Conditionals and Control Flow

Control flow is how programs make decisions. Bad branching — pyramid else chains, magic booleans, nested ternaries — is one of the fastest ways to create unmaintainable modules. This lesson covers the syntax (if, switch, ternary) and the style senior teams expect: guard clauses, explicit comparisons, and choosing the right construct for readability.

if / else — the decision tree

switch compares one expression against many case labels using strict equality (===). When a case matches, execution falls through to the next case unless you break or return. Fall-through is powerful for grouping (401 and 403 share handling) but dangerous when accidental — always comment intentional fall-through. default catches no match; omitting it means silent no-ops when a new enum value appears.

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switch — multi-way branching

switch compares one value against many case labels using strict equality (===). Every case needs break unless fall-through is intentional.

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Ternary operator — expression, not statement

The example below demonstrates "Ternary operator — expression, not statement" in runnable form. Read the comment lines first — they map each step to the mental model from earlier sections, then trace execution top to bottom as the engine would.

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Truthy checks vs explicit comparisons

The ternary operator (condition ? a : b) is an expression — it produces a value you can assign or return. That makes it ideal for simple UI labels and config picks. The moment you nest ternaries three levels deep, switch to if/else or a lookup table. Expressions should fit on one screen line in code review.

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Optional chaining in conditions

Truthy checks (if (value)) are concise but imprecise. Empty arrays, empty objects, and the string "0" are all truthy. APIs should validate shape explicitly: Array.isArray, typeof x === "string", Number.isFinite. Optional chaining (?.) short-circuits property access when intermediate values are null or undefined — it prevents the classic "Cannot read property of undefined" without six nested ifs.

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When branches grow beyond three levels, extract named functions or lookup tables instead of nesting. Switch works well for discrete enums; if/else chains suit ranged conditions like age tiers. The goal is a control flow readers can scan in one pass during incident response.

1. 1Prefer guard clauses over deep else nesting
2. 2switch uses ===; always break or comment fall-through
3. 3Ternary for simple value picks; if/else for complex branches
4. 4Check arrays with .length, not bare truthiness
5. 5Next lesson: for, while, and iteration patterns