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What happens if we re-declare a variable in JavaScript?
When you re-declare a variable in JavaScript using var, the original value is preserved. The re-declaration doesn't reset or change the variable's value.
Example
Let's see an example where we declare the variable age twice:
<html>
<head>
<title>Variable Re-declaration Example</title>
</head>
<body>
<script>
var age = 20;
var age; // Re-declaration without assignment
if (age > 18) {
document.write("Qualifies for driving");
}
document.write("<br>Age value: " + age);
</script>
</body>
</html>
Qualifies for driving Age value: 20
What Happens During Re-declaration
When you re-declare a variable with var, JavaScript treats it as if you just mentioned the variable name again. The declaration is ignored, and the value remains unchanged.
var name = "John";
console.log("Before re-declaration:", name);
var name; // Re-declaration
console.log("After re-declaration:", name);
var name = "Alice"; // Re-declaration with new value
console.log("After assignment:", name);
Before re-declaration: John After re-declaration: John After assignment: Alice
Behavior with let and const
Unlike var, re-declaring variables with let or const in the same scope throws an error:
let count = 5; let count = 10; // SyntaxError: Identifier 'count' has already been declared const PI = 3.14; const PI = 3.14159; // SyntaxError: Identifier 'PI' has already been declared
Key Points
-
varallows re-declaration without errors - Re-declaring with
varpreserves the original value -
letandconstprevent re-declaration in the same scope - Assignment during re-declaration updates the value
Conclusion
Re-declaring a var variable in JavaScript doesn't change its value unless you also assign a new value. Modern JavaScript prefers let and const which prevent accidental re-declarations.
Updated on: 2026-03-15T21:27:35+05:30
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