this.state.something发生变化并引起了问题

Question

For some reason I can't do this

this.state.something.map(obj => obj.id)

because it caused extra rerendering. How to get something from state, without assign it to a new variable?

I used to do this ugly hack

something_state_holder = this.state.something
something_state_holder.map(obj => obj.id)

but is there any better / more elegant way?

Answer 1

As @mayank-shukla said, what you are doing here is perfectly valid. The map() method iterates through an array and returns and array, but without mutating the iterated one. Here's what the MDN documentation says:

The map() method creates a new array with the results of calling a provided function on every element in this array.

map does not mutate the array on which it is called (although callback, if invoked, may do so).

However, should you want to make a shallow copy on an array, you could use slice:

something_state_holder = this.state.something.slice();
var resulting_array = something_state_holder.map(obj => obj.id);

This iterates through the copy, and not the state. What you did is not creating an array, you are simply assigning the reference to the same array to a new variable. So mutating that will mutate the state.