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JavaScript React Components (2018) Stateful Components and Lifecycle Methods Resetting the Stopwatch

Posted by Janaka Bandara
Janaka Bandara
1,329 Points

When you store seconds in a const, how come that value gets changed? Isn't const an immutable (for primitive) in ES2015?

In this video, when the teacher wanted to calculate the time in seconds, he declared a const second in render method. Now in previous ES2015 tutorials, it says when you declare a variable as const and if it holds a primitive data type, that variable is immutable. JS suppose to give you a warning, but in React this doesn't happens. Any idea why?

Thanks

3 Answers

Daniel Totten
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Daniel Totten
Full Stack JavaScript Techdegree Graduate 21,008 Points
Daniel Totten
.a{fill-rule:evenodd;}techdegree seal-36
Daniel Totten
Full Stack JavaScript Techdegree Graduate 21,008 Points

If I'm understanding this all correctly, I think the render() method gets called every time there is a state change. Therefore, the seconds variable is created each time the render() method is called, not updated, and therefore is still immutable. I think if we put the seconds variable in a different scope, we would have to use let instead of const.

Janaka Bandara
Janaka Bandara
1,329 Points

Got it, I missed that completely. Thanks Daniel

Kevin Nguyen
Kevin Nguyen
1,317 Points
Kevin Nguyen
Kevin Nguyen
1,317 Points

Not sure what you need, but it can be:

const obj = {}

We can't change it, but we can modify its parameters like:

// Allowed
obj.name = 'me'
// Not allow
var obj = {
  name: 'me'
}
Janaka Bandara
Janaka Bandara
1,329 Points

Hi thanks for the reply, But in the video (~1:30 mins)

const seconds = Math.floor(this.state.elapsedTime / 1000);

This is a number type . So it should not get changed right?

Nicopixel Nicopixel
PLUS
Nicopixel Nicopixel
Courses Plus Student 91 Points
Nicopixel Nicopixel
Nicopixel Nicopixel
Courses Plus Student 91 Points

The variables declared with "const" and "let" are "scopped", means that "seconds" doesn't exits outside any scope http://es6-features.org/#BlockScopedVariables