no-param-reassign

Disallow reassigning function parameters

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 when not in strict mode (see When Not To Use It below). 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:

Open in Playground

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

var foo = function(bar) {
    bar = 13;
}

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

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

var foo = function(bar) {
    for (bar of baz) {}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

Examples of correct code for this rule:

Open in Playground

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

var foo = function(bar) {
    var baz = bar;
}
1
2
3
4
5

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:

Open in Playground

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

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

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

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

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

var foo = function(bar) {
    for (bar.aaa of baz) {}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21

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

Open in Playground

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

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

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

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

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

var foo = function(bar) {
    for (bar.aaa of baz) {}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21

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

Open in Playground

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

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

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

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

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

var foo = function(bar) {
    for (bar.aaa of baz) {}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21

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

Open in Playground

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

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

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

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

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

var foo = function(barBaz) {
    for (barBaz.aaa of baz) {}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21

When Not To Use It

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

Strict mode code doesn’t sync indices of the arguments object with each parameter binding. Therefore, this rule is not necessary to protect against arguments object mutation in ESM modules or other strict mode functions.

Version

This rule was introduced in ESLint v0.18.0.

Resources

Edit this page

no-param-reassign

Rule Details

Options

props

When Not To Use It

Version

Further Reading

Resources