no-implicit-coercion

Disallow shorthand type conversions

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 += ``;
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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);
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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:

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

var b = !!foo;
var b = ~foo.indexOf(".");
// bitwise not is incorrect only with `indexOf`/`lastIndexOf` method calling.
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Examples of correct code for the default { "boolean": true } option:

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

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

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

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

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

var n = +foo;
var n = -(-foo);
var n = foo - 0;
var n = 1 * foo;
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Examples of correct code for the default { "number": true } option:

/*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
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string

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

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

var s = "" + foo;
var s = `` + foo;
foo += "";
foo += ``;
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Examples of correct code for the default { "string": true } option:

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

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

This option is not affected by the string option.

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

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

var s = `${foo}`;
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Examples of correct code for the { "disallowTemplateShorthand": true } option:

/*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}`;
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Examples of correct code for the default { "disallowTemplateShorthand": false } option:

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

var s = `${foo}`;
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allow

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

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

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

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

Version

This rule was introduced in ESLint v1.0.0-rc-2.

Resources

• Rule source
• Tests source