Question 1

Why does 0.1 + 0.2 not equal 0.3?

Accepted Answer

Because computers store most decimals in **binary floating point**, and 0.1 has no exact binary representation — exactly as 1/3 has no exact decimal representation.

**The mechanism.** Binary fractions are sums of 1/2, 1/4, 1/8, 1/16 and so on. Some decimals map cleanly: 0.5 is 1/2, 0.75 is 1/2 + 1/4. But 0.1 requires an infinitely repeating binary expansion, and the standard format (IEEE 754 double precision) has only 53 bits of significand. The value is therefore **rounded** to the nearest representable number.

So 0.1 is not really 0.1, and neither is 0.2. Adding two slightly-wrong numbers produces a slightly-wrong sum, and in this case it lands on a value that displays as `0.30000000000000004`.

**This is not a bug**, and it is not specific to JavaScript. It affects every language using IEEE 754 — Python, Java, C, Go, Rust, Excel. Languages differ only in how aggressively they round for display, which is why some appear to get it right.

**What to do about it:**

**Never compare floats with `==`.** Compare against a tolerance — check whether the absolute difference is below a small epsilon appropriate to your magnitudes.

**Never use floats for money.** This is the most consequential rule. Use integer **minor units** — store pence or cents as whole numbers — or a **decimal** type designed for exact base-10 arithmetic (`BigDecimal` in Java, `decimal` in Python and C#, dedicated libraries in JavaScript).

**Beware accumulation.** Summing many floats compounds error; summing in sorted order or using compensated summation reduces it.

**Format at the boundary**, rounding only when displaying, not during calculation.

**Where floats are appropriate:** measurements, graphics, scientific computation — anywhere the inputs are approximate anyway and the range matters more than exactness.

Question 2

Why does JavaScript sort not order numbers correctly?

Accepted Answer

The default sort converts every element to a string and compares those strings. So ten sorts before nine, because the character one comes before nine. The array is correctly sorted, just

Question 3

What is the difference between null and undefined in JavaScript?

Accepted Answer

Undefined means a value was never assigned. Null means a value was deliberately set to nothing. The language draws that distinction, and whether your code should is a style decision worth making

Question 4

Why are null references called a billion-dollar mistake?

Accepted Answer

Because Tony Hoare, who introduced the null reference into ALGOL W in 1965, later described it exactly that way — apologising publicly for a design decision that has produced an enormous quantity of

Question 5

What causes Cannot read properties of undefined in JavaScript?

Accepted Answer

The error means you accessed a property on a value that was undefined or null. Modern engines name the property in the message, and that name is the most useful clue available, because it tells you

Question 6

What is Unicode and why does text still break?

Accepted Answer

Unicode is a standard assigning a unique number — a code point — to every character across every writing system. It exists because the earlier situation was chaos: dozens of incompatible encodings,

Question 7

What do I need to know about TypeScript types as a beginner?

Accepted Answer

Understanding TypeScript types is crucial for beginners to write error-free code. Key concepts include basic types, arrays, enums, interfaces, and function types.

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