Var Let Const Javascript Difference
Understanding the Difference Between var, let, and const in JavaScript
Var Let Const Javascript Difference
In JavaScript, `var`, `let`, and `const` are used to declare variables but they have some key differences. `var` is function-scoped or globally-scoped and can be hoisted, which means it can be accessed before it's declared. `let` and `const` are block-scoped, meaning they are only accessible within the block they are defined in. `let` can be reassigned, while `const` variables are read-only and cannot be reassigned. It's recommended to use `const` by default and only use `let` when you need to reassign a variable.
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1 - Declaration and Scope:
`var`: Has function level scope, meaning it is function scoped and can be accessible throughout the function in which it is declared.
`let`: Has block level scope, meaning it is block scoped and can only be accessed within the block it is declared in, such as within loops or if statements.
`const`: Also has block level scope like `let`, but the value cannot be changed once it is assigned.
2) Hoisting:
`var` is hoisted to the top of its scope and can be used before it is declared, although it will return `undefined`.
`let` and `const` are hoisted as well, but they are not initialized. Trying to access them before the declaration results in a `ReferenceError`.
3) Reassignment and Immutability:
Variables declared with `var` and `let` can be reassigned with new values.
`const` variables cannot be reassigned. However, if the value is an object or array, the properties or elements of the object or array can still be mutated.
4) Temporal Dead Zone (TDZ):
`let` and `const` variables are in the TDZ from the start of their containing block until the actual declaration is encountered in the code. Accessing them in this zone will result in a `ReferenceError`.
5) Global Object Property:
Variables declared with `var` become properties of the global object (`window` in browsers).
`let` and `const` variables are not added as properties to the global object, providing better encapsulation and avoiding potential naming conflicts.
For training students, it is important to emphasize these differences as they form the foundation of understanding variable declaration in JavaScript and help in writing more predictable and maintainable code.
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