword } console.log(x(1, 2)); // undefined function y(a, b) { return a + b; // A value is returned via the return keyword } console.log(y(1, 2)); // 3

# undefined in Objects

Referencing a non-existent property on an object also returns undefined.

const obj = {}; console.log(obj.prop); // undefined

Creating a property and assigning a value returns the assigned value.

const obj = {}; console.log(obj.prop); // undefined /* Create a property and assign a value */ obj.prop = 1; console.log(obj.prop); // 1

# undefined in Arrays

Referencing a non-existent element on an array also returns undefined.

let arr = []; console.log(arr[0]); // undefined

Assigning a value to the array returns the assigned value.

let arr = []; console.log(arr[0]); // undefined /* Assign a value to the array */ arr[0] = 1; console.log(arr[0]); // 1

# Data Type of undefined

The data type of the undefined value is Undefined.
In JavaScript, you can check the data type of a variable using the typeof operator.

let x; console.log(typeof x); // "undefined"

This means that undefined is a value with a unique data type.

# Evaluating undefined in Conditional Statements

The following example shows how the undefined value is evaluated in a conditional statement.

let x; if (x === undefined) { console.log("Evaluates to true."); } else { console.log("Evaluates to false."); } // Output: "Evaluates to true."

However, when the undefined value is evaluated as the boolean type, it is false.

let x; if (x) { console.log("Evaluates to true."); } else { console.log("Evaluates to false."); } // Output: "Evaluates to false." // Evaluate using the Boolean() function console.log(Boolean(x)); // false