prefer-ternary.md

Prefer ternary expressions over simple if-else statements

💼 This rule is enabled in the ✅ recommended config.

🔧 This rule is automatically fixable by the --fix CLI option.

This rule enforces the use of ternary expressions over 'simple' if-else statements, where 'simple' means the consequent and alternate are each one line and have the same basic type and form.

Using an if-else statement typically results in more lines of code than a single-line ternary expression, which leads to an unnecessarily larger codebase that is more difficult to maintain.

Additionally, using an if-else statement can result in defining variables using let or var solely to be reassigned within the blocks. This leads to variables being unnecessarily mutable and prevents prefer-const from flagging the variable.

Fail

function unicorn() {
	if (test) {
		return a;
	} else {
		return b;
	}
}

function* unicorn() {
	if (test) {
		yield a;
	} else {
		yield b;
	}
}

async function unicorn() {
	if (test) {
		await a();
	} else {
		await b();
	}
}

if (test) {
	throw new Error('foo');
} else {
	throw new Error('bar');
}

let foo;
if (test) {
	foo = 1;
} else {
	foo = 2;
}

Pass

function unicorn() {
	return test ? a : b;
}

function* unicorn() {
	yield (test ? a : b);
}

async function unicorn() {
	await (test ? a() : b());
}

const error = test ? new Error('foo') : new Error('bar');
throw error;

let foo;
foo = test ? 1 : 2;

// Multiple expressions
let foo;
let bar;
if (test) {
	foo = 1;
	bar = 2;
} else{
	foo = 2;
}

// Different expressions
function unicorn() {
	if (test) {
		return a;
	} else {
		throw new Error('error');
	}
}

// Assign to different variable
let foo;
let bar;
if (test) {
	foo = 1;
} else{
	baz = 2;
}

Options

Type: string
Default: 'always'

• 'always' (default)
  • Always report when using an IfStatement where a ternary expression can be used.
• 'only-single-line'
  • Only check if the content of the if and/or else block is less than one line long.

The following case is considered valid:

// eslint unicorn/prefer-ternary: ["error", "only-single-line"]
if (test) {
	foo = [
		'multiple line array'
	];
} else {
	bar = baz;
}