Function context scope doubt

Hi everybody,
Hope you all are doing great.
I have a question regarding how object's context works on functions at Node.JS.
For example:
- function nestedFunctions() {
- var a = "I am a variable"
- console.log("Initial context:")
- console.log(this)
- function one() {
- console.log("First nested function, this is my this context:")
- console.log(this)
- console.log("First nested function, this is my a value:")
- console.log(a)
- function two() {
- console.log("Second nested function, this is my this context:")
- console.log(this)
- console.log("Second nested function, this is my a value:")
- console.log(a)
- }
- two()
- }
- one()
- }
- nestedFunctions()
The output of that code is:
- Initial context:
- Object Global
- First nested function, this is my this context:
- Object Global
- First nested function, this is my a value:
- I am a variable
- Second nested function, this is my this context:
- Object Global
- Second nested function, this is my a value:
- I am a variable
Everything OK so far, because the function context (this) is created on global scope, and so the nested functions. But my doubt is when running the following code:
- class Dog {
- constructor(name) {
- this.name = name
- }
- bark() {
- console.log(`Woof, my name is ${this.name}`);
- }
- nestedFunctions() {
- var a = "I am a variable"
- console.log("Initial context:")
- console.log(this)
- function one() {
- console.log("First nested function, this is my this context:")
- console.log(this)
- console.log("First nested function, this is my a value:")
- console.log(a)
- function two() {
- console.log("Second nested function, this is my this context:")
- console.log(this)
- console.log("Second nested function, this is my a value:")
- console.log(a)
- }
- two()
- }
- one()
- }
- }
- let rex = new Dog('Rex')
- //rex.sitAndBark()
- rex.nestedFunctions()
Output:
- Initial context:
- Dog { name: 'Rex' }
- First nested function, this is my this context:
- undefined
- First nested function, this is my a value:
- I am a variable
- Second nested function, this is my this context:
- undefined
- Second nested function, this is my a value:
- I am a variable
I am having troubles understanding that last one, and why the this of the nested functions is undefined. Why isn't the global context instead of undefined (like when declared outside a class, i.e, the first example)?
Best regards to all
Comments
-
Another example with the function defined on an object:
- var rex = {
- nestedFunctions() {
- var a = "I am a variable"
- console.log("Initial context:")
- console.log(this)
- function one() {
- console.log("First nested function, this is my this context:")
- console.log(this)
- console.log("First nested function, this is my a value:")
- console.log(a)
- function two() {
- console.log("Second nested function, this is my this context:")
- console.log(this)
- console.log("Second nested function, this is my a value:")
- console.log(a)
- }
- two()
- }
- one()
- }
- }
- rex.nestedFunctions()
The output in that case is:
- Initial context:
- { nestedFunctions: [Function: nestedFunctions] }
- First nested function, this is my this context:
- Object Global
- First nested function, this is my a value:
- I am a variable
- Second nested function, this is my this context:
- Object Global
- Second nested function, this is my a value:
- I am a variable
Which is the same behaviour of the first example. I don't get why in the second case, the one on a class definition, the result of the nested "this" is undefined instead of the global context.
0 -
Got the answer:
That's because the body of a JavaScript class executes in strict mode. This mode, as its name suggests, is a bit more restrictive. Among the features of this execution mode, this doesn't refer to window inside a function that is being called as a bare function, it returns undefined instead.Object's context ('this') between nested functions at a class
0 -
@IgnacioAcuna Exactly correct
0
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