JS: Array.prototype.indexOf

By Xah Lee. Date: . Last updated: .
myArray.indexOf(value)
  • Return the index of first element that is value.
  • Return -1 if not found.

🛑 WARNING: It cannot find NaN. To check NaN, use JS: Array.prototype.includes. 

console.log([3, 4, 5, 6].indexOf(5) === 2);
console.log(["b", 5, 6, "b"].indexOf("b", 1) === 3);
// cannot find NaN
console.log([NaN, NaN].indexOf(NaN) === -1);
// finds undefined that exist as array element
console.log([3, undefined].indexOf(undefined) === 1);

// HHHH------------------------------

// sparse array
const xx = [3, 4];
xx.length = 100;

// cannot find non-existent element
console.log(xx.indexOf(undefined) === -1);
// true
myArray.indexOf(value, startIndex)

Search starts at startIndex. 

console.log([3, 3].indexOf(3, -1) === 1);

JavaScript, Array Search