behaviour of Array.indexOf()

Question

Array.IndexOf() function behaves different for strings and objects.

stringBehaviour();
objectBehaviour();

function stringBehaviour() {
    let s1 = "helmet";
    let s2 = "trousers";    
    let collection = [];  
    collection.push(s1);
    collection.push(s2);
    console.info(collection.indexOf("trousers"));
};

function objectBehaviour() {
    let obj1 = {item: "helmet"};
    let obj2 = {item: "trousers"};
    let collection = [];
    collection.push(obj1);
    collection.push(obj2);
    console.info(collection.indexOf({item: "trousers"}));
};

what can i do to have indexOf() not looking for the same instance but for equality of the objects elements so that the function objectBehaviour() would return 1?

it's because in JS, object's equality works by the reference value. In `console.info(collection.indexOf({item: "trousers"}));`, the object `{item: "trousers"}` is a new object with a different reference than `obj2`. Try with `console.info(collection.indexOf(obj2));` and you'll see it's the same behaviour. The problem is the reference — Kaddath, Aug 20 '19 at 16:05
in addition to the question which is marked as duplicated by yours, you should decide how to compare your objects (for your custom `indexOf` implementation). For instance, `{item: "trousers"} != {item: "trousers"}` (these are not the same object, just objects with the same fields) but `JSON.stringify({item: "trousers"}) == JSON.stringify({item: "trousers"})`; but this won't work if your objects may contain methods — YakovL, Aug 20 '19 at 16:07
thanks, i got that. can i somehow define how js will check for equality while searching a collection after a `indexOf()` call, like you could override the equals method in other programming languages? — Bobbey, Aug 20 '19 at 16:08
hardly so. But you can do `array.map(item => JSON.stringify(item)).indexOf(JSON.stringify(itemToBeFound))` (or you can define and use and `toComparable` helper instead of `JSON.stringify` — YakovL, Aug 20 '19 at 16:11
One solution is to use [findIndex](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/findIndex) which you call with a comparison function. For simple cases, you can use `JSON.stringify` as in @YakovL comment (it can be trickier with objects automatically timed with date objects -dates will differ- or recursive structure -JSON will error-) — Kaddath, Aug 20 '19 at 16:12

score 1 · Accepted Answer · answered Aug 20 '19 at 16:09

You could use findIndex instead and pass a function which compares objects by their properties.

stringBehaviour();
objectBehaviour();

function stringBehaviour() {
    let s1 = "helmet";
    let s2 = "trousers";    
    let collection = [];  
    collection.push(s1);
    collection.push(s2);
    console.info(collection.indexOf("trousers"));
};

function objectBehaviour() {
    let obj1 = {item: "helmet"};
    let obj2 = {item: "trousers"};
    let collection = [];
    collection.push(obj1);
    collection.push(obj2);
    console.info(collection.findIndex(el => isIdentical(el, { item: "trousers"})));
};

function isIdentical(current, given) {
  return Object.keys(given).every((key) => current[key] === given[key]);
}

behaviour of Array.indexOf()

1 Answers1