Understanding Var, Let, and Const in JavaScript for Salesforce Lightning Developers

As a Salesforce Lightning Developer, mastering JavaScript is essential, especially when dealing with variable declarations. ES6 introduced two new ways to create variables: let and const. Before diving into the differences between var, let, and const, let's cover some prerequisites: variable declarations vs. initialization, scope, and hoisting. Remember, const signals that the identifier won’t be reassigned, while let indicates that the variable may be reassigned.

JavaScript Var

In JavaScript, a variable can hold a value that can vary over time. The var keyword is used to declare a variable. A variable must have a unique name, and var is function-scoped.

Example of var

var temp = 'Hello';
console.log(window.temp); 
// Output: 'Hello'

var temp = 'Hello Code';
console.log(window.temp);  
// Output: 'Hello Code'

In this example, we see that var allows us to redeclare the variable temp, which overwrites the previous value.

JavaScript Let

The let keyword is used to declare variables in JavaScript. Unlike var, which is function-scoped, let is block-scoped. This means that a variable declared with let will only be accessible within the block it is defined.

Example of let

let temp = 'Hello';
console.log(window.temp); 
// Output: 'undefined'

let temp = 'Hello Code';
console.log(window.temp);  
// Output: SyntaxError: Identifier 'temp' has already been declared.

In this case, if you try to redeclare the variable temp with let, a SyntaxError occurs because let does not allow redeclaration within the same block.

JavaScript Const

The const keyword is used to define constants that are also block-scoped. Variables defined with const behave like let variables, but the key difference is that their values cannot be changed through reassignment, nor can they be redeclared.

Example of const

const temp = 1; 
try {
    temp = 2;
} catch (err) {
    console.log(err);
}
// Output: TypeError: Assignment to constant variable.

Here, an attempt to reassign the value of temp results in a TypeError, indicating that the value of a constant cannot be changed.

Conclusion

Understanding the differences between var, let, and const is crucial for effective JavaScript programming, particularly in the context of Salesforce Lightning development. Use var when you need function-scoped variables, let for block-scoped variables that may need reassignment, and const for block-scoped constants that should not change. By mastering these variable declarations, you can write cleaner and more efficient JavaScript code in your Salesforce applications. Happy coding!