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.

This rule can be also configured to fail when function parameters are modified. Side effects on parameters can cause counter-intuitive execution flow and make errors difficult to track down.

Rule Details

This rule aims to prevent unintended behavior caused by modification or reassignment of function parameters.

Examples of incorrect code for this rule:

/*eslint no-param-reassign: "error"*/

function foo(bar) {
    bar = 13;
}

function foo(bar) {
    bar++;
}

function foo(bar) {
    for (bar in baz) {}
}

function foo(bar) {
    for (bar of baz) {}
}

Examples of correct code for this rule:

/*eslint no-param-reassign: "error"*/

function foo(bar) {
    var baz = bar;
}

Options

This rule takes one option, an object, with a boolean property "props", and arrays "ignorePropertyModificationsFor" and "ignorePropertyModificationsForRegex". "props" is false by default. If "props" is set to true, this rule warns against the modification of parameter properties unless they're included in "ignorePropertyModificationsFor" or "ignorePropertyModificationsForRegex", which is an empty array by default.

props

Examples of correct code for the default { "props": false } option:

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

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

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

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

function foo(bar) {
    for (bar.aaa in baz) {}
}

function foo(bar) {
    for (bar.aaa of baz) {}
}

Examples of incorrect code for the { "props": true } option:

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

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

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

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

function foo(bar) {
    for (bar.aaa in baz) {}
}

function foo(bar) {
    for (bar.aaa of baz) {}
}

Examples of correct code for the { "props": true } option with "ignorePropertyModificationsFor" set:

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

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

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

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

function foo(bar) {
    for (bar.aaa in baz) {}
}

function foo(bar) {
    for (bar.aaa of baz) {}
}

Examples of correct code for the { "props": true } option with "ignorePropertyModificationsForRegex" set:

/*eslint no-param-reassign: ["error", { "props": true, "ignorePropertyModificationsForRegex": ["^bar"] }]*/

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

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

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

function foo(barBaz) {
    for (barBaz.aaa in baz) {}
}

function foo(barBaz) {
    for (barBaz.aaa of baz) {}
}

When Not To Use It

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

Further Reading

• JavaScript: Don’t Reassign Your Function Arguments

Version

This rule was introduced in ESLint 0.18.0.

Resources

• Rule source
• Documentation source