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31The Seven Primitive Types
32undefined: The Default Value
33null: Intentional Absence
34Boolean: true and false
35Number: Floating-Point Only
36Special Number Values: NaN, Infinity
37String: Sequences of Characters
38Symbol: Unique Identifiers
39BigInt: Arbitrary-Precision Integers
40The typeof Operator
41Primitives Are Immutable
42Primitives Are Copied by Value
43Primitive Wrapper Objects
44When Not to Use new String(), new Number()
45Objects Store References, Not Values
46Reference Equality vs Value Equality
47Assignment Copies References
48Mutating Objects Through Multiple References
49Functions Receive References to Objects
50Arrays Are Objects Too
51Const Prevents Reassignment, Not Mutation
52Pitfall: Unintended Shared State
53When Primitives Behave Differently
54Equality vs Identity: == and ===
55How === Works: Type and Value
56When == Performs Type Coercion
57Comparing Primitives
58Special Cases: NaN, -0, and +0
59Object.is for Same-Value Equality
60Comparing Objects by Reference
61Why Two Empty Objects Are Not Equal
62Inequality Operators: != and !==
63Relational Operators: <, >, <=, >=
64String and Number Comparisons
65Common Equality Pitfalls

31The Seven Primitive Types
32undefined: The Default Value
33null: Intentional Absence
34Boolean: true and false
35Number: Floating-Point Only
36Special Number Values: NaN, Infinity
37String: Sequences of Characters
38Symbol: Unique Identifiers
39BigInt: Arbitrary-Precision Integers
40The typeof Operator
41Primitives Are Immutable
42Primitives Are Copied by Value
43Primitive Wrapper Objects
44When Not to Use new String(), new Number()
45Objects Store References, Not Values
46Reference Equality vs Value Equality
47Assignment Copies References
48Mutating Objects Through Multiple References
49Functions Receive References to Objects
50Arrays Are Objects Too
51Const Prevents Reassignment, Not Mutation
52Pitfall: Unintended Shared State
53When Primitives Behave Differently
54Equality vs Identity: == and ===
55How === Works: Type and Value
56When == Performs Type Coercion
57Comparing Primitives
58Special Cases: NaN, -0, and +0
59Object.is for Same-Value Equality
60Comparing Objects by Reference
61Why Two Empty Objects Are Not Equal
62Inequality Operators: != and !==
63Relational Operators: <, >, <=, >=
64String and Number Comparisons
65Common Equality Pitfalls

Lesson 32 of 2,723·2. Values and TypesPro lesson

undefined: The Default Value

Understand when and why variables are undefined, and how undefined differs from undeclared.

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