Interview question

How do you check if an array includes a value, and what's the difference between includes() and indexOf()? किसी array में value है या नहीं कैसे check करें, includes() और indexOf() में क्या अंतर है?

Answer

Method	Returns	Detects NaN?
includes()	Boolean (true/false)	Yes
indexOf()	Index number, or -1	No - cannot find NaN

const arr = [1, 2, 3, NaN, 5];

// includes - simple boolean check, more readable
console.log(arr.includes(3));   // true
console.log(arr.includes(99));  // false

// indexOf - returns position, -1 if not found
console.log(arr.indexOf(3));    // 2
console.log(arr.indexOf(99));   // -1

// Old pattern before includes() existed - checking with indexOf
if (arr.indexOf(3) !== -1) {
    console.log('Found using indexOf');
}

// Modern, more readable equivalent
if (arr.includes(3)) {
    console.log('Found using includes');
}

// KEY DIFFERENCE: NaN handling
console.log(arr.indexOf(NaN));  // -1 - indexOf uses === which NaN fails against itself
console.log(arr.includes(NaN)); // true - includes uses SameValueZero algorithm

// includes with a starting search position (2nd argument)
console.log([1, 2, 3, 2].includes(2, 2)); // true - searches from index 2 onward

// String includes() works similarly (unrelated to array, but same concept)
console.log('Hello World'.includes('World')); // true

Method	Return	NaN detect करता है?
includes()	Boolean	हाँ
indexOf()	Index या -1	नहीं

const arr = [1, 2, 3, NaN, 5];

// includes - simple boolean check
console.log(arr.includes(3));   // true
console.log(arr.includes(99));  // false

// indexOf - position return करता है
console.log(arr.indexOf(3));    // 2
console.log(arr.indexOf(99));   // -1

// पुराना pattern
if (arr.indexOf(3) !== -1) {
    console.log('indexOf से मिला');
}

// Modern तरीका
if (arr.includes(3)) {
    console.log('includes से मिला');
}

// KEY DIFFERENCE: NaN handling
console.log(arr.indexOf(NaN));  // -1 - indexOf === use करता है
console.log(arr.includes(NaN)); // true - includes SameValueZero use करता है

// Search शुरू करने की position
console.log([1, 2, 3, 2].includes(2, 2)); // true

Was this answer clear?

Subjects

Related questions