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In JavaScript, is undefined actually undefined? - DEV Community

How to check for undefined in JavaScript

Undefined Slope - Definition, Equation, Graph, Examples

Lines on Graphs with Zero Slope and Undefined Slope

Undefined Slope, Definition, Equation & Examples - Lesson

How to check for an undefined or null variable in JavaScript? - Stack Overflow

Null and Undefined in Javascript. What's the difference between null and…, by Megan Lo, Webtips

Undefined & Zero Slope Graph, Definition & Examples - Lesson

Undefined Terms: Point, Line, and Plane (Video & Examples)

Undefined symbols and where to find them, by Alex Nadein