freeCodeCamp/curriculum/challenges/chinese/02-javascript-algorithms-and-data-structures/basic-data-structures/check-for-the-presence-of-an-element-with-indexof.md

---
id: 587d7b7b367417b2b2512b14
title: Check For The Presence of an Element With indexOf()
challengeType: 1
forumTopicId: 301154
dashedName: check-for-the-presence-of-an-element-with-indexof
---

# --description--

Since arrays can be changed, or *mutated*, at any time, there's no guarantee about where a particular piece of data will be on a given array, or if that element even still exists. Luckily, JavaScript provides us with another built-in method, `indexOf()`, that allows us to quickly and easily check for the presence of an element on an array. `indexOf()` takes an element as a parameter, and when called, it returns the position, or index, of that element, or `-1` if the element does not exist on the array.

For example:

```js
let fruits = ['apples', 'pears', 'oranges', 'peaches', 'pears'];

fruits.indexOf('dates'); // returns -1
fruits.indexOf('oranges'); // returns 2
fruits.indexOf('pears'); // returns 1, the first index at which the element exists
```

# --instructions--

`indexOf()` can be incredibly useful for quickly checking for the presence of an element on an array. We have defined a function, `quickCheck`, that takes an array and an element as arguments. Modify the function using `indexOf()` so that it returns `true` if the passed element exists on the array, and `false` if it does not.

# --hints--

The `quickCheck` function should return a boolean (`true` or `false`), not a string (`"true"` or `"false"`)

```js
assert.isBoolean(quickCheck(['squash', 'onions', 'shallots'], 'mushrooms'));
```

`quickCheck(["squash", "onions", "shallots"], "mushrooms")` should return `false`

```js
assert.strictEqual(
  quickCheck(['squash', 'onions', 'shallots'], 'mushrooms'),
  false
);
```

`quickCheck(["onions", "squash", "shallots"], "onions")` should return `true`

```js
assert.strictEqual(
  quickCheck(['onions', 'squash', 'shallots'], 'onions'),
  true
);
```

`quickCheck([3, 5, 9, 125, 45, 2], 125)` should return `true`

```js
assert.strictEqual(quickCheck([3, 5, 9, 125, 45, 2], 125), true);
```

`quickCheck([true, false, false], undefined)` should return `false`

```js
assert.strictEqual(quickCheck([true, false, false], undefined), false);
```

The `quickCheck` function should utilize the `indexOf()` method

```js
assert.notStrictEqual(quickCheck.toString().search(/\.indexOf\(/), -1);
```

# --seed--

## --seed-contents--

```js
function quickCheck(arr, elem) {
  // Only change code below this line

  // Only change code above this line
}

console.log(quickCheck(['squash', 'onions', 'shallots'], 'mushrooms'));
```

# --solutions--

```js
function quickCheck(arr, elem) {
  return arr.indexOf(elem) >= 0; 
}
```