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
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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
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@IgnacioAcuna Exactly correct
0
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