Booleans & Comparisons
Booleans have only two possible values: true and false. They are essential for making decisions in your code.
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📋 Copy JavaScript Booleans & Coalescing Cheat Sheet
// Truthiness & Falsy Values
// Falsy: false, 0, -0, 0n, "", null, undefined, NaN
// Nullish Coalescing Operator (??) vs Logical OR (||)
const count = 0;
const safeCount = count ?? 10; // 0 (preserves 0 and "")
const wrongCount = count || 10; // 10 (treats 0 as falsy!)
// Explicit Boolean Conversion
const isReady = Boolean(userToken);
const hasItems = !!items.length;⚠️ 5 Fatal Traps & JavaScript Pitfalls
Trap #1: Logical OR (||) Overwriting Legitimate Falsy Values
Using
val || fallback treats 0, false, and "" as missing values and replaces them with the fallback. Use the nullish coalescing operator val ?? fallback instead.Trap #2: Empty Arrays and Objects Evaluating to Truthy
In JavaScript,
Boolean([]) and Boolean({}) are both true! Checking if (items) does not test whether an array has elements; check items.length > 0.Trap #3: Loose Equality (==) Type Coercion Hazards
The loose equality operator performs bizarre type coercions:
"" == 0 is true, 0 == "0" is true, but "" == "0" is false! Always enforce strict equality ===.Trap #4: new Boolean(false) Object Wrapper Trap
Invoking
new Boolean(false) creates an object wrapper. Because all objects are truthy, if (new Boolean(false)) executes the branch! Use Boolean(val) without new.Trap #5: Short-Circuiting in React/JSX Rendering 0
Writing
count && <Badge /> in JSX renders 0 directly onto the webpage when count is 0. Use count > 0 && <Badge /> or a ternary.💬 Frequently Asked Questions
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