What is the difference between let, const, and var?

const declares a constant binding, let a block-scoped variable, and var a function-scoped variable. Prefer const, then let; avoid var in modern code.

What is the difference between == and ===?

== compares with type coercion; === compares value and type without coercion. Prefer === to avoid surprising conversions.

How do map, filter, and reduce differ?

map transforms each item, filter keeps items that pass a test, and reduce combines items into a single value.

Syntax for working with promises: async functions return a promise, and await pauses until a promise resolves, making asynchronous code read like synchronous code.

What is the difference between null and undefined?

undefined means a variable has not been assigned; null is an intentional empty value. typeof null is 'object', a historical quirk.

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Reference

JavaScript Cheat Sheet

Modern JavaScript essentials — ES6+, DOM, async, and more. Click any card to copy.