Disallow assigning to imported bindings (no-import-assign)

[feross]

https://eslint.org/docs/rules/no-import-assign

The updates of imported bindings by ES Modules cause runtime errors.

Rule Details

This rule warns the assignments, increments, and decrements of imported bindings.

Examples of incorrect code for this rule:

/*eslint no-import-assign: "error"*/

import mod, { named } from "./mod.mjs"
import * as mod_ns from "./mod.mjs"

mod = 1          // ERROR: 'mod' is readonly.
named = 2        // ERROR: 'named' is readonly.
mod_ns.named = 3 // ERROR: the members of 'mod_ns' is readonly.
mod_ns = {}      // ERROR: 'mod_ns' is readonly.

Examples of correct code for this rule:

/*eslint no-import-assign: "error"*/

import mod, { named } from "./mod.mjs"
import * as mod_ns from "./mod.mjs"

mod.prop = 1
named.prop = 2
mod_ns.named.prop = 3

// Known Limitation
function test(obj) {
    obj.named = 4 // Not errored because 'obj' is not namespace objects.
}
test(mod_ns) // Not errored because it doesn't know that 'test' updates the member of the argument.

[mightyiam]

I'm pro this rule. Disallows code that should probably never be written.

Regarding the limitation: it seems like with TypeScript that would be possible to prevent as well.

[feross]

Only 1 failing ecosystem repo. This is the code:

import '../../../polyfill'

import fetch, * as ponyfill from '../../..'
import addModuleSuite from '../../module.spec'

ponyfill.defaultExport = fetch

addModuleSuite('Browser: import on Webpack bundle', ponyfill)

Seems easy enough for them to add an // eslint-ignore comment when doing something hacky like this. Otherwise, this is a good rule.

This will ship in standard 16!