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Values, Types, And Coercion

This file covers the JavaScript type system, coercion rules, and equality behavior. These topics show up constantly in interviews because they expose whether someone understands how the language actually behaves.

TL;DR

Values have types, variables do not. Coercion is rule-based, not random. Equality rules matter because JavaScript performs implicit conversions in specific cases.

JavaScript Types

Primitive Types

  • string
  • number
  • boolean
  • null
  • undefined
  • symbol
  • bigint

Non-Primitive Type

  • object

Important Clarifications

  • Arrays, functions, and dates are all objects.
  • typeof null is "object", which is a long-standing language bug.
  • Variables can hold values of different types over time.

Truthy And Falsy Values

Falsy Values

  • false
  • 0
  • -0
  • 0n
  • ""
  • null
  • undefined
  • NaN

Everything else is truthy.

Common Trap

An empty array [] and empty object {} are both truthy.

Coercion

TL;DR

Coercion is automatic type conversion. It can be explicit or implicit.

Explicit Coercion

String(42); // "42"
Number("42"); // 42
Boolean("hello"); // true

Implicit Coercion

"5" + 1; // "51"
"5" - 1; // 4
if ("hello") {
	// truthy
}

Common Traps

  • Assuming + always means numeric addition.
  • Forgetting that -, *, and / coerce toward numbers.
  • Treating coercion as unpredictable instead of rule-based.

Equality

== Vs ===

  • === checks value and type without coercion.
  • == allows coercion before comparison.

Example

42 === "42"; // false
42 == "42"; // true
null == undefined; // true
null === undefined; // false

Interview Answer

I prefer === by default because it avoids implicit conversion surprises. I still understand == because it follows defined coercion rules and appears in real code.

Common Interview Gotchas

Why is [] == false true?

Because the array is coerced to an empty string, then to 0, and false also becomes 0.

Why is NaN !== NaN?

Because NaN is defined as a value that is not equal to anything, including itself.

How do you check for NaN correctly?

Use Number.isNaN(value).

Objects And References

TL;DR

Objects are compared by reference, not by structural equality.

Example

{} === {}; // false
const a = { x: 1 };
const b = a;
a === b; // true

Common Trap

Thinking two objects with the same shape and values are equal with ===.

Rapid-Fire Questions

How many types are in JavaScript?

Seven primitive types and the object type.

What are falsy values?

The small set of values that coerce to false in boolean contexts.

Why prefer ===?

Because it avoids implicit coercion and is easier to reason about.

Why is typeof null weird?

Because it returns "object" due to a long-standing bug in the language.