JavaScript Best Practices

Loose equality performs type coercion before comparing, producing surprising results (0 == '' is true, null == undefined is true) that rarely match actual intent. Strict equality compares both value and type, avoiding an entire category of subtle bugs.

var has function-scoping and hoisting behavior that causes real confusion, especially inside loops and closures. const and let are block-scoped and behave predictably — default to const, and only use let when a variable genuinely needs to be reassigned.

An unhandled Promise rejection can fail silently in some environments or crash a Node process in others, and the error's actual cause is often lost by the time anyone notices. Every async operation should have explicit error handling — a .catch(), or a try/catch around an await.

Modifying an object or array passed into a function changes it for whatever code holds another reference to it, producing bugs that are hard to trace because the mutation happens far from where the unexpected behavior shows up. Return a new value instead of mutating in place where practical.

Deeply nested property access on data that might be partially missing (a common shape for API responses) is both safer and more readable with ?. and ?? than a chain of manual && checks.

Callback-based asynchronous code nested several levels deep ('callback hell') is hard to read and even harder to add proper error handling to correctly at every level. async/await, built on Promises, expresses the same logic in a flat, sequential, far more readable form.

Two objects or arrays with identical contents are not === equal to each other in JavaScript — equality compares references, not structure. Comparing them with === will almost always be false even when they 'look the same', which surprises people coming from languages with value-based equality.