The answer
By how the function was called. A plain call gives this as undefined (strict mode) or the global object. A method call gives the object. call, apply, and bind set it explicitly. Arrow functions inherit this from their lexical scope.
How JavaScript Creates Execution Context for Functions
How is this value set in a function execution context?
More FAQs in How JavaScript Creates Execution Context for Functions
A new function execution context with a variable environment, an outer environment reference (for the scope chain), and a value for this. It is pushed onto the call stack.
A pointer to the parent function's environment. The engine follows these references up the chain to resolve variables not found locally. This is the scope chain.
When the function returns. Its call stack frame is popped and local variables are gone, unless a closure keeps a reference to the variable environment.
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