CODEWITHSUNDEEP
javascriptscopev8
tychomag149
tychomag149

Reputation: 21

Is the global execution context the same as the global object in JavaScript?

As I understand it, every time a JavaScript program begins running, the engine first creates an execution context, pushes this execution context into the call stack/execution stack, and then it creates a global object (window in the browser and global in Node) as well.

To create the execution context, the engine first goes through the creation phase, where it allocates space in memory for entire function definitions and variable declarations (hoisting). It maintains a reference to the outer scope (this creates the scope chain, but in the global execution context there isn't anything above it), and it also creates the this property within the execution context and sets it to the window object in the browser and module.exports in Node. Lastly, the engine then goes through the execution phase, where it executes the code line by line and assigns a value to each variable.

Am I right in differentiating the global execution context creation from the creation of the global object itself? I view both of them as operations that happen side by side but are not the exact same thing.

Upvotes: 2

Views: 852

Answers (1)

jmrk
jmrk

Reputation: 40501

Yes, it's fair to say that the global context and the global object are separate concepts. One illustrating distinction is the this binding: a context defines what this refers to (in case of the global context: to the global object); whereas the global object has no property named "this".

At the same time, global context and global object are somewhat coupled insofar as local variables in the former are properties on the latter.

Note that "execution context" is mostly an abstract concept, that means an engine only has to behave "as if" it did what the spec describes. Chances are that high-performance engines will take certain shortcuts (e.g., optimized code might keep some local variables in registers or on the machine stack, never putting them into any context at all).

Upvotes: 1

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