Let vs. Const in JavaScript: When to Use What

Doubt:

Why bother with const if I can reassign with let?

Insight: When to Use const and let in JavaScript for More Reliable Code

In JavaScript, let and const are both block-scoped, modern variable declarations introduced to replace the more error-prone var. They serve distinct roles:

• const: This keyword creates a constant reference, so you cannot reassign the variable to a different value after its initial assignment. However, for objects and arrays, you can still modify the contents they reference because const only prevents reassignment of the reference, not changes to the object or array’s contents.
• let: This keyword allows reassignment, making it useful for variables whose values will change over time (e.g., counters, accumulators, or configurations).

Example Code

Imagine Tony Stark is tracking some suit systems. Certain settings, like the suit model, should remain constant, while others, like suit power level, will need updates.

const suitModel = "Mark XLVII"; // This suit model cannot be reassigned
let powerLevel = 100; // Power level will decrease over time

// Attempting reassignment
suitModel = "Mark L"; // Error: Assignment to constant variable
powerLevel = 85; // Allowed - powerLevel can be reassigned

console.log(suitModel); // Outputs: "Mark XLVII"
console.log(powerLevel); // Outputs: 85

Working with const and Objects or Arrays

It’s important to note that const prevents reassignment of the variable, not modification of the object or array it references. Here’s an example where Tony is tracking his team members in the Avengers.

const avengers = ["Iron Man", "Thor", "Hulk"];<br>avengers.push("Black Widow"); // Allowed - modifying the array contents<br>console.log(avengers); // Outputs: ["Iron Man", "Thor", "Hulk", "Black Widow"]<br><br>// Reassigning the array itself will throw an error<br>avengers = ["Spider-Man", "Doctor Strange"]; // Error: Assignment to constant variable

In this case, const prevents reassigning avengers to a new array but allows operations that modify the original array’s contents, such as adding or removing items.

Why This Matters

• Predictability: Using const whenever possible creates more predictable code. You know that a const variable won’t change, which reduces cognitive load and helps avoid accidental bugs.
• Intentional Reassignment: When a variable is declared with let, it signals that reassignment is intentional and likely necessary within that context, making the code more readable and intention-driven.

Best Practice: Optimal Usage Tips for let and const

• Default to const for variables that shouldn’t be reassigned, and use let only for variables that require reassignment, such as counters or values in loops.
• Switch to let only when reassignment is required: For example, in loops, counters, or variables that must be updated as the code progresses.

Final Takeaway: const for Stability, let for Flexibility

Think of const as a safeguard that prevents unintended changes. In Tony’s world, this might be like locking down certain suit features once they’re set. By using const as the default, you ensure that certain values remain constant, leading to more stable and predictable code. Reach for let only when you’re working with a variable that needs flexibility.

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Curious about JavaScript’s other quirks? Head over to our article on == vs. === to learn the key differences.