Visualizing `var`, `let`, `const`, and the new `using` declarations.
The three primary ways to declare variables, each with different scoping and mutability rules. `var` is function-scoped, while `let` and `const` are block-scoped. `const` also prevents re-assignment.
const name = "Alice"; // Fixed value let count = 0; // Can be changed var legacy = 1; // Old, but works
`const` userName: Sarah
`let` userAge: 25
`var` highscore: 100
`let` and `const` are confined to the block (`{ ... }`) they are declared in. `var` is hoisted and has function scope, leading to common bugs. `let` and `const` prevent this by being more predictable.
if (true) {
var varVariable = "accessible outside";
let letVariable = "inaccessible outside";
}
console.log(varVariable); // Works!
console.log(letVariable); // Error!
Inside a code block:
`let` status:
`var` status:
The `using` declaration provides a clean syntax for managing resources that need to be explicitly cleaned up (e.g., a file handle or a database connection). It works with objects that implement the `[Symbol.dispose]` method. `await using` is used for asynchronous resources.
// Resource class
class MyResource {
[Symbol.dispose]() {
console.log('Resource cleaned up.');
}
}
{
using myResource = new MyResource();
// ... do work with myResource
} // Resource is automatically disposed here
Resource: Not created
Lifecycle Log: