In JavaScript, there are a lot of different ways to convert value types. Some of them might be hard to read and understand.

Such as:

var b = !!foo;
var b = ~foo.indexOf(".");
var n = +foo;
var n = -(-foo);
var n = foo - 0;
var n = 1 * foo;
var s = "" + foo;
foo += ``;

Those can be replaced with the following code:

var b = Boolean(foo);
var b = foo.indexOf(".") !== -1;
var n = Number(foo);
var n = Number(foo);
var n = Number(foo);
var n = Number(foo);
var s = String(foo);
foo = String(foo);

Rule Details

This rule is aimed to flag shorter notations for the type conversion, then suggest a more self-explanatory notation.

Options

This rule has three main options and one override option to allow some coercions as required.

• "boolean" (true by default) - When this is true, this rule warns shorter type conversions for boolean type.
• "number" (true by default) - When this is true, this rule warns shorter type conversions for number type.
• "string" (true by default) - When this is true, this rule warns shorter type conversions for string type.
• "disallowTemplateShorthand" (false by default) - When this is true, this rule warns string type conversions using ${expression} form.
• "allow" (empty by default) - Each entry in this array can be one of ~, !!, +, - -, -, or * that are to be allowed.

Note that operator + in allow list would allow +foo (number coercion) as well as "" + foo (string coercion).

boolean

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

::: incorrect

/*eslint no-implicit-coercion: "error"*/

var b = !!foo;
var b = ~foo.indexOf(".");
// bitwise not is incorrect only with `indexOf`/`lastIndexOf` method calling.

:::

Examples of correct code for the default { "boolean": true } option:

::: correct

/*eslint no-implicit-coercion: "error"*/

var b = Boolean(foo);
var b = foo.indexOf(".") !== -1;

var n = ~foo; // This is a just bitwise not.

:::

number

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

::: incorrect

/*eslint no-implicit-coercion: "error"*/

var n = +foo;
var n = -(-foo);
var n = foo - 0;
var n = 1 * foo;

:::

Examples of correct code for the default { "number": true } option:

::: correct

/*eslint no-implicit-coercion: "error"*/

var n = Number(foo);
var n = parseFloat(foo);
var n = parseInt(foo, 10);

var n = foo * 1/4; // `* 1` is allowed when followed by the `/` operator

:::

string

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

::: incorrect

/*eslint no-implicit-coercion: "error"*/

var s = "" + foo;
var s = `` + foo;
foo += "";
foo += ``;

:::

Examples of correct code for the default { "string": true } option:

::: correct

/*eslint no-implicit-coercion: "error"*/

var s = String(foo);
foo = String(foo);

:::

disallowTemplateShorthand

This option is not affected by the string option.

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

::: incorrect

/*eslint no-implicit-coercion: ["error", { "disallowTemplateShorthand": true }]*/

var s = `${foo}`;

:::

Examples of correct code for the { "disallowTemplateShorthand": true } option:

::: correct

/*eslint no-implicit-coercion: ["error", { "disallowTemplateShorthand": true }]*/

var s = String(foo);

var s = `a${foo}`;

var s = `${foo}b`;

var s = `${foo}${bar}`;

var s = tag`${foo}`;

:::

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

::: correct

/*eslint no-implicit-coercion: ["error", { "disallowTemplateShorthand": false }]*/

var s = `${foo}`;

:::

allow

Using allow list, we can override and allow specific operators.

Examples of correct code for the sample { "allow": ["!!", "~"] } option:

::: correct

/*eslint no-implicit-coercion: [2, { "allow": ["!!", "~"] } ]*/

var b = !!foo;
var b = ~foo.indexOf(".");

:::

When Not To Use It

If you don't want to be notified about shorter notations for the type conversion, you can safely disable this rule.