Require or disallow the Unicode Byte Order Mark (BOM) (unicode-bom)

The --fix option on the command line can automatically fix some of the problems reported by this rule.

The Unicode Byte Order Mark (BOM) is used to specify whether code units are big endian or little endian. That is, whether the most significant or least significant bytes come first. UTF-8 does not require a BOM because byte ordering does not matter when characters are a single byte. Since UTF-8 is the dominant encoding of the web, we make "never" the default option.

Rule Details

If the "always" option is used, this rule requires that files always begin with the Unicode BOM character U+FEFF. If "never" is used, files must never begin with U+FEFF.

Options

This rule has a string option:

• "always" files must begin with the Unicode BOM
• "never" (default) files must not begin with the Unicode BOM

always

Example of correct code for this rule with the "always" option:

/*eslint unicode-bom: ["error", "always"]*/

U+FEFF
var abc;

Example of incorrect code for this rule with the "always" option:

/*eslint unicode-bom: ["error", "always"]*/

var abc;

never

Example of correct code for this rule with the default "never" option:

/*eslint unicode-bom: ["error", "never"]*/

var abc;

Example of incorrect code for this rule with the "never" option:

/*eslint unicode-bom: ["error", "never"]*/

U+FEFF
var abc;

When Not To Use It

If you use some UTF-16 or UTF-32 files and you want to allow a file to optionally begin with a Unicode BOM, you should turn this rule off.

Version

This rule was introduced in ESLint 2.11.0.

Resources

• Rule source
• Documentation source