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Real programs make decisions. JavaScript's if runs a block when a condition is truthy, skips it otherwise. Chain with else if; finish with else for the fallback.

Basic shape

The first matching branch runs; the rest are skipped. Order matters — check the most specific or strictest condition FIRST.

Comparison operators

⚠️ Always use === and !==

Never use == or !=. They do silent type coercion that causes real bugs:

=== checks both type AND value. ESLint warns on == for a reason — make it a habit.

Logical operators

Note && and || return values, not booleans! "hello" || "world" is "hello", not true.

Truthy and falsy

In a boolean context (like if), JavaScript checks if a value is truthy or falsy:

Falsy (just these 7): false, 0, -0, "", null, undefined, NaN.

Truthy: everything else, including "0", "false", [], {}. Yes, even empty arrays and objects are TRUTHY.

The ternary

For short decisions:

Reads as: "score >= 60 ? then 'pass' : else 'fail'." Avoid nesting — chained ternaries get ugly fast.

Nullish coalescing — ?? vs ||

?? only treats null and undefined as missing. || treats 0 and "" as missing too. Use ?? when 0 or "" are valid values.

Switch — for many discrete cases

Don't forget break — without it, execution "falls through" to the next case. (Sometimes this is intentional, as in the example.)

Common mistakes

• Using == instead of === — type coercion bites.
• Forgetting break in switch — silent fall-through bugs.
• Treating [] and {} as falsy — they're truthy.
• if (a = b) — a single = is assignment! Always returns the assigned value.