What is an Execution Context?

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Devs Hive · Accepted Answer

This question comes up in almost every Frontend or Backend interview at the junior/middle level.

In short, an Execution Context is the environment in which our code is currently running; put simply, it is the value of this at the moment of execution. There are 3 types of execution context:

1. Global Execution Context

This context is created once, when the file is loaded, and it holds all the code that is not inside functions: global variables, global functions, and this at the top level.

In the browser, the global object is usually window.

var x = 10;

function sayHi() {

console.log("hi");

}

console.log(x); // 10

console.log(this === window); // true (але залежить від режиму)

2. Function Execution Context

The key thing to remember here: every function call creates a new context. Even if it is the same function, the context will be different each time. In it, JS stores the function parameters, local variables (ті, що всередині {}), and this for that specific call.

function sum(a, b) {

const result = a + b;

return result;

}

sum(1, 2); // один контекст

sum(5, 10); // інший контекст

3. Module Execution Context

If the code runs as an ES module, it is executed in a module context. It is similar to the global one, but there are 2 important differences:

Variables do not automatically become global
this at the top level will be undefined

const x = 123;

console.log(this); // undefined

// x не з'явиться як window.x

</script>

What is an Execution Context?

1. Global Execution Context

2. Function Execution Context

3. Module Execution Context

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