JavaScript Interview Question – Difference between VAR, LET and CONST

One of the most common interview questions for JavaScript developers is the difference between var, let and const. These three keywords are used for declaring variables in JavaScript, but they have some key differences that every developer should be aware of.

1. var

The var keyword was the only way to declare variables in JavaScript before ES6. It has function-level scope, which means that variables declared with var are function-scoped, rather than block-scoped. This can lead to unexpected behavior and bugs, especially in larger codebases.

2. let

The let keyword was introduced in ES6 and is now the preferred way to declare variables in JavaScript. It has block-level scope, meaning that variables declared with let are only accessible within the block in which they are defined. This makes code more predictable and easier to reason about.

3. const

The const keyword is also introduced in ES6 and is used for variables that are meant to be constant, or not reassignable. It has block-level scope, like let, and must be initialized at the time of declaration. While the value of a const variable cannot be changed, it does not make the value immutable. For example, if a const variable holds an object, the object’s properties can still be modified.

Conclusion

In summary, var, let and const are all used for declaring variables in JavaScript, but they have different scoping rules and use cases. var has function-level scope, while let and const have block-level scope. let is preferred for variable declarations in modern JavaScript, while const is used for constants that should not be reassignable.