prefer-string-starts-ends-with

Enforce using String#startsWith and String#endsWith over other equivalent methods of checking substrings.

🎨

Extending "plugin:@typescript-eslint/stylistic-type-checked" in an ESLint configuration enables this rule.

🔧

Some problems reported by this rule are automatically fixable by the --fix ESLint command line option.

💭

This rule requires type information to run.

There are multiple ways to verify if a string starts or ends with a specific string, such as foo.indexOf('bar') === 0. As of ES2015, the most common way in JavaScript is to use String#startsWith and String#endsWith. Keeping to those methods consistently helps with code readability.

This rule reports when a string method can be replaced safely with String#startsWith or String#endsWith.

.eslintrc.cjs

module.exports = {
  "rules": {
    "@typescript-eslint/prefer-string-starts-ends-with": "error"
  }
};

Try this rule in the playground ↗

Examples

❌ Incorrect

declare const foo: string;

// starts with
foo[0] === 'b';
foo.charAt(0) === 'b';
foo.indexOf('bar') === 0;
foo.slice(0, 3) === 'bar';
foo.substring(0, 3) === 'bar';
foo.match(/^bar/) != null;
/^bar/.test(foo);

// ends with
foo[foo.length - 1] === 'b';
foo.charAt(foo.length - 1) === 'b';
foo.lastIndexOf('bar') === foo.length - 3;
foo.slice(-3) === 'bar';
foo.substring(foo.length - 3) === 'bar';
foo.match(/bar$/) != null;
/bar$/.test(foo);

Open in Playground

✅ Correct

declare const foo: string;

// starts with
foo.startsWith('bar');

// ends with
foo.endsWith('bar');

Open in Playground

Options

This rule is not configurable.

When Not To Use It

If you don't mind that style, you can turn this rule off safely.

Resources

• Rule source
• Test source