Disallow Reassignment of Function Parameters (no-param-reassign)

Assignment to variables declared as function parameters can be misleading and lead to confusing behavior, as modifying function parameters will also mutate the arguments object. Often, assignment to function parameters is unintended and indicative of a mistake or programmer error.

Rule Details

This rule aims to prevent unintended behavior caused by overwriting function parameters.

Options

This rule takes one option, an object, with a property "props".

{
    "no-param-reassign": [2, {"props": false}]
}

props (false by default) - If true is set, this rule warns modifying of properties of parameters.

The following patterns are considered problems:

/*eslint no-param-reassign: 2*/

function foo(bar) {
    bar = 13;       /*error Assignment to function parameter 'bar'.*/
}

function foo(bar) {
    bar++;          /*error Assignment to function parameter 'bar'.*/
}

When {"props": true}:

/*eslint no-param-reassign: [2, { "props": true }]*/

function foo(bar) {
    bar.prop = "value"; /*error Assignment to function parameter 'bar'.*/
}

function foo(bar) {
    delete bar.aaa;     /*error Assignment to function parameter 'bar'.*/
}

function foo(bar) {
    bar.aaa++;          /*error Assignment to function parameter 'bar'.*/
}

The following patterns are not considered problems:

/*eslint no-param-reassign: 2*/

function foo(a) {
    var b = a;
}

When {"props": false}:

/*eslint no-param-reassign: [2, { "props": false }]*/

function foo(bar) {
    bar.prop = "value";
}

function foo(bar) {
    delete bar.aaa;
}

function foo(bar) {
    bar.aaa++;
}

When Not To Use It

If you want to allow assignment to function parameters, then you can safely disable this rule.

Version

This rule was introduced in ESLint 0.18.0.