Proposal: add 'allowOnTop' option to no-param-reassign rule

When allowOnTop is true, reassign is allowed at the beginning of function scope (same concept like var statement in vars-on-top rule).

The reason - I don’t want to reassign params except intended setting default like:

function (data) {  
   data = data || 0;
   ...
}

This can be avoided in ES6 with default arguments, but they can’t used in all cases, for instance making a defensive copy:

function (data) {  
   data = _.clone(data);
   ...
}

You can use _ prefix or new variable dataCopy but i don’t like it much. In fact I want to hide original variable to prevent unwanted access to unsafe origin.

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[ljharb]

For future explorers: v8 severely deoptimizes a function in sloppy mode when you assign arguments, even on top, so it’s potentially a big performance hazard - not “just” style.

[ljharb]

@cipri-tom reassignment doesn’t have side effects; primitives are immutable (so “changing them” doesn’t apply). Reassignment isn’t bad because it has side effects; it’s bad because you should just make a new variable, for clarity.