How do I use indexOf method with multiple Boolean values in an array (Javascript)? - Stack Overflow

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I'm just new to javascript and I'm having trouble getting the position of multiple boolean values using indexOf() method in an array. I've seen this similar post "indexOf method with multiple values" but it seems this only works on strings:

let arr = ["true", "true", "false", "false", "false", "false", "false", "true"];
let newArr = [];

for (let i = 0; i < arr.length; i++) {
  if (arr[i].indexOf("true") >= 0) {
    newArr.push(i);
  }
}
console.log(newArr) // result: (3) [0, 1, 7]

I'm just new to javascript and I'm having trouble getting the position of multiple boolean values using indexOf() method in an array. I've seen this similar post "indexOf method with multiple values" but it seems this only works on strings:

let arr = ["true", "true", "false", "false", "false", "false", "false", "true"];
let newArr = [];

for (let i = 0; i < arr.length; i++) {
  if (arr[i].indexOf("true") >= 0) {
    newArr.push(i);
  }
}
console.log(newArr) // result: (3) [0, 1, 7]

But, my codes look like below using boolean values in an array:

let arr = [true, true, false, false, false, false, false, true];
let newArr = [];

for (let i = 0; i < arr.length; i++) {
  if (arr[i].indexOf(true) >= 0) {
    newArr.push(i);
  }
}
console.log(newArr) // result: Uncaught TypeError: arr[i].indexOf is not a function

How could I make the same result (position: [0, 1, 7) like on string values in an array? I did try use join()method like ('"' + arr.join('","') + '"') to convert boolean into string. It only looks the same but now in one string value, so, it does not still work. Please help with my Javascript journey. Thanks for your help!

• javascript
• arrays
• boolean
• indexof

Share Improve this question Follow edited Aug 24, 2020 at 10:00 codecumber asked Aug 24, 2020 at 9:52 codecumbercodecumber 11233 silver badges99 bronze badges 2

• In the first example, you're calling indexOf on the first element of the array. This means that you're actually doing "true".indexOf("true"), which is a (weird) way of paring two strings. You don't need any of that. You can just use a[i] == "true" – Michele Lambertucci Commented Aug 24, 2020 at 9:58
• you can same functionality by type-checking , instead of using indexOf(); arr[i] === true like that. – Papai from BEKOAIL Commented Aug 24, 2020 at 9:58

Add a ment  | 

4 Answers 4

Sorted by: Reset to default 3

IndexOf returns the first index at which given element can be found in the array, or -1 if it is not present, meaning, you'll only find the first index (0).

let arr = [true, true, false, false, false, false, false, false];
let newArr = [];

for (let i = 0; i < arr.length; i++) {
    if (arr[i]) {
        newArr.push(i);
    }
}

console.log(newArr);

When you refer to arr[i] in your first code block, your referring to the string value at index i. This means that when you do:

if (arr[i].indexOf("true") >= 0) {

... you're performing .indexOf() on arr[i] which is either "true" or "false" (not your array). When you perform indexOf() on a string it will give you the first index of where the passed in string appears. In the case of "true", if arr[i] is "true" then you'll get index 0 as the string true starts at index 0. In the case of arr[i] being "false" then .indexOf() will return -1 since "true" doesn't appear in your string. As you can see, your .index() is just checking if "true" appears in one of your string array values. However, since your array values only consist of either "true" or "false" there is no need to use .indexOf() in your first example, but rather you can just check whether the current item in the array is equal to "true" and if it is, then push the current index to your array.

In the case of your second example, your values are booleans. This means arr[i] now is either the boolean value true or false. Booleans do not have an .indexOf() method, hence why you're getting an error. However, as pointed out above, you don't need to use .indexOf() checking if arr[i] is true, and instead, just need to check if arr[i] is true and if it is, then .push() the current index like so:

let arr = [true, true, false, false, false, false, false, true];
let newArr = [];

for (let i = 0; i < arr.length; i++) {
  if (arr[i]) { // roughly the same as if(arr[i] == true)
    newArr.push(i);
  }
}
console.log(newArr);

Hmm you could reduce it. There is an third argument the current index i

let arr = [true, true, false, false, false, false, false, true];

let result = arr.reduce((a,v,i) => {
   if(v === true) a.push(i);
   return a;
},[])

console.log(result);

Or with forEach it also has an index argument

let arr = [true, true, false, false, false, false, false, false];
let newArr = [];

arr.forEach((el, i) => {
   if(el === true) newArr.push(i);
})

console.log(newArr);

Basically indexOf is the method of Array and String type. So when you do is with Boolean value, it would not work

Array.prototype.indexOf()

String.prototype.indexOf()

So in your case, just case the value to String

let arr = [true, true, false, false, false, false, false, false]
let newArr = []

for (let i = 0; i < arr.length; i++) {
  if (String(arr[i]).indexOf(true) >= 0) {
    newArr.push(i)
  }
}
console.log(newArr)

No need to worry of casting the value passed to indexOf param to String, because this method would cast that for you

^{String.prototype.indexOf() > Syntax > Parameters}

^{ECMAScript Language Specification}

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