@typescript-eslint/prefer-string-starts-ends-with

There are many ways to ask whether a string starts or ends with some text: indexOf(...) === 0, slice(0, 3) === "...", charAt(0), s[0], a regular expression with ^ or $. Each one makes the reader work out the indexes. startsWith and endsWith say what you mean.

--fix rewrites the check to startsWith or endsWith where that can't change the result.

Configure#

Off by default. This rule needs types, so turning it on makes bun lint type check your code first.

bunfig.toml

[lint.rules]
"@typescript-eslint/prefer-string-starts-ends-with" = "error" # or "warn"

Every option, set to its default:

bunfig.toml

[lint.rules]
"@typescript-eslint/prefer-string-starts-ends-with" = ["error", {
  allowSingleElementEquality = "never", # "always" or "never"
}]

Examples#

example.ts4 problems

function isApiRoute(path: string) {
  return path.indexOf("/api") === 0;
}

function isHidden(name: string) {
  return name[0] === ".";
}

function isTypeScript(file: string) {
  return file.slice(-3) === ".ts" || /\.tsx$/.test(file);
}

example.tsNo problems

function isApiRoute(path: string) {
  return path.startsWith("/api");
}

function isHidden(name: string) {
  return name.startsWith(".");
}

function isTypeScript(file: string) {
  return file.endsWith(".ts") || file.endsWith(".tsx");
}

Indexes and methods such as indexOf are only reported on a string. On an array they're fine:

example.tsNo problems

function startsWithHome(segments: string[]) {
  return segments[0] === "~" || segments.indexOf("home") === 0;
}

Options#

Option	Default	What it does
`allowSingleElementEquality`	`"never"`	With `"always"`, allow comparing the first or last character, such as `s[0] === "a"` or `s.charAt(0) === "a"`.

example.tswith { "allowSingleElementEquality": "always" }No problems

function isHidden(name: string) {
  return name[0] === "." || name.charAt(name.length - 1) === "~";
}