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One Reply to "Immer and Immutable.js: How do they compare?"
It is not correct to say that assignment does not imply mutation. Mutation simply means changing something. Assignment involves a mutation of the environment in which code executes; either a new name is installed which maps to the value corresponding to the right hand side of the assignment expression, or else the value installed under that name is updated to the new value.
Before you say this is pedantic and unimportant, consider that the example given for assignment used the var keyword, which can easily result in updates to the global environment. The effect could be that a function in some other module which could have previously been idempotent in effect loses this property, ie. that now running it before the assignment occurs has a different effect than running it after.
This is analogous to modifying prototypes owned by other modules, such as that of Array or Object; as it may cause undesirable behaviour it should not be done.
The scope of a mutation’s effects may be limited sufficiently by use of strict mode, and for example, the use of const and let instead of var.