For Loops
Loops allow you to run the same block of code multiple times with varying values.
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📋 Copy JavaScript For-Loops & Iteration Cheat Sheet
const items = ["alpha", "beta", "gamma"];
// for...of (Values in Iterables: Arrays, Sets, Maps)
for (const [index, val] of items.entries()) {
console.log(`${index}: ${val}`);
}
// for...in (Keys in Objects ONLY)
const config = { host: "localhost", port: 8080 };
for (const key in config) {
if (Object.hasOwn(config, key)) {
console.log(`${key} = ${config[key]}`);
}
}⚠️ 5 Fatal Traps & JavaScript Pitfalls
Trap #1: Using for...in on Arrays
for...in iterates over all enumerable property keys, including prototype methods and string indices in arbitrary order. Always use for...of or array methods for arrays.Trap #2: Asynchronous Code Inside Array.prototype.forEach
forEach does NOT await promises! Passing an async callback runs promises concurrently and exits immediately before completion. Use for (const x of items) await ....Trap #3: Closure Variable Leaks with var in Loops
Declaring
for (var i = 0; ...) shares a single variable across all loop iterations, so asynchronous callbacks (like setTimeout) all log the final value. Use let to create per-iteration bindings.Trap #4: Modifying Array Length During Iteration
Calling
splice() or push() inside an index-based for loop alters the array length and shifts elements, causing items to be skipped.Trap #5: Infinite Loop from Stagnant Counter Increment
Typing
i-- instead of i++ locks the main browser thread in an infinite loop, crashing the tab.💬 Frequently Asked Questions
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