Core JavaScript / Execution Context
Execution Context
The environment a piece of code runs in — its variables, its scope, and what this means there.
Every time JavaScript runs code, it does so inside an execution context: a container holding that code's variables, its lexical environment (which scope chain it can see), and what `this` refers to there. The Global Execution Context is created once, when the program starts. A new Function Execution Context is created every single time a function is called — even calling the same function twice creates two separate contexts, each with its own variables.
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State
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Why?
Each call to add() builds a brand-new execution context with its own a, b, and sum — the two calls never see or interfere with each other's variables.