Question 1

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 explicitly rather than by accident.

Undefined appears on its own in several situations: a declared variable before assignment, a missing function parameter, a property that does not exist on an object, an array index out of range, and the return value of a function with no return statement. You rarely write it yourself.

Null only appears when something puts it there. Your own code, or an API returning it to represent an intentionally empty field, most commonly a database column that is genuinely empty rather than absent.

The practical consequences show up in comparisons. Loose equality treats them as equal to each other but not to anything else, which is one of the few defensible uses of loose equality. Strict equality distinguishes them. A quirk worth knowing is that the type check on null reports object, a long-standing bug in the language kept for compatibility.

Default parameter values trigger for undefined but not for null, so passing null explicitly overrides a default while omitting the argument does not. This surprises people regularly.

The nullish coalescing operator exists precisely for this area. Unlike the logical or operator it only falls back for null and undefined, not for empty strings, zero or false. That difference matters whenever zero or an empty string is a legitimate value, which is a common source of subtle bugs when the older operator is used for defaults.

Many teams simply standardise on one for absence, usually undefined internally and null at API boundaries.

Question 2

Why does TypeScript say Object is possibly undefined?

Accepted Answer

The compiler is telling you that a value type includes undefined and you are using it as though it does not. With strict null checking enabled, undefined and null are not silently assignable to other

Question 3

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 4

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 5

Why does this become undefined inside a callback in JavaScript?

Accepted Answer

In JavaScript the value of this is determined by how a function is called, not by where it was written. When you pass a method somewhere as a callback, you pass the function itself and leave the

Question 6

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.

Question 7

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

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