What are undeclared variables?

Undeclared variables in JavaScript are variables that have not been declared using the keywords var, let, or const, but have been assigned a value.

// Explicit declaration
let declaredVariable = "I am declared";

// Implicit assignment (creates an undeclared variable in non-strict mode)
// In strict mode ('use strict') this will throw a ReferenceError
undeclaredVariable = "I am not declared";

The behavior of undeclared variables depends on the execution mode:

• Non-strict mode: Assigning a value to an undeclared variable creates a global variable with that name in Window (for browsers). This is considered bad practice as it pollutes the global scope and makes debugging harder.
• Strict mode ('use strict'): Assigning a value to an undeclared variable throws a ReferenceError. This is the preferred behavior, preventing accidental creation of global variables and leading to more predictable code.

Checking for an undeclared variable can be done using typeof. An undeclared variable in non-strict mode will have the type 'undefined', just like a variable declared but not initialized.

let initializedButUndefined;
// console.log(typeof initializedButUndefined); // => 'undefined'

// In non-strict mode
// undeclaredVariable = "Check";
// console.log(typeof undeclaredVariable); // => 'undefined'

// Check if a variable exists before accessing
if (typeof potentiallyUndeclaredVariable !== 'undefined') {
    // Code to work with potentiallyUndeclaredVariable
}

Using undeclared variables is a source of potential errors and makes code maintenance difficult. It is always recommended to explicitly declare variables using let, const, or var (although var has its own quirks and modern code more often uses let and const). Using strict mode ('use strict') is a standard practice to prevent such issues.