In JavaScript, NaN is a special value of the Number type. It's used to represent any of the "not-a-number" values represented by the double-precision 64-bit format as specified by the IEEE Standard for Binary Floating-Point Arithmetic.

Because NaN is unique in JavaScript by not being equal to anything, including itself, the results of comparisons to NaN are confusing:

• NaN === NaN or NaN == NaN evaluate to false
• NaN !== NaN or NaN != NaN evaluate to true

Therefore, use Number.isNaN() or global isNaN() functions to test whether a value is NaN.

Rule Details

This rule disallows comparisons to NaN.

Examples of incorrect code for this rule:

::: incorrect

/*eslint use-isnan: "error"*/

if (foo == NaN) {
    // ...
}

if (foo != NaN) {
    // ...
}

if (foo == Number.NaN) {
    // ...
}

if (foo != Number.NaN) {
    // ...
}

:::

Examples of correct code for this rule:

::: correct

/*eslint use-isnan: "error"*/

if (isNaN(foo)) {
    // ...
}

if (!isNaN(foo)) {
    // ...
}

:::

Options

This rule has an object option, with two options:

• "enforceForSwitchCase": true (default) additionally disallows case NaN and switch(NaN) in switch statements.
• "enforceForIndexOf": true additionally disallows the use of indexOf and lastIndexOf methods with NaN. Default is false, meaning that this rule by default does not warn about indexOf(NaN) or lastIndexOf(NaN) method calls.

enforceForSwitchCase

The switch statement internally uses the === comparison to match the expression's value to a case clause. Therefore, it can never match case NaN. Also, switch(NaN) can never match a case clause.

Examples of incorrect code for this rule with "enforceForSwitchCase" option set to true (default):

::: incorrect

/*eslint use-isnan: ["error", {"enforceForSwitchCase": true}]*/

switch (foo) {
    case NaN:
        bar();
        break;
    case 1:
        baz();
        break;
    // ...
}

switch (NaN) {
    case a:
        bar();
        break;
    case b:
        baz();
        break;
    // ...
}

switch (foo) {
    case Number.NaN:
        bar();
        break;
    case 1:
        baz();
        break;
    // ...
}

switch (Number.NaN) {
    case a:
        bar();
        break;
    case b:
        baz();
        break;
    // ...
}

:::

Examples of correct code for this rule with "enforceForSwitchCase" option set to true (default):

::: correct

/*eslint use-isnan: ["error", {"enforceForSwitchCase": true}]*/

if (Number.isNaN(foo)) {
    bar();
} else {
    switch (foo) {
        case 1:
            baz();
            break;
        // ...
    }
}

if (Number.isNaN(a)) {
    bar();
} else if (Number.isNaN(b)) {
    baz();
} // ...

:::

Examples of correct code for this rule with "enforceForSwitchCase" option set to false:

::: correct

/*eslint use-isnan: ["error", {"enforceForSwitchCase": false}]*/

switch (foo) {
    case NaN:
        bar();
        break;
    case 1:
        baz();
        break;
    // ...
}

switch (NaN) {
    case a:
        bar();
        break;
    case b:
        baz();
        break;
    // ...
}

switch (foo) {
    case Number.NaN:
        bar();
        break;
    case 1:
        baz();
        break;
    // ...
}

switch (Number.NaN) {
    case a:
        bar();
        break;
    case b:
        baz();
        break;
    // ...
}

:::

enforceForIndexOf

The following methods internally use the === comparison to match the given value with an array element:

• Array.prototype.indexOf
• Array.prototype.lastIndexOf

Therefore, for any array foo, foo.indexOf(NaN) and foo.lastIndexOf(NaN) will always return -1.

Set "enforceForIndexOf" to true if you want this rule to report indexOf(NaN) and lastIndexOf(NaN) method calls.

Examples of incorrect code for this rule with "enforceForIndexOf" option set to true:

::: incorrect

/*eslint use-isnan: ["error", {"enforceForIndexOf": true}]*/

const hasNaN = myArray.indexOf(NaN) >= 0;

const firstIndex = myArray.indexOf(NaN);

const lastIndex = myArray.lastIndexOf(NaN);

const indexWithSequenceExpression = myArray.indexOf((doStuff(), NaN));

const firstIndexFromSecondElement = myArray.indexOf(NaN, 1);

const lastIndexFromSecondElement = myArray.lastIndexOf(NaN, 1);

:::

Examples of correct code for this rule with "enforceForIndexOf" option set to true:

::: correct

/*eslint use-isnan: ["error", {"enforceForIndexOf": true}]*/

function myIsNaN(val) {
    return typeof val === "number" && isNaN(val);
}

function indexOfNaN(arr) {
    for (let i = 0; i < arr.length; i++) {
        if (myIsNaN(arr[i])) {
            return i;
        }
    }
    return -1;
}

function lastIndexOfNaN(arr) {
    for (let i = arr.length - 1; i >= 0; i--) {
        if (myIsNaN(arr[i])) {
            return i;
        }
    }
    return -1;
}

const hasNaN = myArray.some(myIsNaN);

const hasNaN1 = indexOfNaN(myArray) >= 0;

const firstIndex = indexOfNaN(myArray);

const lastIndex = lastIndexOfNaN(myArray);

// ES2015
const hasNaN2 = myArray.some(Number.isNaN);

// ES2015
const firstIndex1 = myArray.findIndex(Number.isNaN);

// ES2016
const hasNaN3 = myArray.includes(NaN);

:::

Known Limitations

This option checks methods with the given names, even if the object which has the method is not an array.